Monday, December 26, 2011

12/26 - 12:30pm - Post-Christmas Snow Chance?

While many did not have a white Christmas yesterday, it certainly was a nice day across much of the East US with warm temperatures. We now turn our attention to a system that will be bringing much of the Southeast rain and possibly some snow as you head northward. This system is brewing in the Gulf Coast states right now and will bring West Tennessee rain this afternoon through tomorrow and the Louisville area rain tonight through tomorrow. While West Tennessee will likely stay all rain throughout the duration of this system due to warmer temperatures, Kentuckiana may have to deal with some snowfall during the afternoon tomorrow. The track of this storm from Tennessee to Eastern Kentucky is usually one that gives us our bigger snow events during the winter, but this time that won't be the case because of the lack of cold air we'll have to work with.

The forecast for snow is a tough one because we'll be dealing with colder air coming in on the backside of this system to give us our snow. This means that the atmosphere will cool from top to bottom and while snow will be falling it will likely not accumulate much, if any, because the air at the surface will still be warm while the upper atmosphere cools down. Nevertheless, a switchover to snow in the Louisville area will likely happen sometime tomorrow during the early to mid afternoon hours as cold air penetrates enough of the atmosphere to keep snow falling all the way to the surface. The NAM (left) seems to be the most aggressive on this switchover right now due to the surge of cold air it brings in early on in the afternoon in Louisville but the forecast sounding, or a vertical snapshot of the atmosphere at a given time, still indicates that we'll have temperatures just above freezing near the surface. This means that snow will be making it to the ground, but will likely melt on contact or accumulate lightly on the grass and elevated areas in heavier snow bands. Speaking of banding, that's another issue we could face. Snow banding is when you get a thin strips of heavy snow falling that look like bands due to winds coming together in these areas. These bands can produce snow accumulations even when temperatures are above freezing, just like in Jackson, Tennessee earlier this month when over an inch of snow accumulated with temperatures just above freezing.

Overall, I think areas in and close to Louisville can expect a dusting and maybe up to an inch of snow on grassy and elevated surfaces at some point tomorrow evening. If cooler air does not come in as quickly as expected this will remain a rain/snow mix or even just plain rain. Areas north and west of the Ohio River have the best shot for a light accumulating snow as they'll be slightly cooler and have an earlier switchover during the late morning hours. Most of the models are agreeing that we'll stay above freezing during most of this event, but we will be dipping down below that mark late Tuesday evening. This may create slick spots if any roads are wet from the snowfall during the afternoon. The 12z NAM (top left) is indeed the most aggressive on snowfall totals around here by looking at the snowfall output map, which you can see a larger view of by clicking. The 0z GFS (bottom left) is less aggressive, which is typical for it lately, and the Euro (which I cannot post here to copyright policies) keeps the snowfall along and north of the Ohio River. There are still a lot of small variables like exactly how fast the precipitation will move out of here and how much cold air will actually work into this process, so little details like these can drastically alter who gets snow and how much.

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