After looking and analyzing a few of the latest forecast models (00z runs), the chance of severe weather occurring in south Texas is looking promising. There are still questions as to the intensity and location(s) of severe weather. It should be noted that even if severe weather doesn’t occur, strong thunderstorms will be likely anywhere from the San Antonio area and southward on Tuesday(01/08) and into Wednesday(01/09).
Before I get into the details of the upcoming event, I want to briefly go over what is needed for thunderstorms and severe thunderstorms. One thing that is needed for “surface based thunderstorms” to occur are surface dewpoint temperatures to be above 50°F (generally above 55°F). It is possible to have thunderstorms with dewpoint temperatures lower than 50°F, however, these would most likely be in the form of “elevated thunderstorms”. Elevated thunderstorms are thunderstorms that form above the planetary boundary layer (PBL). Though these could still be very dangerous, it’s the “surface based thunderstorms” that usually do the most damage.
Another thing needed to form thunderstorms is instability. This comes from positive convective available potential energy (CAPE) and a negative lifted index (LI). I’m not going to go into the details of CAPE or LI right now. Just know that CAPE is needed. There is no set value number of how much CAPE is needed. It just depends on the system and atmospheric profile. In order for strong and severe thunderstorms, CAPE greater than 2000 J/kg is sufficient for large hail, strong winds and tornado potential.
There needs to be a trigger mechanism in order for thunderstorms to spark. This trigger mechanism comes in a variety of ways/forms: sea breeze, warm front, cold front, orographic upsloping, frictional convergence(i.e. wind convergence), vorticity, jet streaks, etc.
In order to have severe thunderstorms, there needs to be strong speed and directional shear. Winds need to be veering (turning clockwise with height) from the surface to 250mb. Wind shear tilts the storms, separating the thunderstorm’s updraft and downdraft. It also allows tornadogenesis to occur.
Now that you know what is needed to start a thunderstorm, I can explain my thinking of what may occur on Tuesday(01/08) and Wednesday(01/09):
GFS and ECMWF are in fair agreement that a 500mb cut-off low will be digging into northwest Mexico as early as Monday morning. This low is forecast to continue digging until early Wednesday morning, where it will then lift northeast into central, then northern Texas, and eventually into Oklahoma. Surface winds across all of South Texas will be turning out of the east beginning Monday afternoon. This will allow surface dewpoints to rise into the 50s, and eventually 60s by Tuesday morning, especially along the coast. Below is the GFS forecast model valid Tuesday morning at 6am.
Image Credit: AccuWeather Pro
Dewpoints will remain above 60°F until after the mid-level cut-off low has lifted into northern Texas and the trailing cold front pushes through, which should be by early Wednesday morning.
Remember, there also needs to be a trigger mechanism to start thunderstorms; and there will be by Tuesday:
The mid and upper level low will enable vorticity to “spin up” a surface low pressure on the lee side of the Sierra Madre Oriental Mountains. This is something in the meteorological world called Lee-Side Cyclogenesis. This surface low will have a warm front extended from it and will likely form right over Rio Grande Valley, or just north of the RGV:
Image Credit: Hydrometeorological Prediction Center (HPC)
Image Valid: Tuesday Evening (00z Wed, 6PM CT)
This warm front will definitely be the key in triggering strong to severe thunderstorms across South Texas. My current thinking is the best severe potential for severe weather will be along the warm front and just ahead of it. The image above will not be 100% accurate, it just gives an idea of what will happen. The time and location of development of the warm front will be something to monitor. Regardless where it develops, the chances for strong to severe thunderstorms still persists, even south of the warm front.
Finally, I’d like to discuss the CAPE and wind shear a head of this next system. Earlier runs of the NAM and GFS didn’t have much CAPE a head of this storm system. The latest models have increased CAPE, slightly. Needless to say, this is will NOT be a severe weather outbreak of hail and tornadoes. According to the latest forecast models, CAPE increases to 750-1000 J/kg by Tuesday afternoon. NAM model is pretty consistent with 750-1000 J/kg, with isolated areas reaching 1000+ J/kg. GFS is a little more aggressive with CAPE values, ranging from 750 J/kg to 1500 J/kg, just north of the Valley. Though these numbers seem on the low side when it comes to severe weather, they are pretty impressive for early January.
Image Credit: Twisterdata.com
Valid: Tuesday Evening (00z Wed, 6PM CT)
CAPE is the only issue I had with this event. CAPE will definitely be there for strong thunderstorms, however I think anything that goes severe will be isolated, but still very possible.
As far as the wind shear goes, there will be plenty of it. Low level jet from 850mb to 700mb will be strong Tuesday afternoon and evening. Bulk shear values between 40-60 knots from the surface to 500mb will be present across South Texas before 00z, then increase to 50-80 knots:
Image Credit: Twisterdata.com
Valid: Wednesday Morning(12z Wed, 6AM CT)
Storms look like they will increase in intensity and numbers if this occurs. Would not be surprised if a squall line forms along the cold front. Damaging winds would be the primary factor if this occurs.
Overall, thunderstorm potential is near 100% across South Texas. The potential for severe weather seems likely at this point, just not a guarantee for everyone. Isolated tornadoes will be possible, but the main concern will be damaging winds and hail with any storm that forms. The best chance for thunderstorms will be Tuesday afternoon, with the best chance of severe weather occurring late afternoon, into the evening hours on Tuesday and even early Wednesday morning. Rain and thunderstorm chances diminish by Wednesday afternoon. Please heed any warnings issued for your area!