The Week The South Froze: Meteorological Context

Disclaimer: the views expressed in this post are mine and mine alone and do not represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

Event Summary and Statistics

The southern U.S. just endured the coldest 10 day stretch in modern memory. The statistics below come from my own munging of the Integrated Surface Data downloaded from the National Center for Environmental Information. Countless temperature records were approached or broken:

  • Oklahoma City (KOKC) set a new record for the longest consecutive run of temperatures at or below 20F, edging the previous modern record set in the great 1983 freeze: an INCREDIBLE 210 hours from February 9 to February 17, 2021!
  • Houston’s Bush International Airport (KIAH), with observations going back to 1973, experienced its 7th longest run of temperatures at or below freezing (43 hours; first place remains with 1983 at a whopping 129 hours) and its 3rd longest run of temperatures at or below 20F, which occurred in the 14 hours from 7 pm Feb 15 to 9 am Feb 16.
  • Houston (KIAH) recorded its 6th coldest temperature of record, reaching 13F early in the morning of Feb 16.
  • Dallas (KDFW) experienced its 2nd longest run of temperatures both at or below freezing (140 hours) and at or below 20F (78 hours). It hit its 3rd coldest temperature (0F; ISD shows 0F but there are other reports citing -2F).
  • About 20% of U.S. stations set record low temperatures and 30% set record low MAXIMUM temperatures between Valentine’s Day and February 16:

In addition new all-time record cold temperatures were set or equaled at 80 U.S. stations during the week! Correspondingly, weekly average temperature departures for the entire region were incredible, with negative departures from the 30-year normal of at least THIRTY DEGREES Fahrenheit across a huge chunk of the central U.S., maximized in Oklahoma and northern Texas:

That’s the kind of cold that can dominate an entire seasonal outcome! Prior to this event, 90-day average temperatures were running 2-3 degrees above normal across Texas and Oklahoma. As of February 19, the 90-day average is now 1 to 2 degrees below normal! In other words, the ~10 days of 30 deg F below normal wiped out the prior 80 days of 3 deg F above. Basic math.

Accompanying the arctic cold were numerous bouts of frozen precipitation with snow of nearly a foot occurring in some areas, including Nashville. Serious ice storms hit a large swath of territory from west Texas through the Deep South. Numerous accidents resulted, including one particular horrific event in the Ft. Worth area on Feb 11.

One thing that is really worth underscoring is the incredible event duration. Look at this meteogram (time series of meteorological information) from Yukon, Oklahoma – just west of the OKC metro:

That’s nearly TWO WEEKS of temperatures below freezing!

Impacts:

I cannot begin to do justice to a comprehensive accounting of human and animal impacts from this event. What we can be sure of is that this will be a multi-billion dollar disaster. In my own neighborhood alone (several thousand homes) on the northwest side of the Houston metro, there were many more flooded homes during and immediately after this arctic cold outbreak than during Hurricane Harvey in 2017! This flooding occurred as pipes broke in thousands of homes, sending water cascading through attic and attached garage roofs and into homes. Even those nimble enough to shut off water to the home still suffered damage. Waiting lists for plumbers and renovation companies are miles long, and many many people are still without running water in their homes. DIY plumbing expertise is trending and there has been an epic run on PVC and fittings. Of course, businesses, schools, places of worship and residences are not generally built for this kind of cold down here; rather they’re built with endless summer heat in mind.

Property damage owing to busted pipes only scratches the surface of the damage. Far worse is the loss of life, whose tally is still being counted. There are reports of people that literally froze to death in their homes, millions of which were without power for days leaving many unable to heat their homes (and also accelerating pipe freezes). There are still countless folks in the nation’s second most populous state without WATER! I have no idea what the toll on ranching and agriculture is but it must simply be devastating. There was literally nothing that anybody around here could do to adequately protect flora and fauna from the wrath of one of the most intense and durable cold waves in history.

Much as already been written about why the electrical infrastructure broke down in ERCOT, the state’s electric grid operator. What I’ll say about the matter is that, in a nutshell, electric generators are strongly disincentivized to gird their infrastructure against this kind of event, which comes along once per generation on average. To do so would incur substantial capital costs that would have to be offset via rate increases, making those that took such actions uncompetitive in a market that only awards the right to generate power on a given hour to the lowest bidder. Perhaps government subsidies, ultimately backed by tax dollars of course, could be used to help foot the bill for such improvements. Freeze offs were by far the largest contributor to the severe imbalance in supply and demand that led to sustained power outages for millions during the heart of the cold. Roughly one third of the largely gas-fired power was offline for days as a result. The supply-demand imbalances were plainly evident in day-ahead ERCOT forecasts during the peak of the event:

I want to note that this was not about the failure of the renewables stack. There is more installed capacity for wind generation in the Lone Star State than in the next 4 states combined (almost) and about THIRTEEN TIMES the installed capacity in New York State. Yes, the turbines froze, but there are fixes for that and I think the next time around they’ll be ready for that.

There are many good articles explaining the various complexities of the situation. For example: here and here. Generator installers are going to be busy for a year servicing the fresh demand of thousands of Texans that now have little faith in the grid operator. Despite the recent surge in stock price, Generac holdings may look even more attractive to investors!

Anatomy of historic southern U.S. cold air outbreaks:

It is instructive to illustrate the kind of meteorological pattern that leads to the biggest cold air outbreaks for the south-central U.S. Warning: this section may get a little into the meteorological weeds so if you’re not up for a meteorology lecture, you won’t hurt my feelings if you skip ahead!

Let’s examine the pattern at two atmospheric “slices” for a few of the big cold air outbreaks that have impacted the south and the Lone Star State in particular. These two atmospheric levels will be in the middle of the troposphere about 18,000 feet (roughly 5600 meters) above our heads, and also mean sea level (near Earth’s surface) so that we can visualize the spatial relationships between large north-south displacements of the former and the position and evolution of intense surface high pressure systems (anticyclones) near Earth’s surface.

The big chills of December 1983 (left column), December 1989 (middle column) and February 2021 (right column). Top row: 500 millibar heights (like the topography of the atmospheric mass field). Bottom row: mean sea level pressure. The latter is displayed near or just prior to record-breaking cold entering the southern Plains and the former is the ‘set up’ pattern aloft a couple of days prior.

One thing that this “synoptic” comparison with the other two big chill events in the modern era (December 1983/1989) reveals are some common factors: just prior to the cold air outbreak, a large amplitude upper-level ridge of high pressure is almost always situated over far northwestern North America and/or Alaska (top row; indicated by the black “H” symbols on the maps above) with a downstream low pressure trough aloft. Winds through a deep layer of the atmosphere tend to blow from a northerly direction (white arrows). At the surface (bottom row), intense surface anticyclones (high pressure systems, around which winds blow anticyclonically in the Northern Hemisphere) build over the cold source region of far northwestern North America and are then driven southward. The reason why these surface features are high pressure systems (also shown with black “H” symbols in bottom row) is that the arctic cold air masses are very dense and thus the weight of the atmospheric column is heavier above them, exerting higher pressures on Earth’s surface. I have plotted the recently expired 2021 Arctic chill in the right column. You can see that this shares the common features shown in the other two events (and many others that I could show).

Because these patterns take time to establish, there is almost always several days’ lead time in predicting them. In fact, these Arctic outbreaks are often the culmination of a persistent pattern called ‘blocking’, a situation where transient upper-level disturbances are re-directed around the periphery of a stationary upper-level high pressure ridge. Think of a large boulder in stream that re-directs the water around its mass. That’s actually not what’s happening physically with these atmospheric blocks, but the effect is the same. These same disturbances actually continually re-energize the block by pulling momentum equatorward (southward in our hemisphere; sometimes this happens in reverse in certain kinds of blocking patterns). Because the pattern AROUND the blocking high pressure cell reinforces it, these patterns become durable. And because they are persistent, they tend to be especially predictable until they break down. That’s one reason why the 2021 Arctic cold blast was fairly predictable pretty far in advance. We’ll explore some of the issues related to event predictability below.

