Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201746 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 1246 PM EST SAT DEC 20 2014 .SYNOPSIS...
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SCATTERED SNOW/RAIN SHOWERS WILL GRADUALLY OVERSPREAD THE REGION FROM EAST TO WEST THIS AFTERNOON THROUGH SUN. WEAK LOW PRESSURE WILL PASS SOUTH OF NEW ENGLAND BRINGING ANOTHER PERIOD OF PRECIP MON NIGHT AND TUE. A MUCH STRONGER STORM PASSING WEST OF NEW ENGLAND WILL BRING HEAVY RAINFALL...GUSTY WINDS AND COASTAL FLOODING CONCERNS TO THE REGION WED INTO WED EVENING. DRY BUT VERY WINDY CONDITIONS FOLLOW FOR CHRISTMAS DAY INTO FRI.
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&& .NEAR TERM /THROUGH TONIGHT/...
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1245 PM UPDATE...SNOW SHOWERS CONTINUE TO AFFECT PORTIONS OF EASTERN MASSACHUSETTS THIS AFTERNOON. THEY ARE STRUGGLING TO MOVE WESTWARD OF CAPE ANN AND EASTERN NORFOLK COUNTY DUE TO MUCH DRIER AIR. EVENTUALLY THE SHOWERS WILL OVERCOME THE DRIER AIR BUT EXPECT THIS TO TAKE ANOTHER COUPLE OF HOURS...DEPENDING ON HOW QUICKLY THE WINDS TURN TO THE NORTHEAST. PREVIOUS DISCUSSION FOLLOWS... INTERESTING MESOSCALE SETUP AS AN INVERTED TROUGH COMBINES WITH SOME OCEAN ENHANCEMENT. APPROACHING UPPER LEVEL SHORTWAVE WILL ALLOW FOR EASTERLY FLOW JUST OFF THE GROUND. THIS COMBINED WITH NORTHEAST SURFACE WINDS WILL ALLOW LOW LEVEL OF THE ATMOSPHERE TO MOISTEN. SOUNDINGS ACROSS EASTERN NEW ENGLAND BECOME SATURATED ALL THE WAY UP TO THE -9C LAYER JUST ABOVE 850 MB. THIS IS FAVORABLE FOR SOME DECENT SNOW GROWTH...ITS JUST LOWER IN THE ATMOSPHERE THAN WE TYPICALLY SEE WITH MORE SYNOPTIC SNOW EVENTS. WHILE THIS IS NOT A TRUE OCEAN EFFECT EVENT...DELTA T VALUES FROM THE MIXED LAYER TO THE OCEAN ARE AROUND 15C. THEREFORE...EXPECT SOME OCEAN ENHANCEMENT. TIMING: WHILE FLURRIES/LIGHT SNOW SHOWERS MAY OCCUR ACROSS FAR SOUTHEAST NEW ENGLAND THIS MORNING. THE MORE SUBSTANTIAL PRECIPITATION SHOULD OVERSPREAD PLYMOUTH COUNTY AS WELL AS THE CAPE AND ISLANDS THIS AFTERNOON. GIVEN EASTERLY FLOW JUST OFF THE GROUND...EXPECT SNOW SHOWERS TO OVERSPREAD MOST OF EASTERN NEW ENGLAND THIS EVENING AS LOW LEVELS MOISTEN. IN FACT...SHOULD SEE AT LEAST SOME SCATTERED SNOW SHOWERS AFFECT EVEN OUR FAR WESTERN ZONES. PTYPE: MAINLY SNOW FOR ALL BUT PORTIONS OF COASTAL PLYMOUTH COUNTY AND ONTO THE CAPE/ISLANDS. A SUBTLE COASTAL FRONT LOOKS TO SETUP ACROSS SOUTHEAST COASTAL PLYMOUTH COUNTY. PTYPE WILL BE SNOW TO THE NORTHWEST OF THIS REGION...BUT ENOUGH MILD AIR SHOULD COME IN OFF THE OCEAN FOR A CHANGE TO RAIN SHOWERS SOUTHEAST OF THIS REGION. NOT EXPECTING THE POSITION OF THIS SUBTLE BOUNDARY TO CHANGE MUCH THROUGH SUNDAY MORNING. SNOW ACCUMULATIONS: WE ARE GENERALLY LOOKING AT A COATING TO 2 INCHES OF SNOW ACROSS MOST OF THE REGION. THE MAIN FOCUS FOR 1 TO 2 INCHES WILL BE ACROSS PORTIONS OF SOUTHEAST MA...AWAY FROM THE SOUTHEAST PLYMOUTH COUNTY COAST. ALSO...THE WORCESTER HILLS INTO NORTHWEST RHODE ISLAND MIGHT BE ANOTHER AREA AS A RESULT OF THE SUBTLE UPSLOPE COMPONENT TO THE WIND. YOU CAN ACTUALLY SEE THAT IN SOME OF THE HIGH RESOLUTION MODEL GUIDANCE. THERE IS ONE AREA TO WATCH IN PARTICULAR...FROM NORTHWEST PLYMOUTH COUNTY AND INTO NORFOLK COUNTY BASED ON MESOSCALE MODELS AND PATTERN RECOGNITION. WOULD NOT BE SURPRISED TO SEE VERY LOCALIZED 3 TO 4 INCH SNOW AMOUNTS...GIVEN SOME OCEAN ENHANCEMENT AND A LITTLE FOCUS ON THE COASTAL FRONT. WILL TRY TO HIGHLIGHT THIS AREA IN OUR SNOWFALL AND PROBABILISTIC GRAPHICS.
