Why the wintry mix and not just snow?

The storm that dropped anywhere from a trace up to 6″ of snow and ice on central and eastern Maryland Tuesday and Wednesday is pushing farther north with blizzard warnings posted for much of interior New England. For much of the storm temperatures around the Baltimore area were well below freezing, prompting many to ask why we didn’t have just snow. Why the wintry mix of icy precipitation? The weather balloon that was launched by the National Weather Service down at Sterling, VA near Dulles Airport on Wednesday morning tells the meteorological story. The graphic posted here shows the trace of the temperature and dew point as the balloon climbed up through the atmosphere. The diagonal red line is the freezing line. Any part of the balloon trace (the thin, squiggly black lines) that shows up on the right side of this diagonal red line indicates temperatures, at that level of the atmosphere, that are above freezing. You can see that most of the balloon trace, high in the atmosphere and near the ground, is left of the freezing line, indicating most of the atmosphere is cold enough for snow. However, I have highlighted a thin layer of the trace that shows up on the warm side of the freezing line. It is this thin, 3,130 foot layer, that caused the snowflakes that formed higher up to melt into raindrops as they fell, only to refreeze into ice pellets (sleet) as they hit the cold layer near the ground. These thin layers of warm or cold air will frequently make forecasting winter weather around here a real challenge, especially since there are only a few scattered weather balloons launched in the mid Atlantic region to pinpoint them for forecasters, and the closest one to Baltimore is the one launched a few times each day down in northern Virginia.

Tom Tasselmyer

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