It wasn’t a major deal but a period of light snow managed to cover the roads and highways this morning in eastern Connecticut. While this was a relatively minor event it did catch people off guard and resulted in a few accidents. Considering my forecast at 11 p.m. prior to the event was for “partly cloudy skies” without mentioning flurries I wanted to look back at the event. What did I miss here?
I’ll start with a real basic look at one of the computer models from Sunday night. The high resolution 4km NAM showed no precipitation over Connecticut or Rhode Island. No signal there.
There were signals showing up on the end of the high resolution rapid refresh model or HRRR Sunday night. Here’s a look at the sounding for Willimantic (IJD) valid 8 a.m. Monday morning.
You can see an area of moisture around 800 mb with a zone of lift in that area as well (note the white line jutting to the west). Underneath that area is a very dry lowest 7,000 feet of the atmosphere so I would expect snow flakes to sublimate on the way down.
The actual sounding (or at least a close approximation) from a Monday morning HRRR run shows a more impressive picture.
You can see a much deeper saturated level and slightly deeper and stronger upward vertical motion. All of this is occurring around -15c which is the preferred temperature for efficient ice crystal growth (dendrites).
While actual precipitation amounts were only one or two hundreths of an inch a quick 1/4″ of snow in some towns was enough to really muck up the roads. A good rule of thumb is to always beware when the models show lift and saturation around -15c. While I probably wouldn’t have forecast accumulation snow (however minor) I think it would be reasonable to forecast flurries in retrospect.