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&& .SHORT TERM /SUNDAY/...
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SCATTERED SNOW SHOWERS WILL CONTINUE DURING THE MORNING WITH MOIST EASTERLY FLOW IN THE LOW LEVELS. AGAIN...PTYPE WILL BE SNOW EXCEPT ACROSS SOUTHEAST PLYMOUTH COUNTY/CAPE AND ISLANDS WHERE BOUNDARY LAYER WILL PROBABLY BE WARM ENOUGH FOR RAIN SHOWERS. WHILE SOME LIGHT PRECIPITATION MAY LINGER INTO SUNDAY AFTERNOON...EXPECT A DECREASE IN AREAL COVERAGE THROUGH THE AFTERNOON AS LOW LEVEL LIFT WEAKENS. IT WILL REMAIN CLOUDY THOUGH WITH TEMPS IN THE 30S.
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&& .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
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HIGHLIGHTS... * INVERTED TROUGH WILL BRING RAIN/SNOW SHOWERS TO THE MASS EAST COASTLINE MONDAY INTO TUES MORNING. * WEAK WAVE OF LOW PRESSURE MON NIGHT/TUESDAY COULD BRING WINTRY MIX TO INTERIOR * ANOMALOUS SYSTEM ACROSS THE GREAT LAKES WILL BRING HEAVY RAIN...GUSTY WINDS AND COASTAL FLOODING CONCERNS AROUND CHRISTMAS EVE OVERVIEW... OVERALL 00Z GUIDANCE IS IN AGREEMENT SYNOPTICALLY WITH ISSUES ON TIMING AND SMALLER SCALE FEATURES. IT REMAINS EVIDENT THAT AN AMPLIFIED MID-UPPER LEVEL TROUGH AND PCPN FOCUSING LEAD SURFACE LOW/FRONTAL SYSTEM WILL STEADILY PROGRESS TO THE EAST-CENTRAL U.S. TUE AS EMBEDDED IMPULSES DIG TO THE LEE OF AN AMPLIFIED ERN PACIFIC/WEST COAST RIDGE ALOFT. THE SUPPORTING AND INCREASINGLY NEGATIVE-TILT TROUGH ALOFT WORKS OVER THE ERN US AND OUT THROUGH THE NERN STATES INTO DAYS 4/5 WED/THU. MODELS/ENSEMBLES HAVE BEEN FAIRLY CONSISTENT ON THIS AMPLIFIED PATTERN AND THE MOST RECENT GEFS/ECMWF ENSEMBLE GUIDANCE SHOW GOOD CORRELATION TO NEXT THURSDAY. HIGH PRESSURE AND UPPER LEVEL RIDGE OVER THE REGION WILL BEGIN TO EXIT BY MONDAY. AT THE SURFACE APPEARS ANOTHER INVERTED TROUGH/ONSHORE FLOW WILL YIELD TO PRECIP ALONG THE MASS COASTLINE ON MONDAY. ASIDE FROM THIS QUICK MESOSCALE EVENT THE MAIN FOCUS IS ON THE LONGWAVE DIGGING TROUGH OVER THE CENTRAL CONUS WHICH WILL PUSH A FEW SHORTWAVES THROUGH THE FLOW. FIRST ONE DEVELOPS A WEAK WAVE MOVING TOWARDS THE REGION LATE MONDAY INTO TUESDAY. THERE IS STILL SOME UNCERTAINTY WITH THE LOCATION/STRENGTH OF THIS SYSTEM BUT APPEARS THAT THERE IS THE POSSIBILITY OF A SNOW/WINTRY MIX IN THE INTERIOR. AS THIS SYSTEM DEPARTS AN ANOMALOUS LOW OVER THE GREAT LAKES WILL DEVELOP A SECONDARY LOW JUST WEST OF SOUTHERN NEW ENGLAND PER ENSEMBLE GUIDANCE. STILL A SOME SPREAD BUT IF THIS DOUBLE LOW DOES DEVELOP THEN THE RAIN AND WIND ASPECT OF THE SYSTEM INCREASES...DETAILS BELOW. STILL NEED TO WORK OUT MINOR DETAILS WITH THIS PARTICULAR SYSTEM BUT WITH 3 TO 4 STANDARD DEVIATIONS AWAY AND GENERAL MODEL