Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191755 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 1255 PM EST FRI DEC 19 2014 .SYNOPSIS... HIGH PRESSURE MOVING INTO EASTERN CANADA WILL RESULT IN DRY BUT CHILLY WEATHER TODAY. A WEAK DISTURBANCE AND ONSHORE FLOW WILL RESULT IN A PERIOD OF SOME LIGHT RAIN/SNOW SHOWERS MAINLY ACROSS EASTERN NEW THIS WEEKEND. LOW PRESSURE WILL PASS NEAR THE REGION ON TUESDAY...BRINGING A PERIOD OF PRECIPITATION. A MUCH STRONGER STORM IS LIKELY TO BRING HEAVY RAIN...WIND AND COASTAL FLOODING CONCERNS TO SOUTHERN NEW ENGLAND WEDNESDAY INTO WEDNESDAY EVENING. DRY BUT VERY WINDY CONDITIONS FOLLOW FOR CHRISTMAS DAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1250 PM UPDATE... CLEARING TO THE N SLOWLY MOVING SOUTH INTO ESSEX COUNTY AND EXPECT SOME SUNSHINE THIS AFTERNOON ACROSS NE MA. ELSEWHERE... CLOUDS WILL REMAIN TRAPPED BELOW THE INVERSION. FORECAST ON TRACK. PREVIOUS DISCUSSION... 12Z SOUNDINGS SHOW A SHALLOW LAYER OF LOW LEVEL MOISTURE TRAPPED BELOW A STRONG SUBSIDENCE INVERSION. NAM SOUNDINGS SHOW THIS MOISTURE REMAINING IN PLACE SO THESE LOW CLOUDS WILL LIKELY HANG TOUGH MOST OF THE DAY RESULTING IN MOSTLY CLOUDY SKIES. HOWEVER...NOTING A LARGE CLEAR AREA ACROSS SW MAINE AND EASTERN NH. NAM IS INDICATING A LOW LEVEL DRY SLOT EXPANDING SOUTH INTO E MA LATER THIS AFTERNOON SO BEST CHANCE FOR PTSUNNY SKIES WILL BE ACROSS NE MA. ADJUSTED SKY GRIDS TO REFLECT MORE CLOUDS AND POTENTIAL FOR SOME SUNSHINE ACROSS NE MA. NO CHANGES TO TEMPS. CLOUD COVER WILL LIMIT TEMP RISE WITH MOST LOCATIONS REMAINING IN THE MID/UPPER 30S...BUT CLOSER TO 30 ACROSS THE HIGH TERRAIN IN WESTERN AND N MA. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... TONIGHT... HIGH PRESSURE ACROSS EASTERN CANADA GRADUALLY SLIDES EAST. THIS WILL ALLOW SURFACE WINDS TO TURN TO A MORE NORTHERLY DIRECTION. IN RESPONSE...OCEAN EFFECT CLOUDS WILL MOVE ONTO EASTERN NEW ENGLAND COAST. DELTA T VALUES FROM THE OCEAN TO THE MIXED LAYER IS AROUND 13C...SO MAY SEE SOME LIGHT SNOW SHOWERS/FLURRIES DEVELOP ACROSS THE CAPE/ISLANDS AND PLYMOUTH COUNTY BY DAYBREAK SAT. LOW TEMPS WILL MAINLY BE IN THE 20S...BUT LOWER TO MIDDLE 30S ON THE IMMEDIATE SOUTHEAST NEW ENGLAND COAST. SATURDAY... INTERESTING SETUP. WEAK INVERTED TROUGH DEVELOPS ACROSS THE REGION. NORTHEAST SURFACE WINDS COMBINED WITH WINDS JUST OFF THE DECK TURNING EAST SOUTHEAST WILL MOISTEN UP THE ATMOSPHERE ACROSS THE SOUTHEAST NEW ENGLAND COAST. 