Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 140141 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 941 PM EDT SAT SEP 13 2014 .SYNOPSIS... HIGH PRESSURE OVER THE GREAT LAKES WILL BUILD INTO NEW ENGLAND TONIGHT BRINGING DRIER AIR TO THE REGION. THE HIGH SETTLES OVER THE NORTHEAST SUNDAY AND MONDAY PROVIDING SUNNY SKIES AND CLEAR NIGHTS WITH SLIGHTLY COOLER THAN NORMAL TEMPERATURES. A COLD FRONT WILL LIKELY BRING A PERIOD OF SHOWERS LATE MONDAY NIGHT INTO TUESDAY. MAINLY DRY WEATHER EXPECTED NEXT WEDNESDAY THROUGH FRIDAY WITH SLIGHTLY BELOW NORMAL TEMPERATURES CONTINUING. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 940 PM UPDATE... AT 930 PM BACK EDGE OF RAIN SHIELD WAS ACROSS CAPE COD AND THE ISLANDS AND MOVING STEADILY EAST-NORTHEAST. THE BACK EDGE OF THE CLOUD SHIELD WAS ACROSS THE HUDSON VALLEY AND THE NYC AREA...ALSO MOVING STEADILY EASTWARD. SURFACE OBS...MESO AND STREAMLINE ANALYSIS INDICATE WIND SHIFT/FRONTAL PASSAGE LOCATED ACROSS THE CT RVR VLY WITH MODEST PRES RISES AND ASSOCIATED SUBSIDENCE OVERSPREADING EASTERN NY AND ADVECTING INTO WESTERN NEW ENGLAND. THIS WILL RESULT IN BACK EDGE OF THE CLOUDS/CLEARING LINE CROSSING THE REGION BETWEEN 11 PM AND 2 AM FROM WEST TO EAST. WHATS TRICKY IS HOW MUCH IF ANY FOG DEVELOPS IN THE POST FRONTAL AIRMASS GIVEN RESIDUAL LOW LEVEL MOISTURE FROM TODAY/S RAINFALL. KEY WILL BE IF INTERIOR VALLEYS DECOUPLE. PRES RISES AND ASSOCIATED SUBSIDENCE ALONG WITH DEW PTS FALLING INTO THE 40S MAY BE SUFFICIENT TO PRECLUDE DECOUPLING AND FOG FROM DEVELOPING. IN FACT TOWARD MORNING PGRAD AND CAA MAY GENERATE NNW WINDS GUSTING UP TO 20-25 MPH ESPECIALLY IN THE COASTAL PLAIN AND HIGHER TERRAIN. PREVIOUS FORECAST HANDLES THESE DETAILS NICELY SO NO MAJOR CHANGES WITH THIS FORECAST UPDATE. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... SUNDAY...NORTHWEST WINDS MAY GUST TO 20 TO 25 MPH IN RI AND EASTERN MA EARLY IN THE MORNING FOR A FEW HOURS DUE TO THE GRADIENT BEHIND DEPARTING LOW PRESSURE AND ADVANCING STRONG HIGH PRESSURE...THEN WINDS WILL DROP OFF. OTHERWISE...A BRIGHT SUNNY DAY IS IN STORE WITH HIGH TEMPERATURES 65 TO 70 EXCEPT 60 TO 65 IN SOUTHERN NH. SUNDAY NIGHT...CRYSTAL CLEAR SKIES WILL ALLOW FOR GOOD RADIATIONAL COOLING. OVERNIGHT LOWS WILL MAINLY BE IN THE 40S BUT COULD DIP INTO THE UPPER 30S IN SOUTHWEST NH. THERE IS A VERY LOW PROBABILITY THAT SOME FROST COULD FORM IN SOUTHWEST NH. && .LONG TERM /MONDAY THROUGH SATURDAY/... HIGHLIGHTS... *SHOWERS POSSIBLE FROM COLD FRONTAL PASSAGE MON NIGHT/TUE *BELOW NORMAL TEMPS OBSERVED AFTER LATE WEEK FRONTAL PASSAGE *LESS HUMIDITY AS DEW POINTS ARE BELOW 60F FOR THE ENTIRE