I hope you have had a great Christmas, even though today’s weather hasn’t been optimal for spending time outdoors. Hey, at least it’s not 25 degrees with blowing snow. Lots of low-level overcast and fog are expected again tonight as moisture levels continue to increase. In fact, spotty showers are probable overnight into the morning hours Wednesday across the eastern three-fourths of Texas. Severe storms are not likely tonight, but stronger storms by morning may be capable of producing small hail.
I know there are various colors on the risk map. The primary difference in risk levels is the chance of having severe weather occur. Threats will be similar regardless of being in the dark green or yellow shading. A level one risk (dark green) has a five percent chance of severe weather within 25 miles of your location. The level two risk (yellow) has a fifteen percent chance of severe weather within 25 miles of your location.
A level two risk includes all of the Hill Country, the eastern Big Country into western North Texas. This risk also includes North Texas along and south of Interstate 20 in the D/FW Metroplex extending northeast along and south of Interstate 30 from Dallas to Texarkana. Central Texas, the Brazos Valley, and all of East Texas are included in this level two risk. It runs along and north of Interstate 10 from San Antonio to Houston. The primary severe weather threat will be damaging straight-line wind gusts and isolated tornadoes. Hail will also be possible along with periods of heavy rainfall.
The western edge of the level one risk is where we may have storms ongoing as early as lunchtime tomorrow. These storms would occur on the leading edge of the dryline and on the western edge of the higher moisture values. Should these storms become severe they’d pose a threat of large hail and localized damaging wind gusts. There is a low risk of a tornado, but widespread cloud cover and modest moisture values should keep the tornado threat low with that activity.
Scattered showers and thunderstorms will be occurring most of the afternoon and evening across the eastern half of Texas. These storms should be well east of the developing storms and eventual squall line to the west. Lots of wind shear and modest amounts of instability in the hail-growth region will allow for some hail with stronger storms. However, the threat of tornadoes should be very low given the expectation of storms being elevated above the cap. If any storms did manage to become surface-based (as in, tap into instability at the surface and thus lower cloud bases) the threat of tornadoes would increase. This doesn’t look likely with the afternoon activity right now.
Most of tomorrow’s severe weather threat may not begin in earnest until after 4-5 PM. A surface dryline and associated cold front will spark off a line of thunderstorms in Northwest Texas, the Big Country, Concho Valley, south into the Edwards Plateau tomorrow afternoon. This line is going to move east tomorrow evening, tomorrow night, and into the morning hours on Thursday. Damaging straight-line winds are likely with stronger storms in the squall line. Embedded brief tornadoes are also possible in association with swaths of higher-end straight-line winds.
The highest threat of damaging winds and brief tornadoes will be in the level 2 risk. It is worth noting that a few storms may fire up just ahead of the squall line tomorrow evening in North and Central Texas. Those storms would need to be watched closely for tornado potential.
One key in tomorrow’s eventual severe weather potential will be the amount of instability available for storms at the surface. We’ll have instability in the mid-levels of the atmosphere pretty much everywhere, hence the risk of hail with stronger storms. In terms of any enhanced tornado/damaging wind risk, we’ll have to watch how far north the surface-instability axis can make it.
Wind shear is going to be darn impressive tomorrow, so it may not take much surface instability to cause problems. December 26, 2015, had copious amounts of surface instability in the presence of strong wind shear. We’ve got the strong shear tomorrow, but surface instability will be much lower this time around. The northern delineation of the level two risk is where we think that axis of instability will end up tomorrow night. However, severe storms remain possible in both the level one and level two risks.
Most of the severe weather will come from an eastward moving squall line tomorrow evening and tomorrow night. Damaging straight-line winds are the primary threat, but brief tornadoes are also possible. Hail and heavy rain are secondary threats. If we see supercells develop ahead of the line tomorrow evening they would have an enhanced tornado threat. All of that would be after dark and involving fast-moving storms. Tomorrow and tomorrow night would be a good day and night to remain weather aware.