850 TEMPS AROUND -6C SHOULD BE COLD ENOUGH FOR ICE NUCLEATION...ESPECIALLY GIVEN EASTERLY FLOW BRINGING SALT PARTICLES IN OFF THE OCEAN. THEREFORE...NOT TOO CONCERNED ABOUT FREEZING DRIZZLE AT THIS TIME BUT THINK WILL BE DEALING WITH SNOW SHOWERS. LIGHT SNOW SHOWERS SHOULD AFFECT FAR SOUTHEAST MA...ESPECIALLY PLYMOUTH COUNTY WHERE PATTERN RECOGNITION WARRANTS LIKELY POPS. AS THE AFTERNOON WEARS ON WE MAY SEE THESE LIGHT SNOW SHOWERS DEVELOP ACROSS CAPE ANN AND PERHAPS EVEN FURTHER INTO EASTERN MA BY EARLY EVENING. ENOUGH MILD AIR SHOULD COME IN OFF THE OCEAN TO RESULT IN PTYPE CHANGING TO LIGHT RAIN SHOWERS ACROSS PORTIONS OF THE CAPE/ISLANDS AND PERHAPS ONTO PORTIONS OF THE IMMEDIATE SOUTHEAST PLYMOUTH COUNTY COAST. AS FOR SNOW ACCUMULATIONS...MAINLY JUST LOOKING AT A COATING TO LESS THAN 1 INCH ACROSS SOUTHEAST NEW ENGLAND THROUGH EARLY SATURDAY EVENING. HOWEVER...LOW RISK FOR VERY LOCALIZED 1 TO 2 INCH SNOW AMOUNTS ACROSS PORTIONS OF PLYMOUTH COUNTY IF THE BOUNDARY LAYER REMAINS COLD ENOUGH FOR SNOW. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... HIGHLIGHTS... * SCT SNOW/RAIN SHOWERS SUNDAY WITH AVG TEMPS * 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 CONCERNS AROUND CHRISTMAS EVE OVERVIEW... OVERALL 00Z GUIDANCE IS IN AGREEMENT SYNOPTICALLY WITH ISSUES ON TIMING AND SMALLER SCALE FEATURES. AN AMPLIFIED MID-UPPER LEVEL TROUGH AND PCPN FOCUSING LEAD SURFACE LOW/FRONTAL SYSTEM DEVELOPS OVER THE CENTRAL US BY EARLY NEXT WEEK. THE SUPPORTING AND INCREASINGLY NEG-TILT TROUGH ALOFT WORKS OVER THE ERN US AND OUT THROUGH THE NERN STATES INTO DAYS 5/6 WED/THU. MODELS/ENSEMBLES HAVE BEEN FAIRLY CONSISTENT IN RECENT RUNS ON THIS AMPLIFIED PATTERN AND THE MOST RECENT GEFS/ECMWF ENSEMBLE GUIDANCE SHOW GOOD CORRELATION TO NEXT THURSDAY. DURING THIS SYNOPTIC TRANSITION...HIGH PRESSURE AND UPPER LEVEL RIDGE WILL BE OVER THE NORTHEAST SATURDAY NIGHT BEFORE DEPARTING LATE SUNDAY. LONGWAVE DIGGING TROUGH OVER THE CENTRAL CONUS WILL PUSH A FEW SHORTWAVES THROUGH THE FLOW. FIRST ONE DEVELOPS A WEAK COASTAL LOW MOVING TOWARDS THE REGION LATE MONDAY INTO TUESDAY. THERE IS STILL SOME UNCERTAINTY WITH THE LOCATION OF THIS SYSTEM BUT APPEARS THAT THERE WILL BE SNOW/WINTRY MIX IN THE INTERIOR. AS THIS SYSTEM DEPARTS APPEARS AN ANOMALOUS LOW OVER THE GREAT LAKES WILL DEVELOP A SECONDARY LOW /DOUBLE BARREL LOW/ CLOSER TO NEW ENGLAND...PER ENSEMBLE GUIDANCE. STILL A GOOD AMOUNT OF SPREAD BUT IF THIS DOUBLE LOW DOES DEVELOP THEN THE RAIN AND WIND ASPECT OF THE SYSTEM INCREASES...DETAILS BELOW. STILL SOME UNCERTAINTY WITH THIS PARTICULAR SYSTEM BUT WITH 2 TO 3 SD AWAY AND GENERAL MODEL AGREEMENT HAVE HIGHER CONFIDENCE THIS SYSTEM WILL IMPACT SOUTHERN NEW ENGLAND. DETAILS... SATURDAY NIGHT INTO SUNDAY...HIGH CONFIDENCE. WEAK WAVE WILL MOVE THROUGH THE FLOW ON SATURDAY NIGHT AND WITH HIGH PRESSURE SHIFTING EASTWARD EXPECT ONSHORE EASTERLY FLOW. GUIDANCE CONTINUES TO SHOW QPF MOVING ONSHORE AND WITH MOISTURE AND THE LIFT COULD SEE SNOW SHOWERS MOVING INLAND