041  
FXUS61 KBOX 051434  
AFDBOX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BOSTON/NORTON MA  
934 AM EST SUN DEC 5 2021  
   
SYNOPSIS
 
 
HIGH PRESSURE BUILDS ACROSS THE REGION TODAY, MOVING OFFSHORE  
TONIGHT. A WARM FRONT WILL LIFT NORTH ACROSS SOUTHERN NEW  
ENGLAND MONDAY MORNING WITH SHOWERS AND A FEW THUNDERSTORMS AND  
UNSEASONABLY MILD CONDITIONS. A STRONG COLD FRONT SWEEPS ACROSS  
THE REGION MONDAY EVENING WHICH WILL BE PRECEDED BY A PERIOD  
STRONG WINDS. DRY AND COOLER THAN SEASONABLE AIR WORKS INTO THE  
REGION LATE MONDAY NIGHT AND TUESDAY. WE COULD SEE THE SEASON'S  
FIRST WIDESPREAD ACCUMULATING SNOWFALL WEDNESDAY INTO WEDNESDAY  
NIGHT, BUT SPECIFIC AMOUNTS AND DETAILS ARE UNCERTAIN AT THIS  
TIME. DRY WEATHER RETURNS TO END THE WORK WEEK ALONG WITH A  
WARMING TREND BEFORE THE NEXT POSSIBLE STORM SYSTEM ARRIVES NEXT  
WEEKEND.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
930 AM UPDATE...  
 
HIGH PRES BUILDS INTO NEW ENG TODAY MOVING TO THE EAST BY  
EVENING. SUNSHINE WILL BE FILTERED BY HIGH CLOUDS WHICH WILL  
CONTINUE TO MOVE ACROSS THE REGION. OVERALL, A NICE EARLY  
DECEMBER DAY WITH LIGHT WINDS AND NEAR SEASONABLE TEMPS IN THE  
40S.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/
 
 
HIGH PRESSURE CONTINUES TO MOVE OFFSHORE. WITH A GENERAL  
SOUTHEAST FLOW DEVELOPING AT THE SURFACE, HELICITY VALUE ARE  
PROJECTED TO BE VERY HIGH DUE TO THE STRONGLY VEERING SHEAR WITH  
THE INCREASING WARM AIR ADVECTION. THERE IS ESSENTIALLY NO  
INSTABILITY IN MOST OF THE AVAILABLE MODEL SOUNDINGS. THIS IS A  
SCENARIO WHICH WILL BEAR CLOSER MONITORING. IF THIS WERE SUMMER,  
WOULD BE MUCH MORE CONCERNED ABOUT A MORNING WARM FRONT WITH  
THIS AMOUNT OF FORECAST HELICITY. SOME MIXED MESSAGES FROM THE  
GUIDANCE, WITH QUITE A FEW OF THE CONVECTION-ALLOWING MODELS  
HAVING GENERATED SOME STRONGER CELLS. THE GREATEST RISK FOR ANY  
THUNDERSTORMS LOOKS TO BE BETWEEN 6-10 AM MONDAY.  
 
THIS WARM FRONT SHOULD FINALLY MOVE NORTH DURING MONDAY  
AFTERNOON, WITH A COLD FRONT ARRIVING LATE IN THE DAY. PERIODS  
OF SHOWERS EXPECTED, WITH A BIT OF A BREAK POSSIBLE TOWARDS  
SOUTHEAST MA DURING THE EARLY AFTERNOON.  
 
EXPECTING TEMPERATURES TO START RISING OVERNIGHT, ESPECIALLY  
AFTER MIDNIGHT. TIMING WILL BE EVERYTHING. NOT EXPECTING WINDS  
TO BE THAT STRONG TO SCOUR OUT ALL OF THE LOW LEVEL COLD AIR,  
ESPECIALLY ACROSS THE HILLIER TERRAIN ALONG AN NORTH OF ROUTE 2  
IN NORTHERN MA. THUS, COULD SEE A BRIEF PERIOD OF FREEZING RAIN.  
DO NOT HAVE ENOUGH CONFIDENCE IN THE TIMING OF BOTH  
PRECIPITATION AND TEMPERATURES FOR ANY HEADLINES AT THIS TIME.  
 
