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Jack’s

Freezing Rain & Snow?

Posted on: 11:43 am, January 14, 2013, by , updated on: 01:06pm, January 14, 2013

A cold rain continues this morning behind the cold front that moved across the region yesterday and last night. The front has currently stalled out and is draped across north Georgia, southwest into southeast Alabama. Blustery conditions continue behind the front has a much colder arctic air mass continues to move into the area. Temperatures dropped quickly last night behind the front and have remained steady or have slightly cooled a few degrees since sunrise.  Due to the overcast sky and moisture, many locations may not even reach the 40s later today. Expect temperatures to remain the same or cool a degree or two for the rest of today.  Below is a look at the current temperatures, cloud cover and precipitation across the Tennessee Valley.

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The main focus for this discussion is the potential for freezing rain, especially in the Shoals later today, tonight and early tomorrow.  The National Weather Service in Huntsville has issued a Winter Weather Advisory for; Lauderdale, Colbert, Franklin, Limestone and Lawrence counties in Northwest Alabama and Lincoln and Moore counties in southern middle Tennessee from 3:00 pm today until 9:00 am Tuesday morning, due to the possibility of freezing rain.  The National Weather Service in Nashville has issued a Winter Weather Advisory for; Wayne, Lawrence and Giles counties in southern middle Tennessee from 2:00 pm today, until 8:00 am tomorrow, for the potential of freezing rain as well.

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The Set Up:
A shallow layer of cold air has moved in behind the arctic front, thanks to gusty northwest winds.  Aloft winds are still out of the southwest, which means that warm, moist air will be moving in above the layer of cold air near the surface, this setup is called overrunning, when cold air is overrun by warmer, moist air. This scenario will keep rain chances in the forecast and also create the potential for some freezing rain later today.  This is a very tricky forecast, because for there to be freezing rain, surface temperatures have to be at or below freezing.  The past several days have been unseasonably warm, actually since January 7th, we’ve seen temperatures above average for January and within the past three days we’ve warmed into the 70s.

Due to the wet ground and persistent warm conditions over the past week, the soil is still warmer than the colder air above, which will limit the potential for ice accumulation if freezing rain does occur. The temperature of each individual object at the surface determines if the rain will freeze when it comes into contact with it.  Tree branches, power lines bridges and overpasses will have the best potential for surface temperatures to be at or below freezing, for ice to form.

This looks to be a marginal event for Northwest Alabama, but with that being said, as we all know a small temperature drop of a few degrees, could create more widespread icing problems and temperatures have underperformed (are colder) than models forecast. Below is a look at Hydrometeorological Prediction Center’s 24 Hour Probability of freezing rain accumulation of 0.01” or higher.

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This shows a 50-80% for freezing rain accumulation of equal to our greater than 0.01” for Northwest Alabama. There are a lot of factors in play that could potentially create a more widespread icing problem from freezing ran, but the more likely scenario is that there could be a light glaze on tree branches, power lines, elevated surfaces and roadways, with most of the roads remaining wet with no ice.

The highest potential for freezing rain will be farther west and north, where temperatures are colder and are currently at, or below freezing, especially in North Mississippi and west/central Tennessee and farther southwest in the Mississippi Delta, where Ice Storm Warnings are in place.  We are not expecting widespread icing across the area due to freezing rain.  The greatest concern remains in the far northwest corner of the state, in the Shoals.  Elsewhere, counties surrounding the Shoals will likely only see a light glaze on trees, power lines and elevated surfaces at best.

How To Prepare:
No reason to change your daily routine later today or tomorrow, just be aware of the potential for patches of ice on elevated roadways in Northwest Alabama, especially in the Shoals if freezing rain does occur. Sleet is also possible. We will continue to update the blog, as conditions become clearer throughout the day. Either way low clouds and fog are remaining this morning, and with damp/wet roads…be careful driving and courteous of other drivers. Rain chances will continue Tuesday, overnight into Wednesday, but temperatures are expected to be too warm for freezing rain to develop.  Flooding remains a concern over the next couple of days as rainfall totals will likely range between 2-3” by Wednesday morning.

Snow?
A potent upper-level low could bring the potential for more wintry precipitation on Thursday. The track of the upper low is critical, because if the upper low moves directly overhead, which models are showing it centered just south of the Tennessee River, dynamic cooling could cause wintry precipitation in the form of snow, even if temperatures are above freezing.  This is something we are keeping a close eye on as some models are hinting at a couple of inches of wet snow accumulation.  We will have more updates as the details become clearer over the next 24-48 hours.

For now, hope everyone stays warm and dry, be safe on the roadways.

- Jennifer Watson

Twitter: @JWatson_Wx

Facebook: Jennifer Watson WHNT