Naughty or Nice, You Might Be in for Some Rocky Weather Tonight or Tomorrow

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It’s all over the news: Christmas Day weather is going to be, well – a mixed bag. From a highly probably white Christmas in parts of Oklahoma and Arkansas, to blizzards and cold in the Plains to yet another strong winter storm pushing into the Pacific Northwest and lingering snow in New England, it really feels like Christmas for much of the country. However, it’s the severe weather potential — including tornadoes — from Texas to Alabama that has residents of those states wondering if such a thing could really happen on Christmas Day. The answer unfortunately, is that yes, it can happen on Christmas Day and in some places, likely will happen on Christmas Day.

This image indicates the pressure heights at 500 millibars (approximately 18,000 feet). Note trough of low pressure over Texas and the Plains. Vorticity is also indicated on this image. Vorticity, indicated by the black dashed lines, amplifies instability in the atmosphere. Notice the classic severe weather signature of strong vorticity in the base of the low pressure trough. Typically when meteorologists refer to instability in the upper atmosphere, vorticity is key. Valid time: 12/25/12 0600 CST. Image: Penn State E-wall

The problem has nothing to do with who’s been naughty or nice, the problem is temperature and humidity — and there’s plenty of both. Temperatures across the South, in many areas, are nearing record highs. In Houston, we’re expected to be within just a few degrees of our record high of 82F while current temperatures (as of this writing at 12 PM CST) are well above 70F from Brownsville (77) and Corpus Christi (80) to 75 in Hammond, LA, to 73 in Evergreen, AL. Additionally, for the past several days the pronounced low-level flow from the Gulf of Mexico has been doing nothing but cranking up the relative humidity.

So the stage is set for a severe weather scenario more commonly seen in the spring or fall. The only thing missing is a trigger to set it all in motion. But it’s not missing. The triggers (in this case, three of them) are already on the way: a warm front moving north from the Gulf, an upper-level low approaching from the west, and a cold front moving in from the north. As a result, shortly after midnight tonight, thunderstorms in many areas will have a very good chance of reaching severe thresholds with strong gusty wind, large hail, frequent lightning and even tornadoes.

This GFS numerical model image shows the low pressure area over Texas and the enhanced RH values (green) over Texas, Louisiana and Alabama early tomorrow morning. Image: Penn State E-Wall

It’s been said that nighttime tornadoes are more deadly than daytime tornadoes. That’s true, but not because tornadoes that occur after dark are more ferocious. Tornadoes that form and strike after dark are typically more deadly because they strike while people are asleep — not tuned in to local television or radio stations. There are a couple of reason why that perhaps won’t be the case tonight. First, the severe weather has been well-advertised in the media; I even wrote about it a week ago (read that article here). When storms are well-advertised well in advance, damage, injury and death can be greatly reduced. However, the main reason for a better outcome tonight is because, I think, people won’t be turning in “as usual.” Lots of people will be attending midnight church services. Lots of people will be out late at Christmas Eve dinners and parties. Lots of people will be awake telling their children, “Go to bed!” Lots of people will be awake assembling toys too big to go under the tree. Lots of (young?) people will be faking sleep while they keep one eyed turned to the window and one ear turned to the roof (might those same people be watching their mobile devices for the latest Tweets from Santa and the good folks at the NORAD Santa Tracker?). And, if there are still people left like my grandparents, there may still be a few hearty believers that will be delivering and decorating the Christmas tree – the one Santa brought — after the kiddos went to bed.

It’s important to know that it’s never not severe weather season. It can happen at any time of the year if the conditions are right. The day or month of the calendar, even the time on the clock, is inconsequential. Bring together unusual warmth and humidity with an unusually strong cold front that happens to be early or late in the season and the calendar no longer matters.

If you’re reading this and there are stores still open on this Christmas Eve, there’s no better time to stop by someplace and buy a NOAA weather alert radio. You can find them at RadioShack, Wal-Mart, Target, Walgreens and any number of local merchants for between $20 and $100.  Just make sure when you get it home you don’t wrap it and put it under the tree. Instead, unwrap it and plug it in.

Finally, though I’m focusing on the potential for severe weather on Christmas Day, the threat moves east to the Carolinas on Wednesday.

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