THE OTHER CONCERN IS THE POTENTIAL FOR STRONG TO DAMAGING WIND  
GUSTS. WILL HAVE A ROUGHLY 45-65+ KT LOW S/SW LOW LEVEL JET  
CROSSING THE REGION. THIS REALLY PICKS UP DURING THE AFTERNOON  
AND INTO THE EVENING IMMEDIATELY AHEAD OF THE INCOMING COLD  
FRONT. LATEST GUIDANCE CONTINUED TO INDICATE A STRONG LOW LEVEL  
INVERSION, WHICH WOULD LARGELY PROHIBIT EFFICIENT MIXING. SHOULD  
TEMPERATURES BE HIGHER THAN CURRENT EXPECTATION, WE MAY NEED  
MORE WIND HEADLINES FOR MONDAY.  
 

 
   
LONG TERM /MONDAY NIGHT THROUGH SATURDAY/
 
 
HIGHLIGHTS...  
 
* LINGERING RAIN AND/OR SNOW SHOWERS WITH COLD FRONT PASSAGE  
OVERNIGHT MONDAY.  
* DRY, BLUSTERY AND MUCH COLDER ON TUESDAY.  
* A NOR'EASTER COULD BRING THE SEASON'S FIRST WIDESPREAD  
ACCUMULATING SNOWFALL TO SOUTHERN NEW ENGLAND WEDNESDAY INTO EARLY  
THURSDAY. HOWEVER, CONSIDERABLE UNCERTAINTY REMAINS ON THE LOW  
PRESSURE TRACK AND PLACEMENT OF THE RAIN/SNOW LINE.  
* DRY WEATHER RETURNS TO END THE WORK WEEK BEFORE ANOTHER STORM  
SYSTEM COULD IMPACT THE AREA THIS WEEKEND.  
 
OVERVIEW...  
 
CPC 6-10 DAY OUTLOOK IS NOW ADVERTISING 70 TO 80 PERCENT PROBABILITY  
FOR ABOVE NORMAL TEMPERATURES ALONG WITH 30 TO 40 PERCENT  
PROBABILITY FOR ABOVE NORMAL PRECIPITATION. THIS STRONG SIGNAL FOR A  
WARMING TREND IS DUE TO THE NAO/AO TURNING POSITIVE AND PNA TURNING  
NEGATIVE, WITH A MEAN H5 TROUGH TRANSITIONING TO MORE ZONAL/MEAN  
RIDGING THIS WEEK. AND AS THE NORTHERN JET ENERGY RETREATS A LITTLE  
NORTHWARDS, IT SETS UP A THERMAL GRADIENT ACROSS THE NORTHERN HALF  
OF CONTINENTAL U.S., WHICH ALLOWS FOR STORMS TO RIDE ALONG THE  
BOUNDARY. THIS PART OF THE FORECAST WOULD FOCUS ON THE POTENTIAL MID  
WEEK SYSTEM, WHICH COULD BE THE FIRST WIDESPREAD ACCUMULATING  
SNOWFALL FOR MUCH OF THE REGION THIS SEASON.  
 
DETAILS...  
 
MONDAY NIGHT INTO TUESDAY...  
 
AS COLDER AIR COMES IN, ANY LINGERING PRECIPITATION COULD MIX WITH  
OR CHANGE OVER TO SNOW SHOWERS, ESPECIALLY ACROSS THE HIGHER  
ELEVATION OF MA/CT. LARGE HIGH PRESSURE BUILDS IN FROM THE WEST ON  
TUESDAY. THIS WILL BRING DRY WEATHER ALONG WITH PLENTY OF SUNSHINE.  
HOWEVER, IT WILL BE MUCH COLDER AND BLUSTERY COMPARED TO MONDAY.  
HIGHS ON TUESDAY SHOULD ONLY TOP OUT IN THE UPPER 30S TO LOWER 40S.  
THIS FRESH INJECTION OF COLD AIR WILL BE KEY IN DETERMINING IN  
PLACEMENT OF THE RAIN/SNOW LINE FOR THE MID WEEK STORM.  
 