SATURDAY NIGHT INTO SUNDAY. BIGGEST QUESTION IS WILL THE MOISTURE SATURATE ENOUGH TO -6C. IF NOT THEN THE POTENTIAL FOR FREEZING DRIZZLE IS POSSIBLE. BELIEVE THAT WITH A SOUTHERLY FLOW AT 850MB AND EASTERLY FLOW AT THE SURFACE WILL ALLOW FOR THE PROFILE TO SATURATE. USUALLY YOU WOULD WANT SATURATION UP TO -8C BUT BECAUSE OF SALT NUCLEI THE SNOW GROWTH REGION BEGINS AT -5C OR -6C. APPEARS THAT THERE IS ENOUGH MOISTURE SO A QUICK HALF INCH TO AN INCH IS POSSIBLE PER SOUNDINGS EAST OF THE WORCESTER HILLS. HOWEVER BECAUSE IT IS A MORE ENE FLOW...PLYMOUTH COUNTY MAY SEE 1 TO 2 INCHES IF THEY REMAIN COLD ENOUGH FOR SNOW...ESP DURING THE DAY ON SUNDAY. SNOW MAY OVERSPREAD ACROSS THE WEST DURING THE DAY IF THERE IS ENOUGH MOISTURE. HOWEVER RIDGE BEGINS TO BUILD INTO THE REGION LATE SUNDAY INTO SUNDAY NIGHT SO THE POTENTIAL FOR SNOW SHOWERS WILL BE LIMITED DUE TO PRESSURE RISE. MONDAY INTO TUESDAY...MODERATE CONFIDENCE. WEAK COASTAL LOW PRESSURE SYSTEM WILL DEVELOP AND MOVE CLOSE TO THE SOUTHERN NEW ENGLAND COASTLINE. THERE IS STILL DESCENT MODEL SPREAD ON THE EXACT LOW TRACK BUT APPEARS THAT IT DOES WEAKEN AS IT APPROACHES FROM THE MID-ATLANTIC. REGARDLESS THERE COULD ALLOW FOR SHALLOW COLD AIR FROM THE DEPARTING TO HIGH TO STAY TRAP AND WITH WARM AIR ALOFT MOVING INTO THE REGION...COULD SEE SNOW OR WINTRY PRECIP ESP ACROSS THE HIGHER TERRAIN LATE MONDAY INTO TUESDAY MORNING. BECAUSE OF THE MODEL SPREAD STILL HAVE DETAILS TO WORK OUT BUT SOMETHING TO WATCH CLOSELY. TEMPERATURES WILL SLOWLY BEGIN TO WARM ON TUESDAY AS SOUTHERLY FLOW BECOMES DOMINATE OUT AHEAD OF STRONGER SYSTEM TO THE EAST. EASTERLY FLOW WILL CONTINUE TO DOMINATE AT THE SURFACE WITH A EASTERLY LLJ STRENGTHEN ABOUT 20-30KTS. TUESDAYS HIGH TIDE AT BOSTON WILL BE 11.6 DURING THE LATE MORNING...AND WITH PERSISTENT EASTERLY WINDS ANTICIPATE AT LEAST MINOR COASTAL FLOODING. IF THIS SURFACE LOW STRENGTHENS AND THE EASTERLY FLOW INCREASES THEN THE MASS EAST COASTLINE WILL HAVE TO BE WATCHED CLOSELY DURING THE TUESDAY HIGH TIDE. 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 SHOW THAT THIS SYSTEM IS AROUND 2 TO 3 STANDARD DEVIATIONS ALLOWING FOR 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 MEXICO... PW ANOMALIES FORECAST TO RISE TO BETWEEN +1 TO +3 /OR HIGHER/...AND TEMPERATURES 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 SEE POSSIBLE URBAN FLOODING ISSUES AS WELL AS WIND ISSUES. GUIDANCE RIGHT NOW IS SHOWING A 45 TO 55 KT JET AT 925 MB WHICH MAY WARRANT WIND ADVISORIES BUT THIS SYSTEM COULD STRENGTHEN EVEN MORE SO A STRONGER JET IS POSSIBLE. LASTLY COASTAL FLOODING ISSUES WILL NEED TO BE ADDRESS AS HIGH TIDE ON WED AND THUS ARE 11.7 AND 11.5 RESPECTIVELY FOR BOSTON. ANY EASTERLY COMPONENT WILL PUSH