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How ice storms work/today will not be ice-mageddon

December 15, 2010

If you’re in Nashville you’re no doubt sitting around waiting for this massive ice-mageddon that is supposed to happen today. Make no mistake, ice storms can be extremely dangerous when they actually happen, but more often than not they end up being far less severe than initially forecast. This is because these types of storms are incredibly complex and the most difficult of all weather phenomena to forecast. Here’s how they work, and why I’m usually skeptical of forecasts involving them:

When an ice storm happens there is always a strong mass of cold air in place (just look at how cold it was yesterday and monday!). Then a low pressure system moves in from the south or southwest, creating a mid-level wind flow from the south or southwest. Mid-level in meteorology is loosely defined as 5,000-10,000 or so feet above sea level. This southerly wind flow brings both warm air and moisture with it, creating a layer of warmer air above the cold air at ground level. This means the precipitation starts out high up as snow or rain, then becomes all rain as it falls through the warmer layer, then refreezes when it gets to the stubborn layer of cold air at the surface. Depending on the thickness of this cold layer, the rain drops will freeze while falling (sleet), or will freeze upon contact with the ground (freezing rain). Usually in these situations, the surface temperature is right at or just below freezing. This freezing precipitation only lasts as long as the cold layer stays below freezing, and that surface layer always eventually warms up. Here’s a graphic I’ve posted on this blog before, to illustrate:

Based on the forecast models, and published forecasts I’ve seen, there’s no doubt Nashville will see some frozen precip today, but I have a feeling that it will change to all rain, and the surface temp will creep up to the mid 30’s by late afternoon or so. The current forecast calls for the ice to remain the longest in the northeastern counties of middle TN, so those folks may see enough accumulation to start downing trees and power lines. I doubt there will be enough accumulation for that to be a major problem in Nashville, and all areas south and west. I’m fairly certain that Nashville’s heat island effect will aid in getting the surface temps up sooner rather than later. Furthermore, most roads have been salted heavily, and even the secondary roads have some salt on them transplanted by car tires, and with the surface temp hovering right around freezing the salt will have no problem preventing ice on the roads. Since the temperature is forecast to continue rising as the night goes on, due to the warmer air in the mid-levels working its way down, there shouldn’t be any problems at all by midnight or so.

PLEASE DON’T THINK THAT I’M TELLING YOU TO DRIVE NORMALLY IN WINTER WEATHER. YOU SHOULD ALWAYS ERR ON THE SIDE OF CAUTION WHEN THERE’S A CHANCE OF WINTRY ROAD CONDITIONS. PLEASE BE CAREFUL, ESPECIALLY IF CROSSING A BRIDGE OR OVERPASS AS THOSE FREEZE MUCH EASIER.

What I’m saying here is that this will NOT be ice-mageddon. For Nashville at least.

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