Thursday January 19 - Friday January 20 2012 snow event

...Small accumulation Thursday night with late blooming storm system; favors eastern Massachusetts for highest totals...


Thursday Summary:


A weak low pressure system will be crossing southeast Canada while another piece of energy swings through the Great Lakes and fuels storm redevelopment just south of New England on Thursday night and early Friday morning.  Model consensus appears to indicate around .1 to .3 inches of precipitation from this system, which, with a 10 inches of snow from 1 inch of water would yield 1 to 3 inches of snow for most of Southern New England.

Thursday Detaills:


Below is a surface map from today's NAM computer model.  There is a noticeable kink in the isobars over southern New England which is indicative of a developing low pressure system over the region.  The shaded colors are 6 hour precipitation totals...in this map we are seeing .1 to .25 inches of water in 6 hours...ending at 12z Friday (8am).

Next is the total storm precipitation throughout the period...again we can see .1 to .25, however some higher values are showing up just offshore and on the outer cape.  This enhancement is a product of the late development of the system and some Atlantic inflow becoming established across eastern areas.  Again, this favors "heaviest" storm totals in the east.

These next two images are of vorticity in the middle layers of the atmosphere...vorticity is instantaneous circulation.  I've circled the energy that produces the parent low pressure system in southeast Canada...the arrows show the direction of the incoming energy that will produce our redeveloper.  Because of the dynamics presented here...I've circled the region near us that will have lowering surface pressures based on the upper level energy.

Same image but 12 hours later...you can see how the flow has altered so quickly...again...where vorticity is being forced by the flow...is the area where surface low pressure will form.  I have circled the area that this upper level map would favor for surface low pressure development.

Next, in the top left, the purple line is the 32F degree line.  This is valid at 8am Friday morning.  Much of southeast Mass is above freezing, but I believe this is mostly a snow event for our region.  Top right you can see the slightly heavier totals on the cape, but still light stuff in general...1-3"  In the bottom left you can see that Atlantic inflow from the south over Eastern Mass...if we are to get slightly higher amounts...this would be the most likely cause.

There is another snow event likely on Saturday.  I will post about that this evening.

Thanks for reading, as always!

JBP