WEDNESDAY...  
 
IT APPEARS INCREASINGLY LIKELY THAT AT LEAST PART OF SOUTHERN NEW  
ENGLAND COULD SEE ITS FIRST WIDESPREAD ACCUMULATING SNOW OF THE  
SEASON. THE DYNAMICS ARE IMPRESSIVE. THE 00Z GFS SHOWS A DECENT  
STORM INTENSIFYING CLOSE TO THE 70W/40N BENCHMARK WITH DEFORMATION  
BAND FORMING ACROSS THE CWA. OMEGA WITHIN THE FAVORABLE  
DENDRITIC GROWTH ZONE OF -12 TO -18C LOOKS TO BE IN THE 15-20  
UNIT RANGE, ALONG WITH NEGATIVE 700-500MB EQUIVALENT POTENTIAL  
VORTICITY AND GOOD 700MB FRONTOGENESIS SHOULD HELP SET UP A  
DECENT MESOSCALE BAND SOMETIME WEDNESDAY AFTERNOON INTO EVENING.  
THE QUESTION WILL BE WHETHER IT IS COLD ENOUGH FOR 1 INCH PER  
HOUR SNOWFALL RATE TO MATERIALIZE AND HOW MUCH SNOW CAN  
ACCUMULATE BEFORE POTENTIAL MIXING OR CHANGEOVER TO RAIN  
ESPECIALLY ALONG THE BOSTON- PROVIDENCE CORRIDOR. ENSEMBLE MODEL  
GUIDANCE (GFS, EC AND GEM) HAVE CONTINUED TO ADVERTIZE DECENT  
PROBABILITY OF 24 HOUR 3-PLUS INCHES SNOWFALL, WITH CONSENSUS OF  
AT LEAST 40-50 PERCENT FOR WEDNESDAY INTO WEDNESDAY NIGHT  
ACROSS INTERIOR MA AND CT, USING UNIFORM 10:1 SNOW TO LIQUID  
RATIO. DEPENDENT ON HOW IMPRESSIVE THE COLD AIR INJECTION IS,  
THIS RATIO COULD POTENTIALLY BE A LITTLE HIGHER. IN FACT, THE  
GFS SHOWS SLR OF AROUND 14 TO 16:1.  
 
WHILE IT IS STILL TOO PREMATURE TO GO INTO POSSIBLE SNOWFALL  
ACCUMULATION, WOULD LIKE TO SEE GREATER SIGNAL FOR FORMATION OF  
DEFORMATION BAND ACROSS THE CWA ESPECIALLY WHEN THE CONVECTION-  
ALLOWING MODELS COME INTO RANGE BY SUNDAY NIGHT/MONDAY MORNING.  
GIVEN IT IS A FAST-MOVING SYSTEM, SNOWFALL RATES WILL LIKELY HAVE TO  
APPROACH OR EXCEED AN INCH PER HOUR FOR WARNING CRITERIA SNOWFALL (6  
INCHES AVERAGED OVER A FORECAST ZONE IN A 12 HOUR PERIOD). THERE IS  
ABOVE AVERAGE UNCERTAINTY WITH THE BOSTON-PROVIDENCE CORRIDOR  
BECAUSE EITHER IT COULD BE A MOSTLY RAIN EVENT OR IT COULD BE THE  
SWEET SPOT FOR SIGNIFICANT SNOWFALL ACCUMULATION IF THE COLDER AIR  
HOLDS ITS GROUND AND THE DEFORMATION BAND SETS UP IN THE VICINITY.  
SO PLENTY FOR SNOW LOVERS TO PONDER OVER AND CONTINUALLY MONITORING  
OVER THE NEXT COUPLE OF DAYS. FOR NOW, REALLY DON'T HAVE THE  
CONFIDENCE TO COME UP WITH A SNOWFALL MAP UNTIL THE HI-RES ENSEMBLE  
GUIDANCE (HREF) COMES INTO RANGE BUT WE HAVE TIME TO FINE-TUNE THE  
SPECIFICS.  
 
THURSDAY INTO SATURDAY...  
 