Early Indications of winter’s attack:

The Stratosphere

This full magnitude of the event was predictable over a week in advance. In fact, the potential for a vicious cold outbreak was potentially signaled over a month earlier. The meteorological world was watching a large-scale disturbance develop over the stratosphere in early January. This is a well-known phenomenon caused by wave energy that evolves upward from the troposphere (the lowest slab of Earth’s atmosphere where we live) for the largest atmospheric “Rossby” waves (eponymously named for the great Carl Rossby). As the largest of these waves propagate upward, they decelerate the westerly winds that flow around the periphery of the stratospheric polar vortex and simultaneous warm temperatures very significantly in its core, leading to its disruption. Sparing the details, this breakdown of the westerly wind flow at high altitudes is accompanied by an expansion of these winds, including the jet stream itself, into lower latitudes. The aforementioned warming at the core of the stratospheric polar vortex is why these events are called “Sudden Stratospheric Warming” episodes (SSW for short). It turns out that the occurrence of SSW’s can sometimes set the stage for significant and durable incursions of arctic air into the lower latitudes. Of course, this is not true of all SSWs and I am waiting to see if somebody can establish causal linkages connecting this year’s SSW to the recent impressive cold air outbreak. Nonetheless, there is a statistically elevated risk for severe cold outbreaks within 2-6 weeks of an SSW. Here was how this year’s event looked in a time-vertical cross section:

A time-vertical section of temperatures averaged along latitude circles and between 60 and 90 degrees of latitude north. Time is on the x-axis, running from left to right and atmospheric pressure on y-axis. Since pressure decreases upward, altitude increases upward as well on this plot. The top of the plot extends to the top of the stratosphere and the bottom near the mean sea level on Earth.

You can see the warm temperatures (appropriately shaded with the warmer colors) developing high up in the stratosphere in early January and descending towards Earth until late month. Some meteorologists keyed in on this as a forecast factor more than a month prior. Once such a disturbance begins descending, probability for intense mid-latitude cold outbreaks increase within weeks and catching one becomes a matter of daily monitoring of analyses and predictions. One issue for North America forecasting is that one cannot be sure that the cold air, which almost always builds up over Siberia, will find its way over to our hemisphere at any point. That DID happen this year.

Subseasonal Predictions:

It is noteworthy that even simple statistical models were predicting this over 3 weeks in advance. One such model that had success is called “Linear Inverse Modeling”, or LIM for short. The LIM makes predictions by essentially asking what kind of future temperature (or precipitation or any other predictand of interest) outcomes are most strongly correlated with slowly varying signals in the oceans and atmosphere. Here is Sam Lillo’s LIM prediction from January 26, made a solid 3-4 weeks prior to the event:

Image

Here was the average of a number of numerical weather prediction models, initialized on 1/21 or about a MONTH prior to the cold snap:

So you can see that even weeks in advance, there were indications of what was to come. Of course, the problem is that predictions at this time scale are not generally consistently skillful and thus are not particularly trustworthy and few stakeholders will take action on them. One area of research that has become very active in recent years is the examination of ‘forecasts of opportunity’: extreme outcomes (e.g., tornado outbreaks, active hurricane periods, arctic outbreaks, etc…) that sometimes (but not often) have predictable long-range precursors. The better we get at figuring out when the signals for these events can be trusted – and when they cannot – the more confidence we will have in forecasts like the above.

Medium-to-short range predictions:

By 7-10 days prior to the event, it was possible to see that a truly extraordinary event was in store for many in the southern states. This was also the time range when we could anticipate some of the regional details of the atmospheric pattern that was likely to unfold. There are many nuances to the event lead-up that I could totally geek out on, but I don’t want to make eyes glaze over so I’ll try for an overview. Here is the progression of 500 mb heights (this time shown as anomalies – or departures from a 30-year climatology of the same) in the top row and mean sea level pressure and ‘thickness’ in the bottom row. So this nearly the same display as I provided above in the context of historical cold outbreaks, but for two additional differences (besides the fact that I’m displaying 500 mb height anomalies here): (1) the maps I’m showing below are the actual forecast for the 2021 event (from the GFS) and (2) an extra field, thickness, is displayed below. Thickness is literally just that: the vertical height difference between two atmospheric pressure surfaces, in this case the 500 mb surface aloft and the 1000 mb surface near Earth’s surface. Thickness is proportional to the temperature of the layer: low thicknesses = cold and high thickness values = warm. Thus, the closer together the two pressure surfaces are, the colder the average temperature of the air column between them.


Top row: 500 mb height anomalies. Warm colors = high pressure and cold colors = low pressure. I’ve marked the approximate centers of the Alaskan upper-level high pressure and downstream Canadian low pressure at 500 mb on the top left panel. Bottom row: surface mean sea level pressure (black contours, every 2 millibars; color shading = 1000-500 mb thickness; cold colors = lower thickness values and cold air and warmer colors = higher thicknesses and warm air). The left plot is the actual weather pattern as of the evening of Feb 7, the middle panels are the forecast for 72 hours (3 days) into the future and right column the forecast for 144 hours (6 days) into the future, or near the peak of the event for Oklahoma.

The forecast progression reveals the reason why forecasters were issuing strongly worded warnings about the impact of the cold. Some things to note from the above:

  • The arctic cold air mass (the whites and purply colors in the bottom row) was slow moving and very intense, with low thicknesses literally “off the scale” for days (the white colors in the bottom row).
  • The upper level pattern (again, top row) associated with the cold air dome featured a very strong low pressure system aloft over Canada. This vortex sat in place for many days, facilitating the maintenance/build up of the extremely cold air mass over northwestern North America. This pattern was not forecast to – and in reality did not – change much during the first half of the month, leading to a prediction for an unusually durable event.
  • Once locales were behind the Arctic front, they stayed in the cold air for a long time. The bottom row shows how slowly the surface anticyclone and associated very cold air evolved. Refer back to the beginning of this post to review some of the statistics.

The foregoing provided a fair amount of forecast confidence in the severity and durability of the event and meteorologists amplified their messaging accordingly. Here are some excerpts from my own Facebook feed, where I progressively strengthen my wording as the event draws closer:

Feb 3: “The weather pattern is poised to facilitate the build up and transport of severe arctic air into the CONUS by later next week. The details are not predictable yet but the intensity of the arctic anticyclone and amplitude of the jet stream are likely to favor a very cold and wintery outcome for much of the CONUS east of the Rockies. This risk includes the Gulf Coast and Texas where setups like this can certainly produce unusually severe ice and snow accumulations. Best case our area will see a few days of very cold temps. Still monitoring but the risks warrant some attention, even 7-10 days in advance.

Feb 9: “The possibilities mentioned over the past couple of weeks are materializing: a real blast of winter is coming, rivaling anything we’ve seen in several years in Texas and in the greater Houston metro.”

Feb 10: “*** Near record-breaking arctic cold increasingly likely for Texas just after Valentine’s Day *** …preparations should be made to protect exposed infrastructure/plants/pools, keep pets inside and plan for the inability to travel from midday Monday through Tuesday.

