Thursday, December 15, 2022

Winter arrives! Brace for cold air (...and winter shenanigans?)

First half of December wet and warm

We have dealt with almost two solid weeks of cloud cover, periods of fog, and multiple rainy systems and thunderstorms to start December. The result was 4-6" of rain, saturated ground, and a bunch of Mid-Southerners fighting Season Affective Disorder (making us feel SAD heading into the Christmas season)! Despite all this damp weather, the Drought Index valid this past Tuesday is a bit discouraging: 

The Drought Index valid December 13 indicates we still have a ways to go to get out from under our summer/fall drought, though areas south of I-40 in west TN could be out of the abnormally dry conditions as soon as next week. (UNL)

I expect we'll see improvement in next week's edition as it tends to lag actual conditions just a bit. But it is also proof that it takes prolonged wet weather to dig out from under a long-term drought! I suspect if you were to try and dig a hole in your yard, you would have some trouble once you get below the top 2-4"! That is drought -- not the standing water on top of the yard!

For the first two weeks of December, despite feeling cool and damp, the average temperature is actually 6.8° above normal! That is driven by both above average highs and lows, but low temperatures in particular have been quite warm for December thanks to all the cloud cover keeping us from dropping anywhere close to freezing since the 1st. That will change quickly now following yesterday's cold front. Readings near 60° are over for the month!

Second half of December brings the chill

A large dome of Arctic air has dropped across much of the eastern 2/3 of the nation, resulting in high temperatures locally in the 40s for the next week. We'll also get down to freezing nearly every morning for the next week. High pressure will control the weather for the majority of this time period resulting in mostly sunny skies. A cold front on Monday looked to potentially bring some precipitation, but it is starting to look like it may not be able to tap into enough moisture in the air to produce more than clouds, drying out as it approaches us Monday. If very light precip were to occur, it would likely be rain. 
The European model ensembles from Wednesday morning (supported by other long-range model data) indicate a chilly airmass in the week behind yesterday's front with even colder air poised to dive southeast over the northern Plains. A 5° departure from normal would equate to highs in the 40s and lows near freezing, on average. (WeatherBell)

Behind that front, we'll have a couple more dry and cool days before a massive shot of polar air that will dive into the central U.S. will shove its way southeast into the Mid-South. While the next week will be chilly, the airmass arriving late next week promises to be #StupidCold, just in time for Christmas weekend. Early indications are that we may not rise above freezing for a few days over the Christmas holiday with wind chills probably into the single digits! As for precipitation....


With all the usual caveats - it's a week out, it's the Mid-South, we have bluffs and I-40 (kidding!!) - the front will be potent enough that it could definitely generate precipitation, probably on Thursday the 22nd. 

Weather forecast map valid Thursday morning, Dec. 22. Polar air dropping through the central U.S. reaches the Mid-South with the potential for a quick shot of snow along the front. (NWS)

Factors that will have to be considered: timing (of the day) and near-surface temperatures, available moisture, system dynamics, etc. One negative factor to a potentially "good" snowfall is the speed of this system. The polar airmass will hit quickly and with a fury. Precipitation won't linger around as the very dry air behind the front quickly shuts precipitation off. Long-range models are not super helpful right now, but the ensembles of those models (which we use at longer ranges to detect trends, not details) are hinting at a quick shot of snow with the front. The MWN Forecast carries a chance of rain and snow on Thursday. We'll keep you posted as we get closer! What you should prepare for though is bitterly cold air for Christmas weekend!

The same European model ensemble model temperature anomaly data shown above, only for December 22-29, showing that cold blast from the northern Plains encompasses much of the eastern U.S. A 12° departure from normal would mean highs averaging near 40° and lows in the low 20s. (WeatherBell)

Erik Proseus
MWN Meteorologist

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