PERIOD OVERVIEW... THIS PERIOD IS CHARACTERIZED BY A RIDGE IN THE WEST AND A TROUGH IN THE EAST OR THE TYPICAL POSITIVE PNA PATTERN AT LEAST TO START. WITH THE UPPER LEVEL LOW PRESSURE SYSTEM DROPPING SOUTH OUT OF CANADA...EXPECT THE ARCTIC OSCILLATION TO DROP MORE NEGATIVE. ALSO EXPECTING THE NAO TO DROP FROM POSITIVE TO MORE NEUTRAL INDICATING A SLOWING PATTERN. MODELS HANDLE THIS PERIOD FAIRLY WELL AND WITH ADEQUATE AGREEMENT ASIDE FROM THE HANDLING OF A SHORTWAVE THAT MOVES THROUGH THE REGION. THE 6Z GFS IS FASTER AND WEAKER WITH THE MOVEMENT OF THAT SHORTWAVE THROUGH THE REGION THAN THE ECMWF. THE LATTER HAS A STRONGER SHORTWAVE...WHICH FORMS A LOW PRESSURE SYSTEM SOUTH OF THE AREA. HAVE GONE WITH A BLEND OF THE MODELS TO ELIMINATE THE GFS BIAS TOWARDS QUICKER EVOLUTION THROUGH THE REGION AND THE HEAVIER PRECIPITATION FROM THE ECMWF. LATER IN THE PERIOD...A COLD FRONT PUSHES SOUTH THROUGH NEW ENGLAND WITH MORE MODEL DISAGREEMENT. THE ECMWF ALLOWS FOR A LATER PASSAGE OF THE FRONT LATE THURSDAY NIGHT WHILE THE GFS PUSHES IT THROUGH LATE WEDNESDAY/EARLY THURSDAY. FOLLOWING THE ENSEMBLES AND WPC...HAVE GONE WITH AN EARLY THURSDAY FRONTAL PASSAGE. DAILIES... MONDAY...A HIGH PRESSURE SYSTEM MOVES FROM WEST TO EAST CHANGING WINDS TO THE SOUTHWEST AHEAD OF A SHORTWAVE FROM CANADA. WITH MIXING UP TO AROUND 850 MB THAT WOULD GIVE HIGH TEMPS IN THE MID TO UPPER 60S TUESDAY...EVEN WITH THE SOUTHWEST WARMING WINDS...THE HUMIDITY REMAINS LOW WITH PWATS ONLY AROUND 1 INCH AND THE TOTAL OF TOTALS ONLY AROUND 40. FEEL A LOWER END EVENT MAY OCCUR FROM THIS WITH THE QUICK MOVING LOW PRESSURE SYSTEM. HAVE LEANED AWAY FROM THE ECMWF...BUT FEEL THE GFS MAY BE TOO LOW. ALSO DON/T SEE MUCH IN THE WAY OF INSTABILITY FOR THUNDER. WEDNESDAY THROUGH FRIDAY...AREAS OF HIGH PRESSURE PINWHEEL AROUND THE UPPER LEVEL LOW IN SOUTHEASTERN CANADA. A COLD FRONT LOOKS TO PASS THROUGH EARLY THURSDAY WITH TEMPERATURES AS COLD AS JUST OVER 2 SDS BELOW NORMAL AT 850MB OR 1.5 KM INTO THE SKY. AM EXPECTING A FEW CUMULUS CLOUDS TO BE AROUND...BUT NOT OVERCAST AT ALL. BEST CHANCE TO SEE THURSDAY OVERNIGHT TEMPS DIP BELOW 40F AND WOULD THEREFORE HAVE TO WATCH FOR FROST IN THOSE AREAS...NAMELY NORTHWESTERN MA AND SOUTHEASTERN NH. SATURDAY...THE 6Z GFS IS SHOWING A COLD FRONT...BUT WITH THE NAO BEGINNING TO GO NEGATIVE AND INDICATING A SLOWING PATTERN...WILL STICK WITH WHAT THE ECMWF AND OTHER ENSEMBLE MODELS ARE SHOWING AND ALLOW HIGH PRESSURE TO SLOWLY EXIT THE REGION. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. CONFIDENCE IS LOW FOR TONIGHT...THEN HIGH FOR SUNDAY AND SUNDAY NIGHT. 