As the event approached, temperature forecasts from a large suite of available model guidance were very impressive:

While these temperatures, predicted by the GFS about 3 days prior to the coldest day of the event – February 16 – were a bit too cold, they didn’t verify too far off and underscored the threat. In fact, these kinds of forecasts prompted forecasters to really amp up the wording to ensure that followers were prepared. Here is an excerpt from my own Facebook feed, part of a Feb 12 post:

Conservatively I’d expect temperatures to bottom out not far from 10F at places like Spring, Tomball and The Woodlands! This will be more than cold enough to solidify any ice on surfaces, destroy exposed and poorly insulated pipes and kill pets. So preparation is absolutely essential.

Snow and ice:

One of the things that made this event remarkable was that there was an active storm track across the southern U.S. This set the stage for truly impressive snow and ice accumulations. The latter also helped perpetuate the event:

https://www.nohrsc.noaa.gov/snow_model/images/full/South/nsm_depth/202102/nsm_depth_2021021605_South.jpg

Snow pack to the Gulf Coast is certainly not something you see very often! In fact, for a “last hurrah” the system dumped several inches of snow in southwest Texas and adjacent Mexico!

https://www.nohrsc.noaa.gov/snow_model/images/full/South/ruc_snow_precip_24hr/202102/ruc_snow_precip_24hr_2021021905_South.jpg

Here’s a picture out my front door. Although I only measured about an inch, that’s still a rare occurrence in these parts!

This post is now long enough. There are additional angles that I intended to cover that I don’t have time or inclination to at this time. This includes the climate change context (spoiler: winter can still happen under global warming; keep the difference between climate and weather in mind), a detailed weather-climate analysis and a more in-depth examination of the human impact side of the story. I’ll conclude by empathizing with the many people that have suffered – and continue to suffer – as a result of poor planning and a lack of readiness by the organizations that they depend on to keep the lights on. I am glad that my own little humble efforts to give my followers as much actionable advanced notice as possible helped them prepare as best they could.

Historic winter weather and arctic cold event for the Greater Houston Metro (February 14, 2021)

Disclaimer: the views expressed in this post are mine and mine alone and do not represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

Note: this is simply an informational post meant for linking on social media. I do have planned what I hope will be a far more interesting retrospective on this event in which I’ll attempt to provide historical, meteorological and climatological context for this once-in-a-generation event for these parts.

What may wind up being recorded as the most extreme winter precipitation and thermal event in the modern era appears imminent for a big chunk of the Lonestar State. Not to detract from our neighbors to the north in Oklahoma, who have already been suffering under a truly mind-blowing duration of extreme cold (going on 9 days of sub-20F in parts of the OKC metro area). Preparations should be rushed to completion. Aside from the mandatory steps to protect pipes and pets, this should include plans to deal with *prolonged power outages and water supply disruptions.* See the attached Winter Storm Impact graphic, something I’ve never even thought about consulting for this area before despite following weather in this area for 40 years!

Overview:

There are few substantive changes to the forecast. Expect exceptional winter storm conditions (for our area) to develop overnight with 2-4″ of snow across northern half of the area – possibly more in spots – on top of a layer of ice and sleet. Temperatures will likely remain in the teens all day Monday, bottoming out in the single digits by early Tuesday AM. This combo of non-negligible accumulating multi-modal winter precipitation and durable sub-freezing temperatures makes this an exceptional event for this region.

Precipitation details:

Model-derived soundings from the across area suggest that there remains a warm layer within the lower troposphere about 1.5-2 km deep in this area. As this layer is lifted and cooled by the approaching storm system, a transition through all three major precipitation phases will occur in our region. Cold rain should become widespread across Harris County with some icing on elevated roadways imminent throughout the region. This will transition to freezing rain mixed with sleet by 5-6 pm across northwest and north Harris and Montgomery Counties (which has already experienced some). Roads will very quickly ice over and travel will become increasingly perilous. Precip will most likely transition to a predominant sleet mode sometime in the 9 pm to midnight window (all times should be treated as best guesses). Some snow will likely mix in as well and a heavy sleet/snow mix is likely at some point in the roughly 10 pm through 1 am time frame. Thereafter we should change over to heavy snow and by Monday mid-morning I think that the Magnolia/Tomball/Spring/Klein/Cypress/Aldine/Jersey City/Fairfield area will see anywhere from 2-4 inches of snow with some upside risk. Areas to the north will see as much as 4-6″ with higher pockets. How much snow vs ice vs sleet is entirely dependent on the timing of the transition between phases. The more freezing rain/icing we see before changeover to sleet/snow, the earlier/more widespread/durable power outages will be across the Centerpoint and (to an extent) Entergy districts will be. Skies should quickly clear by mid-late Monday morning, setting the stage for the coldest temperatures of the modern era.

Temperatures:

All-time record lows are in jeopardy. With fresh snow laid down, most of the incoming solar radiation will be reflected while a fresh batch of Arctic air is transported into the area from the north. This means temperatures will likely start out in the teens across the northern part of the Houston metro and low 20s south, falling slowly during the day. By sundown Monday, we will have spent the entire DAY below 20F (since around 1-2 am actually) and won’t crack above until probably around 11-noon Tuesday. If this holds true, this will be WITHOUT PEER in the modern meteorological area. You’re talking the likelihood of spending nearly ONE AND A HALF DAYS BELOW 20F IN HOUSTON. See the attached meteogram (courtesy of WeatherBell) from this morning’s HRRR to see how incredible this looks on a timeline.

By comparison, the great late December 1989 event saw us spend “only” 19 hours below 20F! Temperatures will likely bottom out sub 10F (our area could see anywhere between 4 and 12F) with colder to our north. Sub-zero is possible as close as B/CS and Huntsville. * Mid-week encore?*Hopefully we will warm up enough that the next winter storm will remain north of the Houston metro. Something I’ll dig more into later today before we lose power.

Impacts:

I glossed over in the overview but it’s noteworthy that ERCOT appears not ready to handle the load. Power has been limit up over the weekend and rolling blackouts certainly appear possible – if not probable – by Monday across the area. In addition, any place that gets a heavy initial ice accumulation on overhead lines and/or trees abutting those lines will likely see the power served by said lines disrupted. It’s not clear to me that CenterPoint is going to be able to restore power as quickly as they usually do in the aftermath of the weather modalities that usually knock out power around here (i.e., tropical cyclones and squall lines). Thus, be prepared for prolonged power outages! Charge those cell phones, protect sensitive medications and for goodness sake get pets and homeless folks OFF THE STREETS AND OUT OF THE YARDS. Regardless of efforts made to protect exposed pipes, I do expect that many will suffer very serious damage to their water infra. Plumbers will be quite busy throughout the area.

Summary

While this is normal winter weather for North Dakota, this area never built this kind of event into its plans when designing infrastructure. Thus, expect a very sizable disruption to your daily lives and please have plans to keep people and pets warm and safe. Avoid travel beginning this evening through Tuesday morning. Please, stay safe.

Hell hath no fury like Hurricane Laura

Disclaimer: the views expressed in this post are mine and mine alone and do not represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

Wednesday AM, August 26, 2020:

Now that much of the forecast drama has faded we can turn our full attention to anticipating Hurricane Laura’s full impacts. The only good news here is that, pursuant to deterministic model consensus that developed during the day yesterday, the storm’s worst impacts are going to miss the nation’s fourth largest city to the east. If not for the persistent Texas upper-level trough responsible for laying out the red carpet into the Texas-Louisiana border region, we’d be talking about preparing for the most damaging hurricane in U.S. history. As it stands, this will be the worst hurricane ever to strike the southwestern Louisiana area. While that is absolutely terrible in and of itself, we’re talking about a region where roughly half a million souls reside, versus nearly 7 million in the greater Houston metro. Again, I do not wish to downplay the tragedy about to unfold from roughly the far upper Texas coastal plain through central Louisiana and far inland!

The current state:

Unless you are living a truly spartan lifestyle devoid of internet, you will be inundated with millions of minute-by-minute updates from every Jim, Joe and Sally, including spectacular satellite and radar loops. As anticipated yesterday, Laura has become a major hurricane and is right on the cusp of becoming a Category 4 storm. Given her large size and some of the internal dynamics ongoing, it’s not clear that she’ll get all the way to 5. Either way, this is a violent hurricane!

Of course, recon is buzzing around through the storm (or, rather, bumping around) and the data being gathered paints an impressive picture:

Here we can see estimated flight-level winds WELL in excess of 110 kts (greater than 130 mph) with similar winds near the surface. Surface pressures at last sampling were approaching 950 mb. I cannot find any buoys that are actively reporting wave heights but an analysis I’ve seen suggests anywhere from 24 to 40 feet out there. Given the size of the storm, this is about the right range. More on implications for surge inundation below! Laura will be a strong Category 4 storm around the time of landfall!

Steering wound up being a clean-cut “follow the weakness in the ridge”. A feature responsible for ensuring the presence of this western edge and the establishment of a solid deep-layer south-southeasterly steering flow has been an upper-level trough that has set up residence in Texas:

This upper low was actually part of a deeper disturbance that detached from the main storm track way off to the north and got deposited. This allowed Laura to make a steady right-hand turn and she’s bearing down on the NW Gulf Coast as a result. A quick check of the memory bank tells me that this will be the worst storm of the modern era for this region. Audrey in 1957 was terrible but the area has developed significantly since then and this storm looks to be more powerful at landfall. That brings us to a discussion of impacts.

Impacts:

The latest numerical guidance paints a frightful picture for landfall:

As with all hurricanes – especially larger ones – the most life-threatening aspect is surge. This storm will be no exception. I do not expect many – if any – structures to survive along the immediate coast from roughly Port Bolivar, Texas and eastward with the very worst occurring along Highway 82 across coastal Louisiana. The towns of Sabine Pass, Cameron, Hackberry, Creole and Grand Chenier are in BIG TROUBLE:

That’s a projected surge of 10-15 feet along the extreme upper Texas coast, 15-20 feet across S. Louisiana and 8-12 feet as far east as Morgan City! This will simply be a devastating event reminiscient of Katrina, but this time farther west. In areas experiencing the strongest surge and topping, battering waves, extreme damage will extend MANY MILES INLAND! Even the National Hurricane Center is justifiably referring to this as an “unsurvivable surge”:

Unsurvivable storm surge with large and destructive waves will cause catastrophic damage from Sea Rim State Park, Texas, to Intracoastal City, Louisiana, including Calcasieu and Sabine Lakes. This surge could penetrate up to 30 miles inland from the immediate coastline. Only a few hours remain to protect life and property and all actions should be rushed to completion.

The latest ADCIRC model shows something else ominous:

While this is based on the 0900 UTC advisory and surely will be worse when re-initialized using the 1500 UTC update, it shows the likelihood of serious inundation along important waterways well inland, notably along the Calcasieu River and attendant tributaries and bayous of Lake Charles. I am fearful that the city of Lake Charles is going to be devastated by this!

As if all of this isn’t bad enough, severe winds will extend far inland:

This is an estimate – and probably not a terribly overdone one – of wind gust potential near the surface. Note how hurricane force wind gusts are likely to extend well inland, mowing down the dense deciduous forests that are prevalent in much of that area and doing lots of property damage as trees topple onto structures and autos. I also have another concern, and that is for tropical tornadoes. With the storm coming inland just to the east of a fairly stout belt of southwesterly flow aloft, that’s a recipe for hurricane-spawned mini-supercell tornadoes. The latest prediction from the HRRR is supportive:

One last note about the above: it’s still possible for the eyewall and all of the aforementioned impacts to make small but meaningful shifts. For this reason anybody near the path must stay closed tuned into credible media sources, who will relay the latest from the official NWS/NHC forecasts.

So that’s all I will say about Laura. My attention must turn back to other matters. I pray that all residents in the direct path of the eyewall and ANYWHERE near the outer wind diameter in the surge-prone areas have taken all necessary precautions. This will be a memorable storm.

Forecast closing in on a Laura consensus: Houston may be spared

Disclaimer: the views expressed in this post are mine and mine alone and do not represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

I am providing an update at 5:30 pm CDT August 25. Hurricane Laura is now entering the warmest portion of the Gulf of Mexico in a low shear environment, favoring steady intensification until perhaps just a few hours prior to landfall when some westerly wind shear begins to interact with Laura’s inner core.

Laura’s structure continues to improve with several “hot towers” having blossomed within a rapidly improving central core of convection that is building around a center – soon to be eye:

It appears that for much of the rest of its over-water journey, Laura will be feasting on the warmest waters in the Western Hemisphere:

I’ve rather crudely drawn a few forecast center locations superimposed on the latest high-resolution sea surface temperature (SSTs) map I could find for the Gulf. It appears that Laura will be traversing SSTs in excess of 30C for about the next 24 hours or so. This factor, combined with low vertical wind shear, should allow for robust strengthening, checked only by some dry mid-tropospheric air in the vicinity and the relatively large size of Laura’s developing inner core/eye. These latter factors may be the only ones that stand in the way of a maxi-cane! Keep in mind that we just aren’t very good at intensity forecasting. Laura could still easily attain Category 4 strength in short order based on warm SSTs with a deep oceanic mixed layer and light vertical wind shear alone.

What about that vertical wind shear that I mentioned above? Well it appears that some may begin to disrupt Laura’s inner core just before landfall, which would be a good development if it occurs since this might mean the storm undergoes a weakening trend at landfall:

Confidence in this latter factor is not very high and I wouldn’t put a lot of stock in it.

As the storm has become better defined and recon missions continue to sample the in-storm and surrounding environment, model forecasts appear to be improving in the short-term, lending to higher confidence overall in the track. During the past 12-24 hours the high-resolution numerical weather prediction guidance has developed a tight consensus on which portion of the Gulf coastline will experience landfall:

Specifically, it now looks increasingly likely that landfall will occur somewhere between central/eastern Galveston Island and Sabine Pass, Texas. If I had to weight the possibilities, I’d say the Beaumont/Port Arthur/Orange area (home to about 400,000 souls) is in considerably more danger from Laura than much of the Houston metro area (home to about 7 millions souls).

Of course, the sensitivities to the precise final path are huge! This is especially true in inundation-prone places such as Galveston, Seabrook, Kemah, Dickinson, Texas City, Port Bolivar (where residents should have already heeded local authorities and any evacuation orders that are standing). To get an idea of the impacts that a direct storm surge would have on these regions for a Category 3 storm, take a gander at the output of the Slosh model. While omitting impacts of rain-induced flooding and wave topping, it does a reasonably good job of showing the inundation risks:

Red areas are those where risks are for anything under 9 feet high to be under water if a Category 3 storm surge impacted a given area. Oof! That’s why anybody in those colored areas within or even near the “cone” need to heed evacuation advice!

What about wind impacts? Take this with a hurricane-sized grain of salt, but this is a “wind gust swath” estimate from the 12z ECMWF deterministic run:

The ECMWF is probably significantly overdone (150 mph+ gusts 2 counties inland?!) but a couple of things are noteworthy: (1) notice how FAST the storm is likely to be moving, causing damaging winds to rapidly penetrate inland near the eye and (2) notice how sharp the GRADIENT of predicted winds are. This means even tiny shifts in the path will make a HUGE difference in the sensible weather at any given location! Taken literally, if this forecast were to perfectly verify, this would be a total non-event in the western two-thirds of Harris County and a very big event in the far eastern outposts of the metro area. It would also likely be a disaster for the entire coast from Galveston northeastward.

As it stands right now, it appears that – given the tight consensus – there is likely to be a significant hurricane wind event for the Golden Triangle of Texas! Again, any westward wobble/shift significantly and rapidly increases the threat to the Greater Houston metro. However I am feeling increasingly comfortable in lessening that threat.

Rainfall: if there’s one thing we can breathe a definitive sigh of relief about regarding Laura impacts it’s that the storm is likely to be booking it when she comes ashore. This is in stark contrast to Harvey, which was a simmering durable nightmare caused by a nearly stationary decaying hurricane that inundated Houston with multiple tropical rainbands over a 3.5 day period, the likes of which we’ll likely never see again in our lifetimes. That’s not to say that there will be no problems; quite the contrary as the storm will be capable of dumping up to a foot of rain in some areas. However the overall destructive potential from rainfall-induced flooding is far less in this storm due to its fast movement.

Bottom line:

Hurricane Laura is going to pose a grave threat to life and property along the northwest Gulf Coast. However, it is looking more likely that most of the serious impacts will miss much of the Houston metro to the east. This looks like a very dangerous storm for far southeast Texas and adjacent areas of southwest Louisiana.

All of this said, any wobbles or errant movements of the storm’s center greatly changes this so careful attention should be paid to the official forecast and local outputs from NWS, media and civil authorities. I’ll also be monitoring this storm closely until landfall. If anything radically changes I’ll be quick to update.

Houston, do we have a problem?

Disclaimer: the views expressed in this post are mine and mine alone and do not represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

Hurricane Laura has survived a punishing gauntlet that included persistent northerly wind shear and a direct trek across the Greater Antilles. Now, at long last, the storm has emerged into the south-central Gulf of Mexico and is poised to feast on some of the highest octane fuel on the planet in the form of sea-surface temperatures of 30-32C (86-89F), save for a brief tangle with a cool eddy. If you’re interested, you can see what theory provides as an upper-bound of intensity for a storm in this environment. This is based on realistic conceptual models of how hurricanes convert thermodynamic energy stored in the ocean boundary layer into kinetic energy (wind).

Anyway, enough of the digression. Onto the current situation. Laura is presently organizing and intensifying over the Gulf, recently attaining hurricane status. A recent infrared satellite image shows that Laura has not yet formed an eye but that is likely to materialize soon:

To get an idea of what the storm’s inner structure looks like as of Tuesday morning (August 25) we actually can see data from a sounder, which detects ice crystals in the clouds. We can see a clear “hook” shape in the returns, which indicates that an inner core is forming up:

My suspicion is that we are seeing the beginnings of a fairly broad core/eye and this may lead to fairly slow intensification for a while. However, by tomorrow more rapid intensification (RI) will probably commence at some point and I do believe Laura will become a major hurricane by tomorrow evening some 6-12 hours prior to landfall. I expect that the internet will soon become festooned with all manner of meteorological “eye candy” as it relates to the storm.

Here is the “hot off the press” prediction of the hurricane’s low-level wind field per the 12z HWRF. Please don’t take the position literally – I’m showing this as an indication of what I believe to be a realistic intensity forecast for this storm:

One thing that inspires some confidence in this prediction is the fact that this model appeared to initialize quite well (agree with observed position/intensity):

Note that recon (left) fixed the center at 14z near 23.5N/86.4W and the 12z storm as initialized by the HWRF (right) is very close in central pressure and position (yes they’re lagged by 2 hours; sorry).

So the odds are high that Laura will become a major hurricane prior to landfall. Two things that could limit the upside to intensity: (1) some increased westerly shear from the Texas upper-level low – the same feature I described in previous blog posts as being responsible for Marco’s demise and for figuring in the long-range track prediction of Laura. (2) the internal structure of Laura; research shows that a broader inner core tends to be less favorable for RI.

Where is Laura going??

The all-important track question has confounded and befuddled computer models and humans alike. Take a look at the track bias of late:

Just focus on the squares, which are 72 hour forecast bias values. Nearly every single model has been much too far north! This trend appears to be decreasing a bit today (refer to the HWRF example above) so hopefully we’ll soon be able to put more faith in the higher resolution deterministic model predictions.

The same “players are on the table” with regards to steering during the final 36-42 hours of Laura’s life over water:

This map shows a snapshot of deep-layer steering winds over the Atlantic Basin. It is basically a weighted-average of winds from the lower to upper troposphere and is relevant for a hurricane that becomes as strong as I expect Laura to become. There is the omnipresent subtropical high pressure ridge sprawling across the southern U.S. and a cyclonic circulation (so-called “inverted trough”) across Texas. The deep-layer flow east of this latter feature is from the south-southeast and as Laura’s circulation gets caught up in this, the storm will turn to the north-northwest. This doesn’t help us nail down the precise landfall location, which is something we really want to know 36-42 hours prior to landfall of a likely major hurricane! However, I think we can say that the probabilities of impact to the middle Texas coast are substantially lessened. I believe the eye will cross the coast somewhere between San Luis Pass, Texas and Cameron, Louisiana.

Unfortunately, even this close to landfall such precision is in the noise. That is precisely why everybody in the so-called “cone of uncertainty” – and especially those within the hurricane warnings that were hoisted at the 10 am CDT advisory time – must prepare as if the eyewall is coming to get them! NOBODY can do any better for you! They can’t tell you all of the salacious details that you really want to know! You just have to follow the old adage of prepare for the worst but pray for the best!

For what it’s worth this plot may still be the best we can do. It’s a calibrated track forecast from The Weather Company, an IBM business:

I was engaging in a mild debate with a colleague of mine yesterday – a brilliant meteorologist and forecaster. He pointed out, perhaps correctly, that the ensembles are coarsened versions of the operational models and thus should not really be used for track forecasting in cases where the storm of interest eventually undergoes RI. Interestingly, under calibration, those concerns really lessen. This product is calibrated. Note that the probability swath includes over 10 million people in the coastal plain alone!

What about “my gut”? Just what IS a “gut feeling” anyway? Well I’d argue that in our domain – weather forecasting – it’s a strong inner conviction that synthesizes the historical experiences of a forecaster: “I’ve seen such and such play out before, and the outcome was X”. In this case, my “gut” is telling me that steering is such that a turn should occur and will PROBABLY spare most of the Houston-Galveston Metro area. However, any little deviation or wobble of the eye within the last few hours of approach could radically change the outcome!! My BRAIN, of course, is telling me that the models have been too far to the east and the storm has already gained a lot of westward ground so a direct impact to the largest metro area BY FAR in the entire Gulf Coast is still very much on the table. The difference between a Cat 3/4 striking San Luis Pass and Sabine Pass is billions of dollars and quite a few casualties! Not to downplay the seriousness for whomever gets hit, but that’s the reality here.

Summary:

What to expect: major hurricane impacts along the upper Texas and/or far southwestern Louisiana coastal plain.

How strong: at least Cat 2 but probably Cat 3 and if shear and inner dynamics are not detrimental it could be much stronger than that.

When to expect it: late Wednesday night into Thursday morning/early afternoon.

I will TRY to update this blog again before impact.

