Winter Storm Watch Through Thursday Evening

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2:00 PM Update:

A Winter Storm Watch for most of North-Central and Northeast Alabama is in effect through Thursday evening:

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That’s Limestone, Madison, Jackson, DeKalb, Marshall, Morgan, Cullman, and Lawrence Counties in Alabama as well as Franklin, Moore and Lincoln Counties in Tennessee.

URGENT – WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE HUNTSVILLE AL
108 PM CST WED JAN 16 2013

…WINTRY WEATHER WILL LIKELY AFFECT MUCH OF THE TENNESSEE VALLEY
BEGINNING LATE TONIGHT AND DURING MUCH OF THURSDAY…

.A POWERFUL UPPER LEVEL LOW PRESSURE SYSTEM WILL DEVELOP AND MOVE
ACROSS NORTH CENTRAL ALABAMA. COLD AIR ASSOCIATED WITH THIS SYSTEM
WILL CAUSE PRECIPITATION TO CHANGEOVER FROM RAIN TO SNOW DURING THE
OVERNIGHT. THERE IS ALSO A POTENTIAL FOR ICE FOR FORM ON ELEVATED
SURFACES AND IN THE HIGHER TERRAIN…AS THE COLD AIR RUSHES ACROSS
THE REGION BY EARLY THURSDAY MORNING.

FORECAST INFORMATION:

An upper-air low is a pocket of very cold air through a depth of the middle and upper atmosphere. They create their own weather environment because they act somewhat like a drain by pulling surrounding air into them, lifting that air rapidly, and cooling it. The process that makes the air get colder as it rises is called adiabatic cooling.

That means these things can “manufacture their own cold air” to borrow a phrase one of my professors used in college.

Upper-Air Low in Morning Water Vapor Imagery (this shows the weather at roughly  15,000 to 20,000 feet above the ground).

Upper-Air Low in Morning Water Vapor Imagery (this shows the weather at roughly 15,000 to 20,000 feet above the ground).

There is always a surprise with a system like this. Since they are high above the ground, we don’t have the ability to measure them very well. Satellites make it easier, but there are still gaps in the data because we don’t have a dense network of observations 4 miles above the ground.

What can the surprise be? It can be a forecast for rain that ends up being heavy snow or vice versa. Sometimes we may expect either rain or snow and get nothing at all. In other words, these kinds of systems do not show their hand very well before they get to us; they’ve got a great poker face.

So the best we can do is give you our opinion on what will happen, and we try to do that as honestly as possible with no wish-casting involved (that’s me trying to take my personal desires out of it to give you a “best-look” at the situation).

If you were reading along last night, you know that we started to scale back the talk of snow some; well, the pendulum keeps swinging, and it is looking more and more snowy yet again.

This was the 6 AM NAM Model’s forecast accumulation of snow for tomorrow:

NAM Snow Accumulation forecast for Thursday (12Z/6AM Run)

NAM Snow Accumulation forecast for Thursday (12Z/6AM Run)

And here is the 12AM GFS forecast (the new run is looking similar although not complete):

12 AM GFS Forecast for Thursday's Snow

12 AM GFS Forecast for Thursday’s Snow

So, as you can see, there is definitely some potential there!

The Bottom Line

  • It’s going to rain tonight – we expect another 0.25″ to 0.75″ of rain through Thursday morning.
  • Some snow is possible, but the amount is in question. With systems like this, we forecast something then stick our head out the window and do a lot of now-casting and adjusting on the fly.
  • Some communities above 1200′ in Madison, Jackson, DeKalb, Marshall, Etowah, and Cherokee Counties could get a very healthy snow exceeding 3 inches on the grass, rooftops, cars, etc. Roads may become slick as the heavier snow falls, but they would likely improve quickly as temperatures remain in the lower 30s.
  • In Huntsville, Decatur, Athens, The Shoals, Russellville, and Moulton – this looks like mostly a rain event with some periods of light snow or a rain/snow mix. Widespread travel problems are not expected except in the higher elevations of Madison County.
  • With all that being said, we reserve the right to change this on the fly. It’s really a very difficult kind of forecast.
  • -Jason
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