Winter Weather Forecasting: Science Or Shot In The Dark?
Winter weather forecasting is the trickiest type of weather to forecast. Even the most seasoned meteorologists have struggled with getting a forecast right during the cold, snowy months of the year. Many of you have gone through the hope, (or dismay), of seeing a forecast calling for several inches of snow, only to wind up with a mere dusting after it's all said and done. Are the meteorologists just throwing darts at a "forecast dartboard", and hoping to get it right? The answer is yes and no. While they aren't actually throwing darts, because of all the variables involved, a 1-2 degree difference in air temperature may mean freezing rain, sleet, or snow.
Photo - Baltimore News Journal
The main determining factors in predicting winter weather are the amount of moisture in the air, the temperature of the air at various levels, and the temperature of the ground. Depending how much warm or cold air precipitation must fall through before reaching the surface determines the type of precipitation, and the amount of moisture in the air determines how much will fall. A pocket of dry air, called a dry punch, or dry slot, near the surface may prevent precipitation from reaching the ground entirely. Where it gets tricky: the amount of moisture can fluctuate, and cause less or more than the amount of snow forecast, or cause a forecast to be updated as snow is already underway. While moisture levels may fluctuate as air masses interact, at the same time, temperatures in the upper air levels, mid-levels, and the surface may also change as precipitation is occurring. Both freezing rain and sleet start out in the cold upper atmosphere as snow, but change as they pass through various air levels and temperatures.
If frozen precipitation passes only through cold air, snow is the result at the surface.
If frozen precipitation passes through a large warm air layer, it melts, as it hits cold air again at the surface, turns into freezing rain.
Sleet is the result of frozen precipitation only partially melting in a warmer air layer, then refreezing as ice pellets in the colder air level at the surface. As you can see there are quite a few processes at work during winter weather. Small variations can cause vastly different results, and it's not always possible to predict these variations before they occur. So, while meteorologists can get a good idea of what's going to happen, last minute changes, courtesy of Mother Nature, can lay even the best forecast aside.