Light to moderate snowfall possible across New England Monday night into Tuesday (1/21/13-1/22/13)

An area of low pressure will be developing across the mid-Atlantic states during the day tomorrow and then drift north/northeastward passing just to the east of the Cape/Islands.  Several computer forecast models are all developing an inverted trough or a "norlun" trough.  Typically we see troughs form and develop in a more northerly to southerly type fashion, however, the trough associated with the area of low pressure will actually be oriented in a easterly to westerly fashion.  This will allow for moisture associated with the area of low pressure to be thrown back westward into portions of central/southern New England.

  These norlun trough setups are always highly intriguing and make forecasting precipitation, in this case, snowfall very difficult b/c in the end it all comes down to mesoscale features and more often than not we really can't pin down the mesoscale features until the hours leading up to the event or in some cases not until the event has already gotten underway!  What we can, do however, is take a look at all the computer data available to us and sort of pinpoint an area of where these features may setup.

  The most important mesoscale feature in this case will be with regards to where any heavier bands of snowfall setup.  Given the model data we have it appears quite likely that any heavier bands of snowfall will setup out across far eastern MA and the Cape region with potentially another band of heavier snowfall across extreme northeastern MA extending into southwestern NH.  At this stage we also can't rule out portions of RI and eastern or SE CT getting into the heavier banding as well, however, this would depend on when the heaviest banding begins setting up and how much moisture the trough can throw back.

Thanks to an Arctic front that has been pushing eastward through the region during the afternoon the airmass ahead of this system will be extremely cold from the surface all the way up through the troposphere.  This will lead to some very impressive snowfall ratios, possibly in the order of 15:1 to 20:1 or perhaps even up near 25:1!  Given the high ratios we would be seeing this means we really don't need a great deal of moisture of precipitation to produce moderate amounts of snowfall accumulation as the snow will have a very high fluff factor.

Given with what the computer models have been signaling the past 24-hours this is what I think we should see for snowfall across the region.  Granted, we are still about 24-30 hours away from the onset of the system so we could still see major changes with regards to track/strength and how much moisture gets thrown back westward into the region.  Also, once we get to tomorrow afternoon or early evening we should have a much better handle on the mesoscale features and where the heaviest band or bands will setup and we will be able to fine tune where the axis of heaviest snowfall will be.  

Snowfall should start late Monday evening/early overnight and last through much of the AM hours on Tuesday, especially across eastern sections where light snows could last into the early afternoon hours.  Snowfall rates of 2-3'' per hour are possible, even some isolated 4'' per hour rates.  One interesting to watch out for will be the potential for thundersnow within the heaviest bands.  Computer models are suggesting we could see some elevated instability work into eastern sections of the region.  If we are able to generate thundersnow this would increase the potential for 4''/HR rates.