Risks from Laura increasing for Texas coast

Disclaimer: the views expressed in this post are mine and mine alone and do not necessarily represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

Tropical Storm Laura has been fighting a valiant battle with northerly wind shear and disruption of moist inflow thanks to its trek across and just south of Cuba overnight. For much of the past 12-18 hours, Laura has looked like half a storm, with the northern semicircle largely devoid of convection. However, the center has managed to stay over the warm waters of the far northwestern Caribbean just south of Cuba and, as a result, is acquiring a reinvigorated central core of convection:

This may begin a period renewed intensification prior to a likely trip across the western tip of Cuba overnight. One of the main storylines with Laura is that it has stayed consistently south of the bulk of model guidance. The latest reconnaissance aircraft mission has found an apparent new center about 1 degree west of the previous fix:

This continues the trend of Laura’s center defying predictions by staying on a more westerly heading. The chart to the right in the above image shows that storms that stay south of guidance early tend to wind up staying south later (h/t to Tomer Burg for crunching the numbers and creating this EXCELLENT analysis!).

The implications are troublesome. Although the latest NHC track forecast (as of 11 am CDT August 24) stayed very consistent with the previous package in showing a “cone of uncertainty” that actually avoids a direct strike on the middle-to-upper Texas coast, the clear southward bias during this phase of Laura’s trek makes it clear – at least to me – that this is not really adequately weighting the landfall probabilities optimally. It’s worth pointing out (h/t to Jason Sippel for the reminder) that NHC constructs error bars that reflect their 5-year average forecast errors as a function of forecast horizon (time into the future). As such, they lose the opportunity to incorporate important information from the numerical weather prediction models which, in this case, are more dispersive than the historical error!

What about the steering pattern going forward? Well, while it’s still unclear what impact the broader circulation associated with Marco (which has come much farther east than predicted) will have on the environmental steering flow, one prominent feature that definitely matters is the persistent upper-level trough and attendant circulation over Texas. You can clearly see this on a recent water vapor channel satellite image:

I’ve taken the liberty of annotating this image with important facts about Marco and Laura. This Texas low pressure system is responsible for inducing the southwesterly vertical wind shear that has kept Marco weak and asymmetric. However, short-range model forecasts reasonably show this feature weakening and shifting westward through tomorrow night:

This will also make possible a more westward track for Laura. Let’s look at the 06z ECMWF ensemble positions for the storm center near the time of projected landfall:

The vast majority of these are WELL southwest of the southern edge of NHC’s cone! So while an impact along the Texas coast is not a grand slam, it’s certainly looking more likely despite some of the more reliable high resolution forecast models and global deterministic model forecasts that heighten probabilities closer to Beaumont-Lake Charles.

The punchline is that I am assessing a greater risk to the upper and even middle Texas coast than the official forecast at this time. This is by no means certain but everyone from Corpus Christi to Lafayette, LA should be making preparations for a hurricane!

How about the intensity forecast? Well, with the aforementioned Texas upper-level low likely to drift west, the same vertical wind shear that Marco suffered will not be nearly as detrimental to Laura, which will be taking a prolonged track right across the warmest waters of the Gulf of Mexico. So, needless to say, upside is significant, especially for a storm that tracks farther south. For what it is worth, here is the latest intensity guidance from the potpourri of available models:

The upside implied by these appears to be Cat 3 (update: Jason Sippel informs me that these are misrepresentation of the modeled intensity due to smoothing; the HWRF has been consistently showing a truly violent hurricane making landfall) but in reality if the storm is able to undergo a period of rapid intensification prior to landfall, it could certainly go higher. We’re just not good at intensity forecasting so take all of this with a grain of salt. Given that Laura will experience a very favorable environment for intensification, there is certainly a risk that she will be a major hurricane at landfall. Everybody in the path should prepare accordingly!

Tropical Trouble 2020: Watching Laura

Intro

The 2020 Atlantic hurricane season promised to be a bad one and it appears to be trying hard to live up to expectations. Numbers to date have been significant but metrics speaking to overall season severity, including accumulated cyclone energy and number of hurricanes-to-date, have been average. The season, however, now looks poised to make a run for it in these categories. To wit, Tropical Storm Laura is the most promising prospect to date to help make the season start to live up to the pre-season hype.

I won’t spell out the current details since you can easily keep abreast of the latest happenings on any number of excellent websites. Some that I highly recommend are the National Hurricane Center, Tropical Tidbits and WeatherNerds. My focus will be on the long range forecast for Laura. First, check out the latest visible satellite image in which you can see something pretty remarkable: two tropical cyclones in the same general part of the basin simultaneously.

The lead storm in the Gulf is, of course, Marco (Polo – ha!) and this now minimal hurricane will likely make landfall soon along the north central Gulf Coast and clear the path for Laura’s entry early this week. I’ve superimposed sea surface temperatures (SSTs) and you can see that SSTs across much of the Gulf are near or in excess of 30C. In case you’re wondering, this supports as strong a hurricane as you could ever not hope for.

Intensity prospects

I suppose there is some likelihood that Marco will produce a cooling of the SSTs via upwelling in his wake across the central Gulf of Mexico. However it’s interesting to note that just prior to the storm crossing this portion of the basin, a “cool eddy” was already “observed” by satellite. You can see anomalously low sea-surface heights (which also means a small depth over which warm waters exist beneath the ocean surface) over some of the same areas traversed by Marco. While this might temporarily tamp down on intensity potential, the storm will move into much higher octane waters if/when she gets beyond about 88W. So already there is a dependency of Laura’s ultimate intensity on track: a more westward track allows the storm to enjoy a lengthy stretch of bathwater over the western Gulf whereas a northerly track means less time to intensify. For any outcome, it’s a pretty good bet that whatever circulation emerges from the western portion of Cuba will intensify into at least a moderately strong hurricane prior to landfall.

Ultimately, the actual intensity achieved is strongly dependent upon details involving interaction of the storm with its environment – and dynamics internal to the storm – that are hard to predict. Eyewall replacement cycles, periods of rapid intensification and the like are notoriously elusive predictands. Suffice it to say that given the weakening of the vertical wind shear that is hampering Marco, Laura’s prospects for intensification look significantly better and a major hurricane is quite a possible – if not probable – outcome by landfall.

Want to see the upside as predicted by actual hurricane models? Take a look at this 12z horror show from the NHC’s own HWRF and HMON models (8/23 12z initializations). Keep in mind that we cannot really say which part of the coast will ultimately suffer. The point here is that the models best suited to predict the details of hurricane structure/evolution show the upside very clearly and it’s why this needs to be taken seriously.

Who gets hit?

The short answer based on latest data is anywhere from the middle Texas coast to Louisiana. The environmental steering flow is dominated by a fairly strong subtropical ridge that is building over the Southeast U.S. right now. This will force Laura into the Gulf and on a generally west to northwest heading until landfall.

Unfortunately exactly WHICH portion of the Gulf Coast gets the brunt of Laura is impossible to pin down right now. You can appreciate the challenge by taking a gander at these “spray and pray” charts below:

These are ensemble tracks from two different model systems and both have the highest density of modeled tracks spread from around the middle Texas coast to western Louisiana. That is indeed the swath of coast that I believe is at highest risk. The intense subtropical ridge to the north of the storm will likely have a western extent somewhere near the Texas/Louisiana border. Depending on how much latitude Laura gains during her traverse across the Gulf, the storm may well experience increased deep-layer steering flow from the southeast, impelling the storm to turn northwestward. The sooner that happens, the farther east landfall will be. Landfall timing looks to be anytime between late day Wednesday and Thursday AM depending on location.

Bottom Line:

Details matter, including how much disruption Laura’s circulation experiences while traversing Cuba and how far south the storm emerges into the Gulf. As long as the center is being disrupted and remains relatively disorganized, models will struggle to initialize the storm and to predict its evolution. Thus, these forecasts will change. The only thing I feel confident is stating at this point is that somebody along the Texas or Louisiana coast is probably in big trouble. Stay tuned!