AGREEMENT HAVE HIGH CONFIDENCE THIS SYSTEM WILL IMPACT SOUTHERN NEW ENGLAND. DETAILS... SUNDAY NIGHT INTO TUESDAY...MODERATE CONFIDENCE. HIGH PRESSURE AND UPPER LEVEL RIDGE WILL BEGIN TO MOVE OFFSHORE AS DIGGING TROUGH BEGINS TO SET-UP OVER THE CENTRAL CONUS. APPEARS ANOTHER MESO-SCALE INVERTED TROUGH WILL SET UP ON MONDAY AND LAST INTO EARLY TUESDAY MORNING. GUIDANCE SHOWS THAT RAIN/SNOW SHOWERS WILL DEVELOP ALONG THE MASS EAST COASTLINE THANKS TO ONSHORE EASTERLY FLOW. APPEARS THE COLUMN WILL SATURATE PER BUFKIT SOUNDINGS AND SOUTHERLY FLOW ALOFT. BOTH THE NAM AND GFS SHOW QPF MAKING IT ONSHORE DURING THIS TIME FRAME. HOWEVER THE EC IS SLIGHTLY SLOWER AND KEEP THE BULK OF THE PRECIP JUST OFFSHORE. ADD A CHANCE OF POPS DUE TO THE NAM/GFS MODEL CONTINUITY. BY LATE MONDAY NIGHT INTO TUESDAY A SOUTHERN BRANCH STORM SYSTEM FORMS ALONG THE SE COAST AND TAKES A TRACK OFF THE MID ATLANTIC COAST. THIS SYSTEM GETS CAUGHT UP WITH THE OVERALL LARGE SCALE AMPLIFICATION OF THE UPPER AIR PATTERN. THERE IS DECENT WARM ADVECTION WITH COLD AIR DAMMING ACROSS THE INTERIOR. BIGGEST QUESTION WILL BE IS THERE ENOUGH MOISTURE FOR PRECIP DURING THIS TIME FRAME. IF PRECIP OVERRUNS THEN COULD SEE FREEZING RAIN/DRIZZLE ACROSS THE INTERIOR LATE MONDAY INTO TUESDAY MORNING. DURING THE DAY ON TUESDAY...GOOD AMOUNT OF WARM AIR WILL PUSH SURFACE TEMPS ABOVE FREEZING ALLOWING FOR ANY PRECIP TO BE MORE OF THE LIQUID VARIETY AHEAD OF THE CHRISTMAS EVE STORM. HOWEVER GUIDANCE IS ALWAYS TOO FAST IN SCOURING TEMPS OUT ESP ACROSS THE CT VALLEY SO WE WILL NEED TO MONITOR THIS CLOSELY...ESP DUE TO LIGHT WINDS AND THE HIGH IN EASTERN CANADA. WE COULD ALSO SEE SOME MINOR COASTAL FLOODING CONCERNS ON TUESDAY. SEE BELOW FOR MORE DETAILS. WEDNESDAY INTO THURSDAY...HIGH CONFIDENCE IN SYSTEM LOW CONFIDENCE ON DETAILS. STRONG ANOMALOUS SYSTEM WILL STRENGTHEN OVER THE GREAT LAKES...DEVELOPING A POTENTIAL SECONDARY SURFACE LOW JUST WEST OF SOUTHERN NEW ENGLAND. STILL A LOT OF DETAILS TO WORK OUT AS THIS SYSTEM IS STILL 5 DAYS AWAY...HOWEVER ENSEMBLES CONTINUE TO SHOW STRONG AGREEMENT SO HAVE INCREASING CONFIDENCE THAT A POTENT STORM WILL OCCUR. LATEST 00Z GUIDANCE SHOWS THAT MOST PRECIP WILL BE LIQUID WITH A DEEP SUBTROPICAL CONNECTION BACK ACROSS THE GULF OF MEXICO...PW ANOMALIES FORECAST TO RISE BETWEEN +3 TO +4. CURRENT PWATS ARE PROGGED ABOVE 1.6 INCHES SO HAVE CONTINUED THE CATEGORICAL POPS. TEMPERATURES WILL BE WELL ABOVE AVERAGE...IN FACT WE MAY BE CLOSE AT BREAKING RECORD HIGH TEMPS IF TEMPS WARM TO 60F OR ABOVE ON CHRISTMAS EVE. AS THE SYSTEM WRAPS UP INTO CANADA...COLD AIR ADVECTION SHOULD CIRCULATE AROUND AND BEGIN A TRANSITION FROM SOUTHWEST TO NORTHEAST IN TURNING P-TYPE TO SNOW. HOWEVER LATEST GUIDANCE DOES SHOW