Showers/T'storms possible Friday; Arctic Blast/Snow Squalls/High Winds Behind the Cold Front

A strong cold front will be approaching southern New England Friday afternoon/evening.  Out ahead of the cold front the airmass is expected to become modestly unstable thanks to surface temperatures in the mid to upper 60's, dewpoints near 60F, and the possibility for mid-level lapse rates to approach 6.5 - 7 C/KM.  Current computer model forecast is suggesting that we could see several hundred joules of most-unstable (MUcape) develop across the region, with a few hundred to perhaps several hundred joules of mixed-layer cape (MLcape), and lifted index values as low as -2C.  These values would be suggestive of a modestly unstable airmass:

 
 With an amplifying trough approaching the region (and responsible for sliding the cold front east), winds aloft will begin to increase as well.  Computer forecast models indicate by mid afternoon we will see a strong mid-level jet begin to approach the region with 700mb winds expected to increase to as much as 50-60 knots and the region getting into the right entrance region of a potent 70-80 knot mid-level jet and the region also entering the right entrance region of an intensifying 120-130+knot upper-level jet.  This will really help to enhance upward motion and lift across the region:

While we will not see rain all day long, after some overnight and early morning shower activity there will be a break in the action.  In fact, it is very possible there are some brief breaks of sunshine at times.  If there are indeed any breaks of sunshine this would further work to destabilize the atmosphere locally.  However, given how the airmass will be fairly moist and very little in the way of capping in place, showers could develop at any time. 

As the cold front continues to approach and encounters a modestly unstable airmass in combination with strong winds aloft, the possibility will exist for a line of heavy downpours and thunderstorms to develop along and just ahead of the cold front.  Given the presence of modest instability and strong shear, a few thunderstorms would have the potential to become strong to marginally severe and have the potential to produce strong winds and small hail.  We will also have to watch for any cells which may take on some rotation which could locally enhance the possibility for stronger wind gusts and perhaps some larger hail.  While the threat for any strong to severe thunderstorms is on the low side as the ingredients in place are marginal, they are just high enough to raise some awareness. 

Once this cold front passes we will enter a cold air advection regime and much COLDER temperatures will filter in from the northwest.  While Saturday may warm-up, the cold air advection will really intensify as the afternoon progresses and temperatures will begin to fall rapidly and the winds will increase.  On Sunday, temperatures may struggle to reach the 40°F mark across many locations and we will be dealing with very strong wind gusts, possibly on the order of 60-65 mph!!  This will certainly yield the risk for power outages.  In addition to the strong winds, overnight Saturday into Sunday expected isolated to widely scattered snow squalls to occur across the region.  While not everyone will see snow, a few very localized areas could potentially pick up a couple inches of snow as the atmosphere will be quite unstable thanks to an area of very steep mid-level lapse rates (>7.5-8 C/KM) moving overhead:

Given the very steep mid-level lapse rates which would enhance instability values, we could be looking at the potential for thunder/lightning to occur with the heavier snow squalls as well.  If this does occur this would vastly increase snowfall rates locally and would enhance the potential for a localized area receiving a couple inches of snow.  The greater potential for this occurrence may be along and just north of the MA Pike, however, this all depends on the track of the shortwave energy which will be responsible for the developing snow squalls.