I hope you’re enjoying the warm weather across Texas this afternoon. The gusty south winds are helping pump moisture north from the Gulf of Mexico back into Texas. This will help set the stage for severe thunderstorms on Monday as a upper level disturbance scrapes the region.
The Storm Prediction Center has placed a large portion of East Texas, Southeast Texas, and portions of Central Texas in their standard risk of severe weather for the evening hours on Monday. Those in the orange-shaded zone have a 15 percent chance of experiencing severe weather within 25 miles of their location. The new afternoon outlook has expanded the risk areas compared to this morning’s outlook. The risk now extends along and southeast of a line from Texarkana-Longview-Fairfield-Temple-Austin-San Marcus. It should be noted that the Houston Metro is now included in this risk. A warm layer aloft, called a capping inversion, will prevent strong thunderstorms until the late afternoon and evening hours on Monday. A good time-frame would be 5 PM Monday through 2 AM Tuesday.
Initial thunderstorms would likely be discrete, or by themselves. The strongest of these thunderstorms will be capable of producing golfball size hail and damaging winds over 60 MPH. As thunderstorms begin to combine into a squall line, the threat for hail will begin to decrease while the risk of damaging straight-line winds will increase. This line will likely begin approaching Houston by 12 AM Tuesday.
If you’re not a weather weenie, go ahead and skip the next few images as I’ll be going nerd.
Weather models have become a little more optimistic with the thunderstorm chances over the past 24 hours. The above graphic comes from the morning 4 kilometer North American Model (NAM) and shows surface-based convective available potential energy (instability) at 6 PM on Monday. During the fall and winter months, we usually have what are referred to as high shear–low cape setups. That means wind shear is very strong while instability values are low. In the late spring and summer, we have high cape-low shear setups. It looks like we’ll have instability values around or in some cases exceeding 2,000 joules per kilogram (2,000 J/Kg) by tomorrow evening across portions of Southeast Texas into Western Louisiana. For a late November event, those values are unseasonably high and will definitely support the possibility of organized thunderstorms.
Lets switch over to looking at the wind shear for this setup. I’ve circled the areas that will be the target zone for Monday’s storms and in what we refer to as the warm sector. The main upper-level energy with this system will be passing north of Texas in portions of Arkansas and Missouri. The result will be lower wind shear values across Texas. With low instability values, these low wind shear values would usually help preclude any severe weather threat. However, the high instability values expected on Monday will help compensate for the weaker wind shear. Those two factors combined with a cold front moving southeast across Central and East Texas will help provide the ingredients for thunderstorms. The cold front should provide just enough forcing to lift the cap and help develop thunderstorms. At least that is what we’re expecting right now.
This is a Skew-T which is a 2D representation of all the levels in the atmosphere. I’ve circled what I want you to notice about this setup. The low level winds are coming from one direction with no turning with height. This means that the tornado threat on Monday will be low as a whole. I still can’t rule out a brief tornado if a storm ends up riding an outflow boundary, but the threat for tornadoes is low and I don’t expect a big problem. This type of setup favors a squall line or semi-discrete thunderstorms with large hail and damaging winds. The high instability values and low freezing levels will promote hail growth and we could see a few very large hailstones with the strongest storms on Monday. As the storms begin to line out and form into a squall line or mesoscale convective system (MCS), the threat for straight-line winds will increase. I’m a bit concerned about this threat specifically for the Houston Metro during the late evening hours on Monday.
Overall, I do expect a busy period tomorrow evening for Southeast Texas. I’ll be manning the ship and providing updates regarding this potential event as we head into Monday.