Though we had a couple of weeks between them, the snow events thus far this season have come with relatively high predictability for New England. In fact, though my typical rule is to issue a snowfall map 24 hours prior to an event, I've been able to share estimates earlier than that for the events this winter. Thursday night's chance of snow is the exception - and perhaps more realistic for New England events.
Amazingly, Thrusday evening's weather pattern holds only 10% predictability as of this Wednesday evening writing - that is, of all the possible scenarios, the most agreement one can find is approximately 10%. That's terrible! Truthfully, it means that any forecast - from complete dismissal to strong hit - should be taken with a grain of salt, and I certainly wouldn't argue that my estimate is any less susceptible to error in this scenario.
At issue is the merger of two energetic disturbances - a system screaming across the Great Lakes into New England in the northern stream of wind, and another trucking northeast out of the Carolinas in the southern stream. The southern disturbance holds the moisture - sure to crank out substantial snow in the higher terrain of Virginia and North Carolina. The northern disturbance timing and intensity will determine just how much moisture can be pulled northward into New England.
My accumulation forecast is based on the premise that the bulk of the southern stream moisture stays south of New England, meaning only a fraction of this moisture streams northward ahead of the approaching northern stream energy. That said, I have a rather large area of a dusting to 1" of snow depicted, all the way into Southeast New Hampshire, because the northern disturbance is strong, and seems very likely to produce snow showers in this area, regardless of how much southern stream moisture becomes involved. The farther southeast one is, the more moisture that should be available, and the higher the potential snowfall amount. On Chatham and Nantucket, one really has to consider the warm ocean temperatures, but it would appear a dewpoint in the middle 20s - even on Nantucket - and cold air streaming in will overcome the ocean influence.
In technical considerations, there are a few things worthy of noting:
- The northern stream disturbance is intense, this favors at least some northward tug of moisture.
- There is little evidence of warm or moist advection of air through any significant column of the atmosphere - that is a negative for snow production.
- The past several systems to emerge off the eastern US coast have developed quicker than forecast.
- The past several systems to emerge off the eastern US have verified farther north than forecast.
- The weighting of all ensemble solutions is north - that is, more members are north than along or south of the current forecast track.
The end result has been for me to adjust anticipated storm track slightly northwest, hence the amounts indicated here. Given aforementioned lack of predictability, the forecast is obviously subject to significant change, but this is my best estimate given the above factors - two maps below.
Thursday Night Accumulation Map:
Zoomed in on Cape Cod: