After several days of quiet weather, we’re once again preparing for a severe weather event. Unlike last Saturday we’re not expecting a tornado outbreak today. However, that does not mean the risk of tornadoes is zero. If nothing else, someone could see some giant hail by the late afternoon or early evening. Confidence is high that a line of storms will move east across the eastern third of Texas overnight into Thursday morning. All said and done, we have four separate risks to discuss. Before we get to that let’s start off by reviewing the latest severe weather outlook from the Storm Prediction Center.
The latest outlook has not changed much from the ones we shared with you yesterday. This one was issued at 1 AM this morning with updates coming out at 8 AM and 11:30 AM. Yours truly will be grabbing a quick nap so one of my team members will have to share the 8 AM outlook (if there are any substantial changes). Otherwise, I’ll bring the late morning outlook along with a quick forecast update in a lunch-time update here on the blog.
Today does have several ‘bust’ potentials along with a risk for a few surprises. That’s just what happens when you’re dealing with the dryline in the spring – sometimes the cap ends up stronger or weaker and sometimes the dryline ends up a bit farther west or east than expected. Don’t be shocked if the outlook gets changed around a bit later this morning.
Not everyone in the risk zones will be impacted by severe weather. Some folks may not get a darn thing. Here are the regional maps of the severe weather outlook. Just click an image for the full-screen version. You can always find the latest severe weather outlooks along with a ‘how they’re made’ tidbit here at our Texas Severe Weather Center.
I’m going to use a different format to discuss today’s severe weather chances. Let’s start off with the first round of storms expected up in the Texas Panhandle. This area will likely experience thunderstorms first with a chance increasing by 2-3PM. Storms will tend to grow upscale into line segments or small clusters fairly quickly as they move northeast. Large hail, localized damaging winds, and isolated tornadoes will be possible. The highest severe weather threat will probably exit the Panhandle around dinner-time. Showers and storms will continue into the evening hours across much of the Texas Panhandle.
Everybody else in a severe weather risk late this afternoon into tonight
Ted Ryan, Science and Operations Officer (SOO) at the National Weather Service in Fort Worth put together a marvelous forecast discussion for North and Central Texas. I don’t usually share these discussions on the blog because they can contain technical elements meant for forecasters. However, I’m going to share it because I think it is well-written and gives an excellent size up of the forecast for this afternoon and tonight. Ted is an extremely intelligent individual with many years of experience forecasting severe weather events across Texas. His thoughts are spot on and put together in a way I don’t think I could improve on. I have made minor grammatical improvements and defined a few meteorological phrases. Otherwise, the words below are Ted’s.
“There is high confidence that all of the ingredients for a severe weather event will be in place over the region this afternoon and tonight, but exactly how it plays out remains uncertain primarily due to a complex upper level pattern leading to waves of organized forcing for ascent [rising air] and subsidence [sinking air]. Meteorologists can often get lost in the “parameter space” of forecasting severe weather, meaning that countless numbers and indices relating to the type and severity of the weather dominate the forecast and conversation of it. In reality, the upper-level dynamics and the physical response of the atmosphere to it is extremely important and can make or break a severe weather event. Unfortunately, this piece is extremely difficult to forecast, both because these features are aloft and difficult to sample which can lead to numerical modeling errors. Likewise, it is tough for human forecasters to infer how the numerical model is handling those features. So having said that, let’s dive into the upper-level features that are critical to the forecast and will influence 3 separate potential severe convective events/areas.
The upper-level low currently over Arizona is poised to enter the state later today and pivot northeastward tonight. Rounding the base of the trough will be two shortwaves, the first moving across the region around midday and the second crossing the region late tonight. Forcing for ascent will occur ahead of both of these, but a wave of subsidence will spread across the region late this afternoon and early evening in the wake of the first. The upper trough maintains a positive tilt which means in the wake of the first shortwave it is very likely that the winds near 800 millibars [around 3500-4000 feet above sea level] will veer [become southwesterly] and weaken which will impact the low-level shear profile and help to keep the tornado threat from getting out of hand. But ahead of the first shortwave may be a different story.
So the 3 areas and times of potential severe convection are…
1) Across the eastern and southeastern counties [of North Texas into East Texas], east of a Hearne to Corsicana to Bonham line, between 3 pm and 7 pm:
The early passage of the first shortwave means most of the area will remain too cool and capped for convection. Across this eastern area, however, it’s possible that just enough daytime heating and, more importantly, continued moisture advection will result in enough uncapped instability for convective initiation and severe weather. A lack of any boundary in this area will limit surface convergence and therefore also may result in convection struggling to organize. Typically this warm-advection driven convection takes a long time to develop and intensify even in unstable environments. The forecast soundings and severe weather parameters in this area make us most concerned about a more significant tornado threat than any other area of our CWA [County Warning Area], but we are also concerned about very large hail. We estimate based on a basket of the CAMs [High-resolution weather models that are built to handle convection] that the potential for severe convection anywhere in this region is about 40% meaning the severe weather threat is more conditional (may not happen at all).
