What can I say that hasn’t already been said over the last several days? There have not been significant changes made to the forecast compared to my update this morning. The severe weather risk zones have been expanded slightly farther west again. Our map contains several colors and various risk levels. The higher the risk level, the more widespread severe weather is expected to be. However, it all comes down to the simple fact that if you’re located in any risk level – you’re in a risk for severe storms tomorrow afternoon and tomorrow night. A good chunk of tomorrow’s severe weather probabilities is based on the potential for damaging straight-line winds. A level four risk zone was added earlier today for parts of East Texas where confidence in widespread damaging straight-line winds continues to increase. Again, we expect a lot of damaging wind reports in the higher risk zones. That does not necessarily mean those higher-risk zones will be where the most tornadoes occur tomorrow (if we indeed see several tornadoes in the first place). I know there is a lot of hype and anxiety about tomorrow. I’m going to talk you through the timeline and the possibilities below.
Tomorrow’s severe weather risk will include a lot of territory. The risk for severe weather could begin not long after lunch-time Friday in portions of North Texas. The science and operations officer (SOO) at the National Weather Service in Fort Worth put together a well-written discussion earlier this afternoon to describe a conditional threat for some big problems. I’ll just paste that discussion here for all to read.
The upper level trough to our west will close off Friday and enter the state during the afternoon hours. Strong dynamical forcing will overspread the region well ahead of it and result in the lifting of a weak cap by early afternoon. Although CIN [the cap strength] will be negligible with the cap gone, determining whether we will have convective initiation out ahead of a cold front remains a huge forecast challenge since there will be no level focus to help “trigger” storms. Should vigorous convection develop in this regime we will be particularly concerned about supercells and the tornado potential given the high values of wind shear in place and well organized cyclonically curving hodographs. The supercells that form in these weakly-forced warm advection regimes out ahead of the front are the ones responsible for our noteworthy cool season tornado events (and some warm season events too). The setup for this event certainly checks all the boxes for what we look for in a cool season tornado outbreak, with perhaps the exception of instability being just a little lower than the big events like Dec 29th `06 and Dec 26th `15 (if you believe current model forecasts). And so what keeps us from saying a tornado outbreak is on the horizon is that there are countless unnamed and forgotten events that look similar to this one which produced no tornadoes. So I believe the key tomorrow will be the instability – reflected by the dewpoint readings during the late morning and afternoon hours. If upper 60s dewpoints can make it into the region and temperatures can climb into the mid 70s, then it will likely be a bad day for us. Until then the best we can say is that the severe weather threat will begin in the early afternoon hours when some of these cells MAY start to develop. These early supercells could develop anywhere in North/Central Texas, but probably the eastern half of the region is more favored due to the expected higher moisture. They will be very fast moving, rocketing off to the northeast at 50 mph, and given the expected low cloud cover, visual spotting and tracking will be difficult. By the late afternoon hours a north-south oriented “Pacific” cold front will organize and intensify in our western zones and begin to move rapidly east. With the addition of a well defined low level “trigger” into the mix, strong dynamic forcing, and no capping inversion, a line of severe thunderstorms will develop along the front. Given the strong wind profiles this line will likely result in a moderate threat of damaging winds and some brief spin-up tornadoes. We believe the line will form around 4 pm near Hwy 281 and move eastward – affecting the I-35 corridor between 5 pm and 9 pm, and the far eastern zones before midnight.
Storm timing and hazards
The model animations attached to this post are from the just-generated evening run of the High-Resolution Rapid Refresh (HRRR) weather model. You know the caveats – model data isn’t a forecast, it is a simple simulation-based on ingested parameters. Models usually don’t get everything 100% right, so don’t take their output 100% verbatim. Yet tonight’s data does help provide a narrowed down time-frame for any given location.
The 0Z HRRR has thunderstorm development underway by 3 PM in North Texas from Sherman to Fort Worth to Goldthwaite. Storms will move quickly to the northeast around 50 MPH. We may have to wait a few hours before a more eastward progression materializes, so it is possible some folks in North Texas could have multiple storms move over their location tomorrow afternoon. We call that ‘training’ or a line of storms just moving over the same areas. That training could result in some flash flooding developing during the Friday evening rush hour in the D/FW Metroplex.
