A Wet Christmas with Some White Later?

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This time of year, the weather pattern is very fast-paced. That’s why we see a lot of wild swings in the long-range computer guidance. Those models have certain biases that cause them to do things in the long-range that are somewhat predictable. Take the Christmas forecast for example. We knew an Arctic blast would blow in here near Christmas, but two weeks ago we were a little off on the depth of the cold and the exact date it got here.

Christmas still looks very wet for most of the South; the threat of severe weather seems very limited at this point for Christmas Eve and Christmas Day, but if there are any stronger storms, they would likely be to the south near the Gulf Coast.

Here’s a look at rain chances through the middle of next week:

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Now that we are within five days of Christmas, the forecast is getting a little more concrete, but it is also getting more interesting!

CAUTION: What you are about to read is a model discussion; we are out of the land of GFS craziness and into the land of “if this trend continues, we will be putting a chance of at least a mix of rain and snow in the official forecast.” If all you are interested in is the forecast itself with no speculation, please click here for the main forecast page on WHNT.com/Weather!

Here’s the midday run of the GFS, which is showing a trend toward some winter weather potential in the middle of next week. If you’ve been keeping up lately, you know I refer to trends instead of single model runs. This is a model’s interpretation of where the weather features will be positioned at Noon on Wednesday, December 26th:

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Now that’s something that makes you go “hmmmmmmm.” This is the third model run in a row that shifts the once heavy rain/severe weather threat well south of us and brings in a different kind of weather. Cold air rushing in behind this strong Gulf storm system may get deep enough fast enough for some wintry precipitation. The picture above shows the 540 line; that’s the blue dashed line behind the strong low. It is a good indicator of where you should start looking for a possible change from rain to snow. It is not the only tool you should use, though.

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After looking at the model’s data in more detail, you can see from this forecast sounding that we have most of the ingredients in place for wintry weather in the Tennessee Valley. Moisture is available where temperatures are under -10ºC (14ºF) in the “snow growth zone.” Surface temperatures are forecast to be near 32º F, and we have a storm system that looks like it will be producing precipitation.

What could go wrong? This looks perfect, right? Well, we have to keep ourselves in check here. There is a small, shallow layer of warm air above the surface that could do one or more of the following: (1) melt all snow and bring cold rain at the surface, (2) melt some snow leaving us with a slushy mixture, or (3) if it is cold enough at the surface, it could melt it all and bring icy conditions.

There is also the issue of how much dynamic energy is left behind that main surface low to produce this precipitation. If it is really there and continues to show up in the guidance over the next two days, then I’d say we’ve got a really good chance of some snow. How much are we talking? If we get some, it’s probably not going to all fall as snow, and it may not all stick to the relatively warm ground. Taking into account the chance of mixed precipitation, some melting, and the uncertainty of how much precipitation will fall in general, I’d say we probably are looking at 2-3″ at the most in the higher elevations, and maybe around an inch in the valleys west toward The Shoals.

At this point, we are still in speculation mode. The forecast on the Seven Day Planner reflects this information plus the knowledge that a lot of these systems show a snow look for days then crush the hopes of the snow lover leaving us with cold rain or nothing at all.

If you’re hoping for snow, good luck! This looks like as good a chance as we could get this far in advance. We’ll keep you posted on the TREND. That’s really what we’re looking for in this situation.

-Jason
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