Preliminary Forecast Analysis of the October 20, 2019 Dallas Tornado

Yet another significant nocturnal tornado event unfolded last night across the Dallas Metroplex, something that appears to be happening more frequently lately (topic for another day perhaps). I was paying attention with “half an eye” due to extended family in town and other matters but I watched in horror as the event unfolded last night. I decided to dig into the data in the immediate aftermath to see what I could learn. I’ll put my preliminary findings here. Others, including Jon Davies, can probably do this more justice; specifically, I believe much more detailed analysis is required to really understand why the storms initiated the way they did.

Let’s start with what happened. Here’s a screen shot of one the tornado as it exited the far northeast suburbs of the Big D (sorry I don’t know whose this is but if it is your capture please let me know so I can either remove or credit it):

 

Dallas Tornado

Pretty stout stovepipe and looks as if the tornado was up to half a mile wide and probably attained strong to violent status along its path through Dallas. Here is a radar shot (base reflectivity and velocity, the latter indicating a strong couplet bearing down on Garland):

IMG_1741

Oof. Scary stuff! Correlation coefficient indicated lofted debris, confirming spotter reports at the time of a significant tornado rampaging through a highly populated and expensive part of town! Read this for more good basic info about how to interpret some  of the dual-pol radar products.

The meteorological set up was pretty classic: deep mobile trough aloft with attendant low-level cyclone, creating a highly sheared environment that superposed strong westerlies aloft on low-level southerlies. The latter wind field advected plentiful low-level moisture into the Red River valley. Here are a couple of upper-level maps indicating the key features (250 mb analysis to the left and 850 mb to the right):

upper level and low level 20191020

The observed sounding at DFW is where things get more interesting:

DFW Skewt 2019102100

There is a strong cap evident at 0z, just under 2 hours prior to tornado time and right around the time storms were initiating just to the southwest of the sounding location. The presence of a strong cap indicates that SUBSTANTIAL mesoscale forcing for ascent was required to initiate storms in the area. Apparently that DID happen. Forecast soundings, incidentally, showed far weaker convective inhibition – including point soundings sampled very near the FWD balloon launch site (not shown). Here is a short-term forecast sounding from the RAP a bit farther to the northeast close to where the tornadoes occurred:

2019102100_RAP_000_32.9,-96.26_severe_ml

You can clearly see that the modeled lower tropospheric inversion is gone and adequate CAPE present, along with strong vertical shear and pronounced curvature to the low-level wind shear vectors. The latter is favorable for tornadoes.

What forced the storms in the first place? Surface conditions just prior to tornado time indicate a pretty well organized set up with a strong mesolow just northwest of the DFW area. One can see what look to be convergent surface winds along a dryline extending south-southwest of the mesolow:

SGPsf.fronts.20191021.01

RAP-derived lower-tropospheric analysis of deep layer moisture convergence – the process that actually initiates storms – confirms the likely presence of strong mesoscale forcing along the dryline, a feature that shows up in the strong mixing ratio gradient (green lines). I’ve highlighted all this with a box:

dmc_dallas_Tornado

The storms that resulted (visible also on the radar composite snapshot superimposed on the image above) trekked into an environment supportive of strong tornadoes. The existence of this environment and expectation that storms would develop was the reason the Storm Prediction Center has a tornado watch in place across the entire region prior to storm initiation (great job, SPC!).

I must emphasize that this event – particularly the details of why/how storms initiated – deserve MUCH more scrutiny in light of the intense cap present on the 0z FWD sounding!

How did the model forecasts fare? Well I’ll just look at the High Resolution Rapid Refresh, although a more complete analysis should certainly also consider the convection allowing ensemble forecasts:

HRRR_Collage_Dallas_Tornado

Notice that the mid-afternoon runs of the deterministic HRRR failed to predict any storms in the Dallas metroplex by 3z, by which time significant damage had already been produced across north and northeast areas of the metroplex. It wasn’t until the 22z initialization that the model depicted supercells over the area. This signature persisted and probably gave forecasters confidence that a big event would likely unfold.

The aftermath of the storm looks grim. Damage survey is underway and I expect to see EF3+ over portions of the impacted area. Prayers for speedy healing for all affected!

The severe weather frenzy of May 2019: anatomy of a forecast of opportunity

Note: the views expressed in this post are mine and mine alone and do not necessarily represent the views of the company that I work for, IBM, or its subsidiary, The Weather Company. By reading this, you the reader assume full and sole responsibility for the outcome of any action that you take on the content provided herein. You indemnify and hold me, the writer, and my employer, IBM, blameless.

June 1, 2019

By Dr. David Gold, Dr. Victor Gensini

Special Thanks to Ed Berry for reviewing and commenting on this post pre-publication!

The period May 17-29, 2019 was among the most active and relentless episodes of violent severe weather and flooding the nation has seen in years. While we won’t have a final tally for a while, there were at least 225 tornadoes during this stretch with 762 tornado warnings issued! All_Tor_Warnings_20190515-20190530 The primary driver was a strong and persistent upper-level low pressure system that set up shop across the western U.S., arguably the atmospheric pattern most effective at generating severe storms across the central to Midwest states. The primary goal of this post is to attempt to explain – from a weather-climate linkage framework – why it occurred. Fair warning: while attempts have been made to soften the technical nature of this post, there’s really only so much that can be done given the complex nature of the processes involved.

One of the most active periods of severe storms in recent memory began in earnest on May 17, 2019 as the lead shortwave trough approached the Great Plains, overspreading the region with stout southwesterly flow aloft. Strong southwesterly winds aloft mixed readily towards the surface during the day, forcing the dryline to shift eastward into the High Plains where it intersected a warm front over southwestern Nebraska.

20190517_composite

This so-called “triple point” served as the focal point for a significant supercell thunderstorm that produced several tornadoes. Other more isolated tornadic storms developed across southwestern Kansas and far West Texas, each producing photogenic tornadoes. From this day onward, repeated rounds of severe weather – including several tornado outbreaks – occurred as a similar environment sustained across the Plains. Fueling this activity was a permanent warm sector characterized by deep tropical moisture and warm temperatures. Another notable feature of this outbreak was a weak “capping inversion”. This promoted high concentrations of severe storms that – given the intense parameters – were still able to become very intense despite a tendency to “compete” with one another for the available atmospheric energy sources. The result of this was a frenzy of tornadoes and flooding.

The “anchor” feature of all this was a persistent upper-level low pressure system. This can be seen in the time-averaged 500 mb height anomaly field:

hemispheric_500mb_anom

Think of this as the average topography of the middle troposphere during the height of the active severe weather period. The pattern shown here is an optimal way to generate days of violent severe weather across not only the midsection of the country but into the eastern states as well, as a strong jet stream extends well downstream of the trough in the West. What follows is a weather-climate “story” about how this feature came to develop and the signposts that allowed it to be predicted with a lead time of 3-4 weeks.

The Extended Range Tornado Activity Forecast group, or ERTAF, was formed by Dr. Victor Gensini, Assistant Professor of Meteorology at Northern Illinois University. The ERTAF is comprised of a group of core members that have been part of the effort for nearly five years. You can view this year’s forecasts, learn about the methodology and view the historical members here: http://atlas.niu.edu/ertaf/. The origination of the ERTAF was motivated by the realization that sometimes patterns favorable for severe weather can be predicted at extended lead time which for the purposes of this article will be deemed to be lead times beyond the range at which numerical weather prediction (NWP) models have useful skill at predicting the emergence of atmospheric “ingredients” favorable for tornadoes. This is beyond about 5-7 days. What provides the ability to sometimes make long-range predictions of extreme events, including active severe weather episodes? It starts with knowing the kinds of jet stream configurations that favor severe weather outbreaks and then asking what causes THOSE.