Conditions being depicted by the HRRR would support all modes of severe weather, including a threat for very large hail. If those storms maintain a supercelluar mode versus more of a linear transformation we are going to have problems with tornadoes. I’m not saying that to scare anyone, but we’re going to have to watch the evolution of the small-scale environment carefully tomorrow.
By 5 PM the HRRR has a broken line of strong to severe thunderstorms from near Sherman to Fort Worth all the way down through Burnet to Hondo. Everyone east of that squall line – all the way to Arkansas and Lousiana, will have to deal with it at some point tomorrow night. That means the Interstate 35 urban corridor from the Red River south to San Antonio are in a severe thunderstorm risk late tomorrow afternoon through tomorrow night. Any ‘breaks’ in the squall line could result in some embedded supercelluar storm structures. Those supercelluar storm structures would be capable of producing large hail and a few tornadoes.
We should see a solid squall line underway by 8 PM from Bonham to Dallas south on Interstate 35 through Waco, Austin, to near San Antonio. Any parts of the squall line that ‘bow out’ will have the potential to produce hurricane-force straight-line winds and embedded tornadoes. Large hail may also occur, but the threat for damaging hail will be lower than with any discrete thunderstorm activity earlier in the day. The line of strong to severe thunderstorms will move east tomorrow evening and tomorrow night all the way into Arkansas and Lousiana. The 0Z HRRR has the line arriving in College Station around 9 PM, Tyler and Houston around 10 PM, Texarkana and Beaumont around midnight. These are rough timeframes and storms could try and arrive an hour or two earlier or later. Not all storms in the squall line will be severe. The most intense portions of the squall line tomorrow evening could produce hurricane-force straight-line winds over 80 MPH, embedded tornadoes, a quick one to two inches of rain, and perhaps nickel to quarter size hail. Wind damage may become widespread in East Texas with concern of trees falling on homes and vehicles, so please take severe thunderstorm warnings seriously too! Any storm ahead of or within the squall line that is able to maintain supercelluar characteristics could produce a strong tornado. Once storms move east into Arkansas and Lousiana early Saturday morning our threat of severe weather will end in Texas. That squall line will make it to Georgia and Florida by Saturday night with significant severe weather impacts along the way.
Flooding threat with tomorrow’s storms
Two to four inches of rain may fall with the storms tomorrow in North Texas northeast into Northeast Texas. Flash flooding, including flooded roadways and creeks, could occur for the Friday evening drive home. Widespread one to two-inch rain totals is expected across the eastern 40% of Texas tomorrow. Some folks may get locally higher amounts. Once the squall line is blasting east it’ll produce a quick inch or two of rain with brief flooding possible. Recent dry weather will prevent a more widespread flooding concern, but we’ll certainly have saturated soils after this event. That’ll be something we keep in mind with an active weather pattern expected next week.
Snow potential Friday night into Saturday morning
This is already a long post, so I won’t touch too much on the snow chances. We are going to see colder air filter in behind this storm system as it departs. While severe weather is underway in Eastern Texas, temperatures in the Panhandle and portions of West Texas will become cold enough to support winter precipitation. A band of light snow is anticipated to wrap around the back-side of the storm system. That’s why we could see minor snow accumulations across the Texas Panhandle and perhaps in West Texas Friday night. Locally heavier accumulations are possible in the Texas Panhandle, and perhaps far Northwest Texas. Those will be driven by smaller bands of heavier snow. I honestly cannot tell you exactly where those will set up yet. The positioning of those snow bands will be determined by the eventual track of a surface low pressure. We’ll get a better grasp on this sometime tomorrow. As the system continues to depart on Saturday we may see some light snow make it into Texoma and even North Texas. Very light snow accumulations may occur near the Red River and we’ll have to keep an eye out for any mesoscale banding that may result in very localized, heavier accumulations.