Court Square Subway Snow by Kurt Strahm on Flickr

Today wasn't very interesting weatherwise. After a cool night the forecast for tomorrow is similarly not very interesting: cloudy with a high in the upper 30s. The forecast for Friday into Saturday, especially beginning Friday night is interesting, very interesting and nerdtastic.

When the weather geeks make a forecast they primarily look at synoptic-scale phenomena such as high and low pressure systems and swoops in the jet stream that are hundreds of miles to a couple thousand miles in extent. Often they'll have to consider things that happen on a smaller scale like thunderstorms or the sea breeze. On rare occasions there will be obscure phenomena like the NORLUN instability trough to contend with. Hey, we said it was obscure! Anyway the NORLUN, named after Norgueira and Lundstedt, the guys who first described it, is this weird little phenomenon where a small portion of a relatively benign storm becomes a narrow, stationary convergence zone that can produce heavy snow squalls. The most infamous such event around these parts happened in February 1993, when the forecast for a couple of inches of snow turned into 12-20 across Long Island and Cape Cod.

Conditions look favorable for the formation of a NORLUN event on Friday. However, because these beasts are so rare they are difficult to forecast with any accuracy. Look for intermittent light snow on Friday through Saturday. 1-2 inches of wet snow is likely across the region. If a NORLUN trough sets up it will most likely happen Friday afternoon and evening and it could plop several additional inches of snow across a small portion of the area. Behind the storm we're looking at a windy weekend and many days of colder than normal weather.