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Wednesday February 20, 2019

Assignment: Predict the snow depth at the end of the day (inches) for Washington-Dulles, VA (KIAD)

A fast-moving winter storm is expected to bring snow, or at least a wintery mix of precipitation, to the DC area on Wednesday. It's a tricky forecast as usual there, because there can be a big difference in the impact the storm will have, depending on the timing and relative amounts of rain, snow, sleet, and freezing rain. So let's forecast the depth of snow on the ground at Washington-Dulles at the end of the day on Wednesday and see what happens.

Question: In the NWS discussion of Wednesday's event, they say: ". . this could put down a quick 4-8 inch snowfall thanks to the copious Gulf moisture available. After that, the warm air aloft will overwhelm the region, with snow change to sleet and freezing rain from south to north during the midday and afternoon hours." Why would warm air aloft make the precipitation type change from snow to sleet and freezing rain?

Forecasts for Washington-Dulles, VA (KIAD)
Forecaster Snow depth, inches (error) Comment Day Score
GFS-MOS-06-DAY01 2.9 (-0.1) 3.8
GFS-MOS-00-DAY00 2.2 (-0.8) 3.0
GFS-MOS-18-DAY01 2.2 (-0.8) 3.0
NAM-MOS-00-DAY01 2.2 (-0.8) 3.0
GFS-MOS-12-DAY01 4.5 (1.5) 2.3
GFS-MOS-00-DAY01 4.5 (1.5) 2.3
NAM-MOS-00-DAY00 0.7 (-2.3) 1.4
NAM-MOS-12-DAY01 0.7 (-2.3) 1.4
GFS-MOS-06-DAY02 0.7 (-2.3) 1.4
GFS-MOS-12-DAY02 0.2 (-2.8) 0.9
NAM-MOS-12-DAY02 0.1 (-2.9) 0.7
GFS-MOS-18-DAY02 5.9 (2.9) 0.7