Evening AFD - 4 inches

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAUNTON MA
355 PM EDT FRI MAR 23 2007

...ACCUMULATING WET SNOW EXPECTED FOR A GOOD PORTION OF SOUTHERN NEW 
ENGLAND SATURDAY NIGHT...

.SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/...
STRONG SHORTWAVE PASSING WELL TO OUR SOUTH THIS AFTERNOON HAS SPREAD 
A SHIELD OF MID LEVEL CLOUDINESS OVER MUCH OF THE REGION.  CAN'T 
RULE OUT A PASSING SPRINKLE OVER THE NEXT FEW HOURS ACROSS MARTHAS 
VINEYARD OR NANTUCKET...BUT THE PRECIPITATION SHIELD WILL PASS TO 
OUR SOUTH.  OTHERWISE...MODELS SHOW THE MID LEVELS OF OUR ATMOSPHERE 
DRYING OUT THIS EVENING SO EXPECT SKIES TO BECOME MOSTLY CLEAR FROM 
NORTH TO SOUTH.  WITH DIMINISHING WINDS AND LOW DEWPOINTS EXPECT A 
GOOD NIGHT OF RADIATIONAL COOLING.  WILL SIDE WITH THE COOLER MET 
NUMBERS AND EVEN LOWER A BIT IN A FEW OF THE NORMALLY COOLER 
OUTLYING AREAS.  LOWS SHOULD RANGE FROM THE 20S ACROSS THE OUTLYING 
AREAS...TO THE LOW TO MID 30S IN DOWNTOWN BOSTON AND PROVIDENCE.

SKIES SHOULD START OUT MOSTLY SUNNY ON SATURDAY BUT CLOUDS WILL 
QUICKLY INCREASE FROM WEST TO EAST IN THE AFTERNOON IN ADVANCE OF A 
STRONG FAST MOVING SHORTWAVE.  DRY AIR SHOULD WARMUP QUICKLY IN THE 
MORNING WITH SUNSHINE SO WILL USE A BLEND OF GUIDANCE AND GENERALLY 
GO WITH HIGHS IN THE LOWER 50S.  THE EXCEPTION WILL BE THE IMMEDIATE 
COAST WHERE WE WILL UNDERCUT GUIDANCE WITH EASTERLY FLOW OFF THE 
OCEAN.  GENERALLY EXPECTING HIGHS IN THE MID 40S ALONG THE EASTERN 
MA COAST AND THESE TEMPERATURES MAY FALL INTO THE UPPER 30S TOWARD 
EVENING AS SEA BREEZES INCREASE.  

MEANWHILE...STRONG SHORTWAVE WILL ALLOW RAIN TO QUICKLY BREAK OUT 
ACROSS THE INTERIOR TOWARD EVENING...THEN REACHING THE COASTAL PLAIN 
CLOSE TO 00Z SATURDAY.  WE WILL THEN RUN WITH CATEGORICAL POPS SATURDAY 
NIGHT. 

THE SYSTEM IS QUITE DYNAMICAL SO EXPECT THE RAIN TO QUICKLY CHANGE 
TO WET SNOW ALONG AND NORTH OF THE MA PIKE VERY QUICKLY AFTER THE 
PRECIPITATION STARTS...AS SOUNDINGS SHOW TEMPERATURES BELOW 0C JUST 
A LITTLE OVER 1000 FEET OFF THE GROUND.  THE WARM LAYER SHOULD 
QUICKLY COLLAPSE WITH GOOD VERTICAL MOTION AND EVAPORATIONAL 
COOLING.  WE ALSO THINK THAT THIS WILL GIVE THE HIGHER ELEVATIONS A 
HEAD START WITH SUCH SHALLOW WARM LAYER QUICKLY BEING ERODED.  SOUTH 
OF THE MA PIKE...ITS MORE UNCERTAIN.  THE GFS/NAM WOULD SUGGEST A 
QUICK CHANGE TO WET SNOW IN THESE LOCATION AS WELL.  HOWEVER...THE 
UKMET/ECMWF SHOW A WARMER SOLUTION IN WHICH THEY WOULD TAKE A LONGER 
TIME TO TRANSITION TO WET SNOW.  ALL THE MODELS SHOW THAT THE 
COLDEST AIR IS SLOPED FROM NORTHWEST TO SOUTHEAST.  THAT MEANS THAT 
THE CT VALLEY MAY TAKE THE LONGEST TIME TO TRANSITION TO WET 
SNOW...WHILE NORTHEAST MA AND SOUTHERN NH GO QUICKLY OVER TO WET 
SNOW.

PREDICTING SNOW ACCUMULATIONS IS QUITE CHALLENGING WITH THESE 
DYNAMICAL SYSTEMS.  THIS LOOKS LIKE A QUICK MOVER...BUT SNOW GROWTH 
LOOKS QUITE IMPRESSIVE FOR A COUPLE OF HOURS DURING THE EVENING.  
BASED ON MODEL CONSENSUS...1 TO 4 INCHES OF WET SNOW APPEARS LIKELY 
ALONG AND NORTH OF THE MA PIKE.  THE HIGHER TERRAIN OF THE WORCESTER 
HILLS AND MONADNOCKS OF SOUTHERN NH SHOULD BE ON THE HIGHER END OF 
THESE NUMBERS.  TO THE SOUTH OF THE MA TURNPIKE WE ARE MORE 
UNCERTAIN AT THE TIME OF THE CHANGE OVER.  THEREFORE...WE WILL GO 
WITH ONE HALF TO 2 INCHES OF WET SNOW AND LET LATER SHIFTS TREND UP 
OR DOWN FROM HERE.  MOST OF THIS EVENT LOOKS TO OCCUR AT NIGHT OR 
WHILE THE SUN IS LOWER ON THE HORIZON.  THAT IS DEFINITELY A BIG 
ADVANTAGE IN ALLOWING SNOW TO ACCUMULATE IN LATE MARCH...PARTICULARLY 
WHEN TEMPERATURES HAVE BEEN WELL ABOVE FREEZING.  SINCE THE SNOW 
GROWTH LOOKS VERY GOOD AND MAY COME DOWN HEAVY FOR A FEW 
HOURS...ACCUMULATIONS MAY OCCUR ON ROADWAYS PARTICULARLY IN THE 
HIGHER TERRAIN.  LOW TEMPERATURES SATURDAY NIGHT SHOULD BE AROUND OR 
JUST BELOW FREEZING.  THESE NUMBERS SHOULD BE REACHED IN THE EVENING 
WITH DYNAMICAL COOLING QUICKLY DROPPING TEMPERATURES TO AROUND 
FREEZING.