Snow Friday AM

UPDATE
Winter storm warning now posted
NWS 3 PM update
Change over from snow to rain begins around 3 AM from west to east.  This is a not a cold system so it will be interesting to watch the rain snow line.

Previous discussion below. 

Living in New England means many things including the Jekyll and Hyde nature of our weather.  Take this week for example.  Temperatures have been in the 50's and 60's.  Last night some folks lost power as heavy rain and roaring winds took down tree branches.  Tomorrow we see snow!  The National Weather Service in Taunton has issued a winter storm watch late tonight into tomorrow afternoon.
NWS Boston hazards

Currently a cold front is draped right across New England.
WPC surface analysis 7 AM
The true Arctic air is locked away behind the cold front in Ontario but it will cool off tonight.  As of 10 AM the New England temps look like this.
RTMA 2 m temps 10 AM (image weatherbell)
The upper air shows southwest flow aloft which explains the mild weather this week
12z NAM 500 mb vort 7 AM (image NCEP)
But see that mess in the Central US?  It will consolidate and add energy to the cold front which will stall offshore.  The ridge in the Atlantic will strengthen and force the trough to take on a negative tilt.
12z NAM 500 mb vort valid 4 AM Friday (image NCEP)
A low will form offshore and move up the coast throwing moisture back along the I-95 corridor.  In Southern New England a dusting to an inch is likely all the way to I-91/84 with heavier snows south of the MA Pike and inside 495.
WPC surface forecast 7 AM Friday
 The latest trends have been for more moisture and the precipitation is moving further west.  So more snow? Well not so fast.  The latest trends have been warmer.  The upper level support is strong so that should allow for a heavy thump of snow.  That will allow for snow to accumulate.  But it will be heavy and moisture laden.  It will not be a fluff bomb.  The Outer Cape and Islands can look for a lot of rain with this system.  Here is my snowcast
Snowcast for Friday
To my friends who are educator's I would expect no school in the 3-5 and 5-9 bands.  The precipitation will begin as warm but switch to snow around 3-4 AM.  The AM commute will stink.  It wraps up early afternoon Friday.  If you need more specific information please ask.

I'll watch the trends and update again tonight if/when the Winter Storm Watches become a warning.

-Zack Green

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