The Arctic Clipper came through our region last night, and was well-forecast in terms of snow timing and amount. And yes, the morning rush is a bit of a mess.
Here are the snow amounts tallied by NWS Sterling:
|Thursday AM snow totals. NWS|
The storm did not have much moisture (water vapor) to work with, but it did manage to freeze out the equivalent of 0.25" of liquid over these counties...if this were rain, it would be enough to wet the ground, but probably not form puddles. Yesterday, the NWS predicted the region south of DC would pick up the highest snow amounts. Keep in mind that it is very, very hard to forecast exact snow amounts, to the inch, when the snow:liquid ratio is 20:1.
Nevertheless, I did some "forensic meteorology" this morning and determined that this pocket of snow was caused by a small area of enhanced uplift north of the low pressure center. Anytime you observe a snowband, you look for something called "frontogenetic forcing" which means a region in the lower-middle atmosphere where a weather front is developing. In this case, a mid-level front formed where chilly air to the north was sliding under slightly milder air to the south. The cold air, acting like a narrow wedge, lifted up a narrow strip of moist air, forming the snowband that dumped on St. Marys and Calvert Counties. You can see how this unfolded in the graphic below:
|10,000 foot weather chart showing snowband (green) and pocket of frontogenesis (purple lines). NWS.|
And all this while you and I were sound asleep!