1:30 PM CST | December 25, 2018
Merry Christmas, everyone! I hope everyone is having a good day. Not to take away from today's festivities, but people should be aware of the risk for Severe Weather tomorrow. December 26th is usually a busy travel day. I have been discussing this potential for thunderstorms for two weeks now. Let me show you the latest outlook from the Storm Prediction Center.
There have been a few adjustments since this morning's Facebook update. The biggest change was to bring the Slight Risk farther north-northeast. The Slight Risk now includes the immediate I-20/30 corridors and the cities of Dallas, Fort Worth, Sulphur Springs, Tyler, and nearby areas. The greatest risk appears to be across North Central/East Central Texas in my opinion. This is where the potential for Damaging Winds to 65 mph, Hail, and a few Tornadoes will reside. I have illustrated where the wind streaks above the surface will coincide with the deepest moisture (60*F dew points) by sunset, based on this morning's NAM (North American Mesoscale Model).
When you hear meteorologists talk about "spin" in the atmosphere, there are referring to how much the winds turn clockwise with height. For instance, favorable winds for tornadic development would be southeasterly surface winds, a southerly low-level jet (850 mb), southwesterly winds a bit higher (700 mb), and more westerly winds in the mid-levels (500 mb). If winds are fairly strong and sufficient moisture and instability are present, you have a favorable environment for Severe Weather and Tornadoes. In the winter, winds aloft are stronger given the more southerly placement of the jet stream, but moisture and instability are usually in short supply due to more frequent cold intrusions and less daytime heating. By late June and July, our situation is the opposite. That is why our main severe weather season is typically March through May, where there is an overlap with the changing of the seasons.
What will we have tomorrow? Favorable Winds Aloft - CHECK; Upper-Level Support - CHECK; Surface Boundary To Focus Storm Development - CHECK; Moisture - CHECK*. What about instability? Instability and the lack of buoyancy near the surface will be the main limiting factors for a more substantial risk. Moisture will still be returning and will not have had enough time to modify - this means cloudiness in the warm sector. Increasing lift with the system will result in rain and thunderstorms ahead of the Pacific cold front. This suggests that most will remain overcast. Based on past cool season systems, I believe that the location of the 60*F dew point air will be key to which areas see severe weather. North of this deeper moisture, strong winds and heavy rain will be the main concerns with only a marginal severe risk.
A few adjustments can be expected over the next 24 hours as we watch the rate of moisture return. While some Severe Storms are expected on Wednesday, an "outbreak" is unlikely given the instability issues mentioned. This is good because we do not want a Severe Weather Outbreak, especially around Christmas of all times. You may have heard some folks making comparisons to the December 26, 2015 outbreak, which included that EF-4 Tornado just outside Dallas. Let me be clear - moisture and instability will not be supportive of severe weather to that magnitude. Just keep an eye on your forecast, and make sure you have a way to get the latest watch and warning information, particularly from Wednesday afternoon through the overnight hours. If a watch is issued for your area, it means that conditions are favorable for the development of severe weather and to keep an eye on it. If a warning is issued for your area, it means that severe weather is imminent or already occurring and to seek shelter immediately. For the latest local weather information for your area, please visit the National Weather Service website. You can also find us on Facebook.