Wintry Weather Expectations Tonight

Posted on: 4:27 pm, January 24, 2013, by , updated on: 02:03pm, January 24, 2013

Model guidance across the board is coming into line with reality on this winter weather set-up for tonight. From the beginning, we have not forecast or expected a major ice storm for North Alabama or Southern Tennessee. Some schools have strategically already delayed the start of the school day on Friday; you can see those updated as they come in here on WHNT.com.

While that is not a bad idea due to the chance of some patchy early morning ice, the threat is low enough that students should definitely go ahead and get homework done and prepare for tests: you’ll likely be in school by midday Friday!

I’ve gotten a lot of questions about who gets what with this event. Here’s the “general” idea:

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The Tennessee Valley is a big area with small microclimates that make ice more or less likely for that specific area. Tonight, the microclimates that will have the most favorable temperature profile for a glaze of ice are those in the higher terrain north and east of Huntsville and Arab into Georgia as well as Middle and East Tennessee.

But wait, what about the valleys? Isn’t Valley Head usually colder than Rainsville or Huntsville International colder than Monte Sano on a cold night? That’s a great point; however, in this particular situation when we are dealing with the movement of air masses (advection), it is the mountaintops that are colder and the valleys are slightly milder (just by a few degrees).

This makes a HUGE difference between rain and freezing rain. Rain is liquid above 32.0º F; rain becomes solid ice below 32.0º F. The cold air mass in place is very shallow: only about 1,700′ thick in Huntsville and about 2,500′ thick in Chattanooga:

Screen Shot 2013-01-24 at 2.01.31 PM

The above images are forecast soundings from FSL; they show the temperature and dewpoint profile through the lowest portion of the atmosphere (the troposphere).

See how the cold air is thicker and colder in Chattanooga and how it is thinner and milder in Huntsville at 6 AM tomorrow?

That’s happening because warm, moist air overrunning the layer of Arctic air eventually wins out and pushes the Arctic air north through the morning hours. There is also a small temperature drop from the surface up to the top of that layer of cold air that just happens to be about the top of Monte Sano, Green, Keel, Sand, and Lookout Mountains (and it’s a close call for Brindlee). It’s that little drop to around 31-32º that will make the difference between some glaze in the higher elevations and just cold, wet conditions in the valleys.

The Bottom Line:

    • Be Prepared for a Glaze of Ice: Everyone in the Freezing Rain Advisory has a chance of some light freezing rain. That chance is best in northeast/east Madison, Marshall, Etowah, DeKalb, Cherokee, and Jackson Counties of North Alabama and parts of Lincoln, Franklin, Moore, Bedford, Grundy, Marion, Hamilton, Sequatchie, and Coffee Counties in Southern Tennessee.
    • Most of us will just get rain: The vast majority of the Tennessee Valley will see rain on Friday morning that gives us no more trouble at 34-36º than it would have at 75º in the summertime. It’s those isolated spots (microclimates) in the higher elevations that could have a glaze to around 1/10″ of ice accumulation mainly on trees, powerlines, cars, rooftops, etc. through 9 AM Friday.
    • Temperatures Will Be Rising! As the Arctic air erodes in the morning, temperatures will be above freezing over most of North Alabama by 10 AM. Huntsville, Decatur, Athens, The Shoals, Cullman, Russellville, Hamilton, and Moulton should have no trouble whatsoever after sunrise in the morning.
    • Today’s sunshine is a big help! A good sunny day helps to ward off major travel problems the next day. There is a lag time for temperatures to drop significantly on major highways, and we don’t expect it to be cold enough tonight for them to freeze. That means the Tennessee River bridges on US 231 and I-65 should be open and passable in the morning as well as the O’Neal Bridge between Sheffield and Florence.

This is not a major winter storm, and it does not have the same capacity to sneak up on us like that upper-air low did a week ago. Travel problems will be at a minimum overnight and Friday morning.

If you are traveling to Nashville, Chattanooga, Knoxville or Atlanta, there will be places between here and there that do have some worse problems.

Around here, we are using the hashtag #valleywx on Twitter to keep up with weather information specific to the Tennessee Valley of North Alabama and Southern Middle Tennessee.

In the Nashville area, you can follow the hashtag #tspotter on Twitter for updated information from @NWSNashville and @NashSevereWx. The #tspotter network is designed to relay information for Davidson and Williamson Counties in particular.

For the rest of Alabama, it’s #alwx, Georgia is #gawx, and Tennessee is #tnwx.

I’ll be passing along everything I hear via Twitter and Facebook and on WHNT News 19’s accounts @whnt and WHNT News 19 on Facebook as well!

-Jason
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