A QUICK MOVING DRY SLOT SO SNOWFALL WILL STRUGGLE DUE TO LACK OF MOISTURE. THIS SYSTEM IS QUITE POTENT SO WE WILL NEED TO WATCH FOR POTENTIAL HAZARDS. CURRENTLY THINK THAT WE COULD POSSIBLE URBAN FLOODING ISSUES AS MODELS PROGGED WELL OVER 1.5 INCHES OF QPF. LATEST EC EVEN SHOWS 2.5 INCHES OF QPF IS POSSIBLE. THE GEFS SHOW A MOISTURE FLUX AT 850 MB BEING 4 TO 5 STANDARD DEVIATIONS AWAY...AND WITH THE TROPICAL CONNECTION AND STRONG SOUTHERLY FLOW COULD SEE HIGHER AMOUNTS DUE TO TOPOGRAPHY ENHANCEMENTS. RAIN FALL RECORDS MAY FALL IF EVERYTHING FALLS INTO PLACE. ALSO BELIEVE THAT WE MAY SEE SOME WIND ISSUES WITH THIS SYSTEM. GUIDANCE RIGHT NOW IS SHOWING A 45 TO 55 KT JET AT 925 MB...HOWEVER IT WILL BE HARD TO BREAK THE INVERSION WITH THE SOUTHERLY FLOW. RIGHT NOW BELIEVE THAT WIND ADVISORIES MAY BE NEEDED BUT IF THIS SYSTEM STRENGTHEN EVEN MORE SO A STRONGER JET IS POSSIBLE. CONTINUED THE MENTIONED OF ISOLATED THUNDER IN THE FORECAST PER PREV FORECASTER SUGGESTION. BELIEVE IT COULD BE POSSIBLE AS SOUTHERN NEW ENGLAND WILL BE IN THE WARM SECTOR...WITH THE ANOMALOUS JET AND SHOWALTERS CLOSE TO ZERO. FRIDAY...LOW CONFIDENCE. ONCE UPPER LEVEL LOW MOVES INTO SOUTHERN CANADA...EXPECT A RATHER FAST UPPER W FLOW IN PLACE. MAY SEE LEFTOVER SNOW SHOWERS BUT TEMPS WILL FALL BACK CLOSE TO SEASONAL NORMS.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH SUNDAY/... THROUGH 00Z...MODERATE CONFIDENCE. MVFR CIGS WILL CONTINUE TO OVERSPREAD EASTERN NEW ENGLAND AND EVENTUALLY OVERSPREAD THE INTERIOR THIS AFTERNOON. SCATTERED SHOWERS ACROSS PORTIONS OF EASTERN MASSACHUSETTS THIS AFTERNOON. MAINLY SNOW SHOWERS TO THE NORTH AND WEST OF CAPE COD CANAL...RAIN SHOWERS SOUTH AND EAST. LOCALIZED IFR CONDITIONS POSSIBLE JUST INLAND FROM THE SOUTHEAST NEW ENGLAND COAST...WHERE SNOW IS FALLING. TONIGHT AND SUNDAY...MODERATE CONFIDENCE. MVFR CONDITIONS DOMINATE IN LOWER CLOUDS AND SCATTERED SHOWERS. LOCALIZED IFR CONDITIONS POSSIBLE WITH THE BEST CHANCE IN SNOW SHOWERS. PRECIPITATION AFFECTS MAINLY EASTERN NEW ENGLAND BUT STILL SOME SCATTERED ACTIVITY ACROSS WESTERN NEW ENGLAND. MAINLY SNOW SHOWERS TO THE NORTH AND WEST OF CAPE COD CANAL...RAIN SHOWERS SOUTH AND EAST. KBOS TERMINAL...MODERATE CONFIDENCE IN TAF. FLURRIES AND SNOW SHOWERS MAY AFFECT THE TERMINAL BY 21Z. KBDL TERMINAL...MODERATE CONFIDENCE IN TAF. OUTLOOK...SUNDAY NIGHT THROUGH WEDNESDAY... MONDAY INTO TUESDAY...MODERATE CONFIDENCE. EXPECT MOSTLY VFR...ISOLATED MVFR ALONG THE MASS EAST COASTLINE IN RAIN/SNOW SHOWERS. TUESDAY NIGHT INTO WEDNESDAY...MODERATE CONFIDENCE. CONDITIONS WILL BE DETERIORATING AS HEAVY DOWNPOURS...ISO THUNDER AND GUSTY SOUTHERLY WINDS MOVE INTO THE REGION BY WEDNESDAY. COULD SEE MVFR TO IFR CONDITIONS IN CIGS AND VSBYS.