THERE COULD BE A FEW LINGERING SHOWERS EARLY THURSDAY AS THE LOW  
PRESSURE LIFTS NORTH. BUT OTHERWISE, THURSDAY LOOKS SEASONABLY COLD  
BUT MOSTLY DRY. ANOTHER SHORTWAVE WILL BE APPROACHING FROM THE WEST,  
SO PERHAPS ANOTHER ROUND OF SOME PRECIPITATION THURSDAY NIGHT INTO  
FRIDAY BUT SHOULD BE MOSTLY DRY BEFORE ANOTHER POTENTIAL COASTAL  
STORM SOMETIME NEXT WEEKEND. INITIAL INDICATIONS ARE THAT THIS  
POTENTIAL WEEKEND STORM IS A MOSTLY RAIN EVENT WITH 925MB  
TEMPERATURES PUSHING +8 TO +10C AND THE STRONG SIGNAL FOR ABOVE  
NORMAL TEMPERATURES. GIVEN WE ARE THIS FAR OUT AND HAVE TO SEE HOW  
THE MID WEEK SYSTEM EVOLVES, MAINLY STUCK TO NBM FOR THIS PART OF  
THE FORECAST.  
 

 
   
AVIATION /14Z SUNDAY THROUGH THURSDAY/
 
 
LOW - LESS THAN 30 PERCENT.  
MEDIUM - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
TODAY...HIGH CONFIDENCE  
 
VFR WITH W TO WNW WINDS AROUND 5-10 KTS BECOMING LIGHT AND  
VARIABLE AS HIGH PRESSURE BUILDS IN OVERHEAD. THIS HIGH PRESSURE  
MOVES OFFSHORE THIS AFTERNOON, SHIFTING WINDS TO THE S/SE.  
 
TONIGHT...HIGH CONFIDENCE IN TRENDS, MODERATE ON TIMING.  
 
VFR WITH LOWERING CEILINGS; DETERIORATION TO MVFR-IFR MORE  
LIKELY AFTER 06Z ACROSS THE INTERIOR. EASTERN LOCATIONS COULD  
SEE MVFR CONDITIONS AND SPOTTY IFR TOWARD DAYBREAK. LIGHT RAIN  
FOR MOST AS TEMPERATURES WARM OVERNIGHT, THOUGH A PERIOD OF  
LIGHT FROZEN PRECIP (MAINLY LIGHT FREEZING RAIN/SLEET) MAY BE  
POSSIBLE ACROSS NORTHERN MA, ROUTE 2 CORRIDOR AND THE  
BERKSHIRES.  
 
SE WINDS INCREASE TO 5-10 KT IN THE INTERIOR, AND AROUND 10-15 KT  
FOR EASTERN/SOUTHEAST MA AND SOUTHERN RI. SOUTHERLY LLJ OF 40 KT  
ASSOCIATED WITH WARM FRONT MAY LEAD TO AREAS OF LOW-LEVEL WIND  
SHEAR AND LOCAL TURBULENCE FROM BDL-BED NORTH AND WEST.  
 
MONDAY...MIX OF MVFR/IFR CONDITIONS EXPECTED DURING THE  
MORNING. VFR POSSIBLE ACROSS EASTERN MA AND PORTIONS OF RI  
DURING THE AFTERNOON.  
 
KBOS TAF...HIGH CONFIDENCE IN TAF.  
 
KBDL TAF...HIGH CONFIDENCE IN TAF.  
 
OUTLOOK /MONDAY NIGHT THROUGH THURSDAY/...  
 
MONDAY NIGHT: MAINLY VFR, WITH AREAS MVFR POSSIBLE. STRONG  
WINDS WITH AREAS OF GUSTS UP TO 40 KT. SHRA LIKELY, CHANCE  
FZRA.  
 
TUESDAY: VFR. BREEZY.  
 
TUESDAY NIGHT: MAINLY VFR, WITH LOCAL IFR POSSIBLE. CHANCE  
SHSN.  
 
WEDNESDAY: MAINLY IFR, WITH AREAS MVFR POSSIBLE. BREEZY. SHSN  
LIKELY, SHRA LIKELY.  
 
WEDNESDAY NIGHT: MAINLY IFR, WITH LOCAL MVFR POSSIBLE. BREEZY.  
CHANCE SHSN, CHANCE SHRA.  
 