2) The northwest and north-central part of the CWA, including the DFW Metroplex, from 5 pm to 10 pm:
A dryline will likely reach a Bowie to Comanche line late this afternoon. Unlike areas farther east, strong surface convergence [lift] along the boundary is forecast and will help to provide a favored zone for convective initiation. However, this region will likely be in weak subsidence [sinking air] in the wake of the early day shortwave [piece of upper-level energy] during the peak heating hours of late afternoon. This could lead to false convective starts and struggling convection despite a very unstable and uncapped airmass as temperatures reach the low 80s with dewpoints in the mid-60s. In addition, the early day shortwave will lead to weaker winds in the 800 millibar area and a veer-back-veer profile in the lowest levels with a hodograph [a way to determine the amount of wind speed and turning with height in the lower levels of the atmosphere] a pronounced kink due to these weaker winds. This type of shear profile may be both good and bad. It should lower the potential for strong sustained low-level mesocyclones and thus the tornado threat would be more isolated and brief. On the other hand, it may actually help convection initiate as there will be less turbulent mixing and shear to decapitate infant updrafts. Finally, this shear profile would increase the potential for very large hail as it would result in greater mid and upper-level cloud layer shear which is far more important for copious amounts of large hail growth than just being unstable. Although we should mention that it most certainly will be very unstable with MLCAPE [most unstable instability values] possibly near 3000 J/KG [quite unstable] and a large amount of instability in the hail growth zone. Several sounding analogs [past events which had similar parameters to today’s setup] suggest that any mature supercell in this area will have the potential to produce baseball to softball size hail. If supercells do form and remain isolated past 9 pm, we will have to closely monitor the 800 millibar flow [winds around 3500-4000 feet above sea level] as the approach of the next trough [upper-level storm system] could bring more cyclonic curvature in the low levels and lead to a higher tornado potential. Remember, severe storms are still not a certainty in this region as it is possible the subsidence wins out – but based on the various CAMs we estimate a 60-70% chance there will be severe weather somewhere in this area between 5 and 10 pm.
3) The entire CWA [Fort Worth’s County Warning Area covers much of North Texas and Central Texas], but especially areas south of I-20 after 9 pm:
The approach of the main upper trough axis will bring the second shortwave [upper-level energy and lift] across Central Texas resulting in strong forcing across most of the region, but especially south of I-20. Moderate to high instability and a weak cap will result in a rapidly developing line of storms west of I-35 that should race eastward into the overnight hours. Storm mode suggests that while there will be hail threat – it will not be as extreme as the other 2 potential events. Instead, damaging winds look like the primary threat with this squall line, but the possibility remains there will be some brief embedded tornadoes along the line. Confidence is high across Central and East Texas that this line of storms will occur (with the chance of storms at 80-100%) however the farther north one goes the weaker and less organized the dynamic forcing is and thus these areas may miss out on this round.
In summary, not everyone will see severe weather, but everyone should prepare for it. It’s one of those days we ask people just to stay plugged into your favorite source of weather information so that you can act if need be – whether it’s moving a car somewhere protected or seeking a safe shelter.”
I know that was a lot of reading, but my goodness did Ted put together an excellent forecast discussion from the National Weather Service office in Fort Worth this morning. That line of storms he referred to this evening will develop southward into the Hill Country and Edwards Plateau around 8-10PM. Those storms will move east with a threat of damaging winds, some large hail (not as bad as earlier in the day), and perhaps brief tornadoes. That means those in Central Texas, South-Central Texas, the Brazos Valley, East Texas, and Southeast Texas can expect some storms in their area late tonight and into the morning hours on Thursday. Storms may not exit far East and far Southeast Texas until after sunrise on Thursday, perhaps closer to mid-morning.
So while the threat of tornadoes isn’t as high as last Saturday, it isn’t zero either. Ted described a few scenarios in which we could which may result in a localized enhancement of a tornado threat this afternoon and this evening. That doesn’t mean a tornado outbreak, but if one tornado comes down your street today, it’ll feel like a big outbreak to you. Likewise, we may not have to deal with much of a tornado threat at all. You’re already weather aware by reading this blog post and by keeping up with the latest forecasts. Ensure you stay up to date throughout the day.
Flooding will also be a threat tonight into Thursday across Northeast Texas, East Texas, and the Brazos Valley. Storms may produce 1 to 3 inches of rainfall on saturated soils. The most likely impacts will be flooded low-water crossings, construction zones, and the typical trouble spots. We’ll see all that water flow into streams, rivers, and lakes – something we’ll deal with this weekend into next week.
I’ll be chasing later today in North Texas along with several folks from TSC. That means we’re not going to have many updates once we get started this afternoon. That’s why you should always have multiple sources of weather information. The National Weather Service and your local television meteorologists provide excellent severe weather information and coverage. When things get going today turn to them. We’ll post updates where we can, but during ‘chase days’ I’m unable to do too much.