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&& .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TODAY THROUGH SUNDAY/...HIGH CONFIDENCE. HIGH PRESSURE ACROSS QUEBEC WILL RESULT IN PERSISTENT NORTHEAST WINDS ACROSS THE WATERS THROUGH THE WEEKEND. WIND GUSTS UP TO 20 KNOTS WITH SEAS BETWEEN 2 AND 4 FEET ARE EXPECTED FOR MOST WATERS...SO NO HEADLINES PLANNED. THERE IS A LOW PROBABILITY THAT A BRIEF SURGE OF NORTHEAST WIND GUSTS NEAR 25 KNOTS WITH MARGINAL 5 FOOT SEAS COULD AFFECT OUR NORTHERN WATERS LATE TONIGHT INTO SUN MORNING...BUT NOT CONFIDENT ENOUGH TO ISSUE ANY HEADLINES AT THIS POINT. OUTLOOK...SUNDAY NIGHT INTO WEDNESDAY...MODERATE TO HIGH CONFIDENCE. SUNDAY INTO MONDAY. CONTINUOUS NORTHEAST FLOW WILL KEEP SEAS CHOPPY. LINGERING NORTHEAST SWELL MAY BUMP SEAS ABOVE 5FT ESP ACROSS THE NORTHEASTERN WATERS. SCA MAY BE NEEDED...BUT LOWER CONFIDENCE. TUESDAY INTO WEDNESDAY...NE WINDS WILL INCREASE...GUSTING TO 30 KT TUE THEN SHIFT TO SE GUSTING TO 30-35 KT WED...HIGHEST ON THE OUTER WATERS. SEAS BUILD UP TO 7-11 FT BY WED. GALES WILL BE NEEDED. && .TIDES/COASTAL FLOODING... HIGH ASTRONOMICAL TIDES WILL OCCUR TUESDAY THROUGH THURSDAY. BREAKING IT DOWN...ON TUESDAY HIGH TIDE AT BOSTON WILL BE 11.6 FT. THERE WILL BE ONSHORE FLOW DURING THIS HIGH TIDE ALLOWING FOR MINOR SPLASHOVER. IF WIND GUSTS INCREASE THEN COULD POSSIBLE SEE MINOR COASTAL FLOODING. ON WEDNESDAY...WINDS WILL SWITCH TO A MORE SOUTHEASTERLY FLOW DURING THE DAY AND INCREASE QUICKLY FROM APPROACHING SYSTEM AND DESCENT PRESSURE FALLS. WEDNESDAY AFTERNOON HIGH TIDE IN BOSTON IS 11.7 FT AND WITH ANY STRENGTH ON THE EASTERLY WIND FLOW WILL PUSH THE TIDE OVER. RIGHT NOW BELIEVE MINOR COASTAL FLOODING IS POSSIBLE ALONG THE EAST COASTLINE. HOWEVER WEDNESDAY EVENING...THE WIND GOES IN A MORE SOUTHERLY DIRECTION AND HIGH TIDE IN PROVIDENCE WILL BE 5.2 FT. 925 MB LLJ STRENGTHENS TO 50-60 KTS WHICH COULD PUSH A DESCENT SURGE UP ALONG THE SOUTH COAST DURING THE TIME OF HIGH TIDE. THINKING THAT MINOR COASTAL FLOODING COULD OCCUR BUT CANNOT RULE OUT MODERATE ALONG THE SOUTH COAST...ESP IF LLJ STRENGTHENS EVEN MORE. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...DUNTEN/RLG NEAR TERM...FRANK/RLG SHORT TERM...FRANK LONG TERM...DUNTEN AVIATION...DUNTEN/RLG MARINE...FRANK/DUNTEN TIDES/COASTAL FLOODING...

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