THURSDAY: MAINLY VFR, WITH LOCAL IFR POSSIBLE. BREEZY. SLIGHT  
CHANCE SHSN.  
 

 
   
MARINE
 
 
FORECASTER CONFIDENCE LEVELS.  
 
LOW - LESS THAN 30 PERCENT.  
MODERATE - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
WINDS CONTINUE TO DECREASE THIS MORNING AS A HIGH PRESSURE MOVES  
ACROSS THE WATERS TODAY, AND OFFSHORE TONIGHT. PRESSURE GRADIENT  
INCREASES MONDAY AS FIRST A WARM FRONT MOVES NORTH ACROSS THE  
WATERS, AND THEN A COLD FRONT APPROACHES OUR REGION LATE IN THE  
DAY. STRONG SW LOW LEVEL JET LEADS TO ENOUGH CONFIDENCE IN GALE  
FORCE GUSTS LATER MONDAY. AS SUCH, WILL BE CONVERTING THE GALE  
WATCHES TO GALE WARNINGS SHORTLY.  
 
OUTLOOK /MONDAY NIGHT THROUGH THURSDAY/...  
 
MONDAY NIGHT: MODERATE RISK FOR GALE FORCE WINDS WITH GUSTS UP  
TO 40 KT. ROUGH SEAS UP TO 11 FT. SLIGHT CHANCE OF RAIN  
SHOWERS.  
 
TUESDAY: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH GUSTS UP  
TO 30 KT. AREAS OF ROUGH SEAS.  
 
TUESDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING  
5 FT. SLIGHT CHANCE OF RAIN SHOWERS.  
 
WEDNESDAY: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING  
5 FT. RAIN SHOWERS LIKELY.  
 
WEDNESDAY NIGHT: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH  
GUSTS UP TO 25 KT. ROUGH SEAS UP TO 10 FT. RAIN SHOWERS LIKELY.  
LOCAL VISIBILITY 1 TO 3 NM.  
 
THURSDAY: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH GUSTS UP  
TO 25 KT. ROUGH SEAS UP TO 10 FT. SLIGHT CHANCE OF RAIN  
SHOWERS, SLIGHT CHANCE OF SNOW SHOWERS. LOCAL VISIBILITY 1 TO  
3 NM.  
 

 
   
TIDES/COASTAL FLOODING
 
 
TIDE HEIGHTS ARE RISING TO THEIR DECEMBER MONTHLY HIGHS THIS  
WEEKEND. THE FOLLOWING ARE THE ASTRONOMICAL HEIGHTS OF UPCOMING  
HIGH TIDES AND DO NOT INCLUDE SURGE VALUES FROM LOCAL WIND/WEATHER  
CONDITIONS.  
 
POCKETS OF SPLASH OVER AND MINOR COASTAL FLOODING ARE POSSIBLE  
ON MONDAY, WITH THE GREATEST RISK ALONG THE SOUTH COAST GIVEN  
SOUTHERLY WINDS. HOWEVER, SIGNIFICANT COASTAL FLOODING IS NOT  
EXPECTED.  
 
BOSTON SUNDAY 12.2 FEET 1120 AM  
MONDAY 12.0 FEET 1214 PM  
 
NANTUCKET SUNDAY 4.4 FEET 1224 PM  
MONDAY 4.4 FEET 120 PM  
 
PROVIDENCE SUNDAY 5.9 FEET 824 AM  
MONDAY 5.8 FEET 917 AM  
 

 
   
BOX WATCHES/WARNINGS/ADVISORIES
 
 
CT...NONE.  
MA...NONE.  
RI...NONE.  
MARINE...GALE WARNING FROM 10 AM MONDAY TO 7 AM EST TUESDAY FOR  
ANZ230>237-250-251-254>256.  
 

 
 

 
SYNOPSIS...BELK/CHAI  
NEAR TERM...KJC  
SHORT TERM...BELK  
LONG TERM...CHAI  
AVIATION...BELK/CHAI  
MARINE...BELK/CHAI  
TIDES/COASTAL FLOODING...  
 
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