Wednesday, January 31, 2007

Snow, Ice, and Rain - Tonight and Tomorrow

As mentioned over a week ago, we are encountering a major cold spell with occasional winter weather threats. Tonight is our first threat of significant problems with ice or snow this winter.

It will be interesting to see how long this cold, dry air can fight it out with the precip. It's down to 14.0 as of 6:25 this morning here in northern Cullman County. Even if the low tracks close to us, it seems like the chance of sleet and freezing rain is pretty significant across north Alabama. What if it tracks 50 miles further south?

My thought at this point is that the precipitation will begin as virga (not reaching the ground). You may see the radar showing precipitation at tonight at 8:00, but it won't be reaching the ground. That will aid evaporational cooling, dropping the temperature and raising the dewpoints. Between 9 and midnight precipitation will begin spreading across the state. In Central Alabama it will bring a period of sleet and freezing rain. Temperatures in the Tennessee Valley will be low enough higher in the atmosphere to support snow initially.

I am confident about how the precipitation will start. I am not as confident about when and if it will change to rain in specific parts of North Alabama. That depends on many factors, such as: the amount of sunshine and warming we get today, how much evaporative and radiational cooling occurs this evening, and, perhaps most importantly, the track of the low pressure.

At this point, the National Weather Service is expecting it to change over to rain before ending. That might occur, but I am not as confident about that yet. Keep a close eye on the position of the center of the low pressure. The further south it remains will result in less rain and more frozen precipitation.

As with all weather forecasts, expect the unexpected!

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