Wednesday, February 13, 2013
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Weather columnist Kevin Myatt: Today a tough call, so monitor conditions

Kevin Myatt is The Roanoke Times' weather columnist.

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It's another living-on-the-edge weather system today.

This is just the way the 2012-13 winter rolls. It has played havoc with my weekly snow meter, and produced several odd winter weather events in which some locations in Southwest Virginia get surprising amounts of snow or ice while others get little or nothing besides rain.

Most forecast guidance, as of Tuesday, points to mostly rain with today's system, as low-level cold air seems to be limited. There will be a better chance of wintry mix or snow the farther one travels north and west of Roanoke.

But it's a close call, with temperatures projected to be only a few degrees above freezing in critical layers of the atmosphere.

As we've seen as recently as last Thursday and Friday (a surprise 3- to 5-inch snow for the Roanoke Valley) and even Sunday night (sleet/snow mix for many locations in what was supposed to be all rain), these close-call situations can turn easily.

For snow to occur, temperatures generally must be at or below freezing from cloud to ground, so that crystals that form in the cloud won't melt on the way down.

Snowflakes can survive very thin layers of slightly above-freezing temperatures, usually near the surface. That was evidenced by Roanoke's official temperature getting no lower than 33 during the wet snow Thursday night and early Friday, and some temperatures as warm as the low 40s as fat conglomerated flakes plopped down in some parts of Southwest Virginia on Sunday night.

There are three major ways that temperatures can become cold enough for snow when it starts out just a little too warm.

The first is cold air advection. It's the simplest — winds move cold air in from somewhere else. Often we see it in the form of cold air damming, when northeast winds bank cold air against the mountains. It also happens when a cold front passes and brings Arctic air in from the west or northwest, or when a passing low-pressure system to the south pulls in colder air from the north.

A second is evaporational cooling. This is when there are layers of cold, dry air between the clouds and surface, so that some of the falling precipitation evaporates on the way down. As evaporation occurs, heat is used up, and the atmosphere cools. This was the primary reason many of you saw some snow and sleet Sunday night when temperatures started out in the 40s.

The third is dynamic cooling, in which atmospheric lift with a strong storm system results in a lowering of atmospheric pressure and cooling of the atmosphere. The Jan. 17 quick flip from rain to heavy snow was a prime example of this.

None of these three seems to be a strong suit with today's situation. But all three could play minor roles, and if it comes down to small pockets of above-freezing temperatures between cloud and ground, it is possible that it will cool enough for sleet or snow to reach the surface in some areas.

Today does not look like a widespread wintry precipitation event, but continue to monitor later forecasts, and keep an eye out the window, just in case that suddenly changes.

As is usual, we'll be following it on the Weather Journal blog. Drop by and let us know what it's doing where you live.

Weather Journal runs on Wednesdays.

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