Surprise Snow!

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Snow in Danielson / Courtesy: Scott Cormier

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?

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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.

hires_tprecip_boston_25

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.

ijd_14z

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.

verify15zIJD

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.

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