The basic answer can be illustrated by showing two weekly averaged charts of the jet stream – one just prior and one during the big extended outbreak:

jet_stream

Note how beforehand the jet is nearly zonally oriented (west-to-east) across the Pacific and afterwards it has contorted into a wavy shape. This wavy pattern includes a deep trough in the western U.S. in the weekly average. As shown earlier, this trough aloft is clearly seen by looking at the hemispheric pattern of 500 mb anomalies around the globe.

Understanding why the jet “broke down” into a wavy pattern first requires understanding what drives the jet stream in the first place. That’s a complex topic but one factor that has recently energized the jet stream is an input of “energy” from a few sources, most notably from tropical rainfall patterns. At the risk of greatly oversimplifying matters, one can think of this in terms of how the atmospheric circulation tends to respond to tropical rainfall patterns that include a fairly persistent “hotspot” over the central equatorial Pacific – something that has been the case over the past few months:

olr

This tends to produce zonally elongated high-pressure systems poleward of the rainfall pattern in both hemispheres with strong jet streams at the poleward flanks of these highs. This is one reason why the jet stream was so strong heading into mid-May. In fact, for the week of May 13-19 speeds within the jet were nearly twice the climatological average! So once that jet collapsed into a wavy pattern, the resulting weather systems were very strong.

By mid-April ERTAF was looking for catalysts for this jet to begin breaking down, knowing that when it did at least one very active severe weather period would likely result. The problem – as always – is timing (there’s an old joke: “the forecast was correct except the timing, phase and amplitude!”). One early clue suggested a time scale of mid-late May: we track certain diagnostics developed by Dr. Klaus Weickmann (NCAR scientist, now retired but still interested in how his life’s work is used and extended) and Ed Berry, consulting meteorologist. These two pioneers developed a conceptual framework that allows a systematic approach to monitoring the time/space scales that link climate and weather and that is particularly amenable to catching high impact weather events on subseasonal time scale – again these are “forecasts of opportunity”.

This framework is called the Global Synoptic Dynamic Model of subseasonal atmospheric variability, or GSDM, and is admittedly at first sight a beast to understand, let alone use. The beauty of this approach is that it does not attempt to oversimplify the way in which various components of the earth-atmosphere system interact to create important weather events, but it does provide a framework for monitoring these components in a real-time forecast setting. By so doing, one has a chance to catch important clues about what might happen, and this includes outcomes that can and do “surprise” the NWP models. Thus, understanding the GSDM has been worth the effort.

Like other helpful forecast rubrics, the GSDM has its own built-in “forecast funnel”: it forces the analyst to begin by considering the big picture (global scale), funneling down to smaller scales – all the way to the regional (say weather systems spanning a continent). Without going into the physics, an example of one of the diagnostics that proves helpful is a time-latitude chart of a quantity that is loosely related to the position and strength of the jet stream:

aam

Warm colors generally indicate latitude belts (latitude here runs along the y-axis with the South Pole at the bottom and North Pole at the top of the chart) where the jet stream is stronger than normal for a given time of the year and cool colors vice versa. You can visualize how the position and strength of these features shift north and south and change intensity – even reversing sign – over time by scanning from left to right across the chart (the time dimension). Note that for much of April into mid-May there was a belt of anomalously strong winds persisting roughly along 20N (we’ve overlaid this feature with a thick black arrow to draw your eye to it). By early May (roughly May 5), a branch of this westerly flow anomaly materialized farther north and appears to gradually evolve towards the North Pole. By the end of the period (far right-hand side of chart), the sign of the flow anomalies has reversed with easterlies showing up in the subtropics where westerlies had existed previously. When this happens, regionally this often manifests as the jet stream breaking down into a wavy pattern. It needs to be emphasized that so far no *predictive* information has been presented.

Could the regional jet stream collapse (as inferred from the quantities just referenced) be anticipated beforehand? To understand that, have a look at the even noisier chart here:

tend

Once again, time tracks from left to right and latitudes along the y-axis. The colored blobs on this plot indicate where (loosely) westerly winds are being accelerated (warm colors) or decelerated (cool colors). We’ve annotated the chart with a black arrow, which traces the poleward shift over time of areas where the atmosphere is acting to accelerate the jet stream. These little red blobs shift poleward across the mid-latitudes (roughly 30-50N) during the first half of May, corresponding to the intense jet stream shown previously. By mid-late May, decelerations occur within the latitude band of the extended North pacific jet stream (circled) – again corresponding to a breakdown of that feature. Anyway, occasionally one can track these episodes and note that they have specific time scales. That is, they tend to materialize in the subtropics and then shift poleward over a period of 30-50 days followed by a new episode. When this happens, there is usually a link to the tropics involved, although occasionally these kinds of cyclical episodes can occur on faster (10-20 day) time scales. It’s beyond the scope of this post to explain the dynamics of these faster variations but the interested and technically-minded reader is encouraged to read the citations provided at the end of this post. ERTAF latched on this longer-time scale behavior in late April and early May and posited that it could lead to a jet stream extension-collapse sequence just like the one that culminated in the persistent western upper-level trough that wreaked all the havoc.

We mention the tropical linkage, which was discussed in the context of how (loosely) enhanced rainfall patterns near the equatorial Dateline played a role in producing a stronger-than-normal mid-latitude jet stream leading up to the super active period. Indeed, this strong jet stream helped fuel severe weather outbreaks in April and early May, too. There is a well-known and rather remarkable episodic oscillatory behavior in the tropics called the Madden-Julian Oscillation (MJO). This behavior is also included in the GSDM and it is episodic. When active, considerable refereed literature demonstrates that the MJO can enhance predictability on subseasonal time scales. The active tornado period was (arguably) one such example. The evolution of the MJO is traced on a phase diagram that takes a bit of practice and experience to read. It’s also not advisable to consider this plot without simultaneously monitoring other visualizations of the tropical winds and rainfall patterns that are used in its derivation. However, in the present case the model forecasts of the MJO managed to predict its evolution reasonably well:

mjo

Note the counterclockwise evolution of the trace – the forecast (initialized on May 5 and valid for the period May 6-20 on the left) and the observed (what happened in phase space on the right). Often when this trace evolves through phases 8-1-2, representing an evolution of the MJO from the Western Hemisphere back into the Eastern Hemisphere, the mid-latitude jet stream collapses into a wavy pattern. We anticipated that this might happen back in early May and, taking into the consideration the evolution of processes discussed earlier and supportive runs from NWP ensembles, we decided to issue a forecast of “Above Normal” Tornado Activity for a sizable chunk of the Plains and eastward into the Midwest/Ohio Valley. While our forecast metric is admittedly very simplistic and needs to be improved, this was a good example of how one can use old fashioned atmospheric sleuthing to sniff out a forecast of opportunity for an extreme weather event.

Recommended reading:

Weickmann and Berry 2007: A Synoptic-Dynamic Model of Subseasonal Atmospheric Variability. Link: https://journals.ametsoc.org/doi/full/10.1175/MWR3293.1

Weickmann and Berry 2009: The Tropical Madden-Julian Oscillation and the Global Wind Oscillation. Link: https://journals.ametsoc.org/doi/full/10.1175/2008MWR2686.1

Gensini and Marinaro 2016: Tornado Frequency in the United States Related to Global Relative Angular Momentum. Link: https://journals.ametsoc.org/doi/full/10.1175/MWR-D-15-0289.1