Thursday, December 11, 2014

First snow for the winter of 2014-2015

Another "nor'easter" storm yesterday (named Damon) brought us our first snow for this winter. That makes it 6 Decembers out of the last 7 it has snowed. It snowed pretty steadily from 6-8pm and left about 1-2cm on the ground. Enough to play in.
A "nor'easter" is so-named because it is a storm that tracks northward up the US East coast, the winter version of a summer hurricane like Irene or Sandy. If it moves up the coast slightly inland (as last month's storm did), it brings rain to NYC and the other big cities on the coastal plain (Philadelphia, Baltimore, ...etc.) and snow up in upstate NY and New England (Boston northward). But if it can move slightly offshore, as it did yesterday, it will bring wet snow to the coastal cities from NYC all the way down to Richmond, VA or even parts of North Carolina.
Technical note: Nor'easters develop in response to the sharp contrast in the warm Gulf Stream ocean current coming up from the tropical Atlantic and the cold air masses coming down from NE Canada. When the very cold and dry air rushes southward and meets up with the warm Gulf stream current (often near 70 F/21 C even in mid winter) intense low pressure develops. The divergence or diffluence in the upper atmosphere caused by the Jet Stream removes and disperses the rising air at a faster rate than it is replaced at the surface, which, along with the Coriolis Force, creates and develops a storm. Their northeast track brings them up along the East Coast past the mid-Atlantic and New England coastal states. The counterclockwise flow around a low-pressure system brings the warm moist oceanic air over land. The warm moist air meets cold air carried southward by the trough. The low enhances the surrounding pressure gradient, which acts to spiral the very different air masses toward each other at an even faster rate. The greater the temperature differences between the two air masses the greater the turbulence and instability, and the more severe the storm can become.

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