THE TIDE ABOVE FLOOD SO COASTAL FLOODING IS POSSIBLE AS WELL AS BEACH EROSION. THE SOUTH COASTLINE WILL ALSO HAVE TO BE WATCH CLOSELY. A VERY STRONG SOUTHERLY JET WILL DEVELOP WED INTO THURSDAY AND WITH ASTRONOMICAL HIGH TIDE COULD ALSO SEE MINOR TO MODERATE COASTAL FLOODING. ANOTHER ISSUE IS THE POTENTIAL PRESSURE FALLS IF THIS SYSTEM STRENGTHENS EVEN MORE...IT COULD ADD ANOTHER FOOT ONTO THE SURGE. FRIDAY...LOW CONFIDENCE. ONCE UPPER 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. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TODAY THROUGH SATURDAY/... THROUGH 00Z...LOW VFR AND MVFR CIGS WILL PERSIST...BUT BECOMING VFR ALONG THE E MA COAST AS CLEARING MOVES IN FROM THE N. TONIGHT...MODERATE CONFIDENCE. EXPECT PARTIAL CLEARING AND BECOMING VFR DURING THE NIGHT AS LOW LEVEL DRY AIR MOVES SOUTH ACROSS THE REGION. HOWEVER...LOW CLOUDS AND MVFR CIGS OVER THE OUTER CAPE WILL GRADUALLY BACK IN ACROSS SE MA OVERNIGHT. SATURDAY...MODERATE CONFIDENCE. MVFR CIGS ACROSS SE NEW ENG WILL EXPAND WEST ACROSS THE REST OF THE COASTAL PLAIN THEN INTO THE INTERIOR DURING THE AFTERNOON. CT VALLEY SHOULD REMAIN VFR. SCATTERED RAIN SHOWERS WILL DEVELOP OVER CAPE COD WITH SNOW SHOWERS INTO E MA BY LATE AFTERNOON. SATURDAY NIGHT...MODERATE CONFIDENCE. WIDESPREAD MVFR CIGS WITH POCKETS OF IFR E MA COAST. PERIODS OF LIGHT RAIN OVER CAPE COD WITH LIGHT SNOW FURTHER INLAND ACROSS E MA. MINOR ACCUM POSSIBLE ACROSS E MA JUST INLAND FROM THE IMMEDIATE COAST. KBOS TERMINAL...MODERATE CONFIDENCE IN TAF. EXPECT VFR TONIGHT...BEFORE MVFR CIGS DEVELOP SAT MORNING. KBDL TERMINAL...MODERATE CONFIDENCE IN TAF. OUTLOOK...SUNDAY THROUGH TUESDAY... SUNDAY...HIGH CONFIDENCE. MAINLY VFR ACROSS CENTRAL AND WESTERN AREAS. E OF ORH MAY SEE MVFR CONDITIONS IN SNOW SHOWERS. MONDAY-TUESDAY...MODERATE CONFIDENCE. MAINLY VFR. MAY SEE LOCAL MVFR IN WIDELY SCT RAIN/SNOW SHOWERS MON NIGHT/TUE. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TODAY THROUGH SATURDAY/...HIGH CONFIDENCE. HIGH PRESSURE BUILDING EAST INTO QUEBEC AND NORTHERN NEW ENGLAND WILL KEEP WINDS/SEAS BELOW SCA THRESHOLDS THROUGH SATURDAY. NORTHWEST WINDS TODAY WILL TURN NORTHEAST BY SATURDAY WITH GUSTS BETWEEN 15 AND 20 KNOTS POSSIBLE AT TIMES. OUTLOOK...SATURDAY THROUGH TUESDAY...MODERATE TO HIGH CONFIDENCE. SATURDAY THROUGH MONDAY...WINDS AND SEAS BELOW SMALL CRAFT CRITERIA. MONDAY NIGHT AND TUESDAY...EXPECT E-NE WINDS GUSTING UP TO 25 KT WITH SEAS BUILDING UP TO 5-7 FT...HIGHEST ON THE EASTERN OUTER WATERS FROM COASTAL LOW. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...FRANK/DUNTEN NEAR TERM...KJC SHORT TERM...FRANK LONG TERM...DUNTEN AVIATION...KJC/DUNTEN MARINE...FRANK/DUNTEN

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