930 PM UPDATE... MVFR/IFR CONDITIONS ACROSS NORTHERN MA AND SOUTHWEST NH WILL LIFT TO VFR 03Z TO 06Z FROM WEST TO EAST. RAIN EXITS CAPE COD AND THE ISLANDS BY 0230Z. WIND SHIFT TO THE NNW OVERNIGHT WITH GUSTS UP TO 25 KT TOWARD MORNING ALONG THE COAST AND HIGHER TERRAIN INLAND. EARLIER DISCUSSION BELOW. ==================================================================== CIGS/VSBYS FORECAST IS OF LOW CONFIDENCE TONIGHT. A WIDE VARIETY OF CONDITIONS RANGING FROM IFR TO VFR WERE OCCURRING EARLY THIS EVENING. MVFR PROBABLY DOMINATE FOR THE FIRST PART OF THE NIGHT...WITH SOME LOCATIONS ALONG THE COASTAL PLAIN IMPROVING TO VFR. HOWEVER...PARTIAL CLEARING WILL RESULT IN SOME MVFR-IFR VSBYS TO DEVELOP ACROSS THE INTERIOR. WINDS WILL NOT DECOUPLE IN ALL LOCATIONS SO UNSURE HOW WIDESPREAD ANY FOG WILL BE. SUNDAY AND SUNDAY NIGHT...VFR WITH CLEAR SKIES AND EXCELLENT VISIBILITY AFTER ANY EARLY MORNING LOW CLOUDS/FOG PATCHES. KBOS TERMINAL...MODERATE CONFIDENCE. MVFR CONDITIONS WILL LINGER INTO THE EVENING BUT SHOULD IMPROVE TO VFR. RAIN ENDS BY AROUND 1Z. KBDL TERMINAL...LOW CONFIDENCE. IFR CONDITIONS SHOULD IMPROVE AT LEAST TEMPORARILY LATER THIS EVENING. HOWEVER...WE MAY SEE SOME FOG DEVELOP AFTER MIDNIGHT BUT THAT IS UNCERTAIN BECAUSE UNSURE IF THE TERMINAL WILL BE ABLE TO COMPLETELY DECOUPLE. OUTLOOK...MONDAY THROUGH THURSDAY...HIGH CONFIDENCE. MONDAY...VFR CONDITIONS EXPECTED. TUESDAY...SHOWERS BEGIN IN THE EARLY MORNING HOURS WITH MVFR CONDITIONS EXPECTED. THEN BECOMING VFR AS SHOWERS EXIT IN THE MID AFTERNOON HOURS. WEDNESDAY AND THURSDAY...VFR CONDITIONS EXPECTED. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. FORECAST CONFIDENCE IS MODERATE TONIGHT THROUGH SUNDAY NIGHT. TONIGHT...E WINDS 15 TO 20 KT WITH SOME HIGHER GUSTS ALONG THE COAST WILL SHIFT TO THE N AND NW WELL AFTER MIDNIGHT AS LOW PRESSURE SOUTH OF LONG ISLAND MOVES NE AND PASSES TO THE E OF NANTUCKET. A BAND OF LIGHT TO OCCASIONALLY MODERATE RAIN WILL PASS OVER THE WATERS AND MAY RESTRICT VISIBILITIES MAINLY TO 3 TO 5 MILES. SUNDAY...THE GRADIENT BETWEEN THE DEPARTING LOW PRESSURE AND ADVANCING STRONG HIGH PRESSURE MAY CAUSE A BRIEF PERIOD OF NW WIND GUSTS TO 25 KT WITHIN A FEW HOURS EITHER SIDE OF DAYBREAK SUNDAY. AS A RESULT A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR THE EASTERN MA COASTAL WATERS. THE WINDS SHOULD DIMINISH BY LATE MORNING...BUT SEAS COULD LINGER NEAR 5 FT OVER THE OUTER WATERS EAST OF MA...SO THE SMALL CRAFT ADVISORY EXTENDS INTO THE AFTERNOON FOR THOSE AREAS. SUNDAY NIGHT...WINDS AND SEAS BELOW SMALL CRAFT ADVISORY CRITERIA. OUTLOOK...MONDAY THROUGH THURSDAY... MONDAY INTO TUE MORNING...MODERATE TO HIGH CONFIDENCE. HIGH PRESSURE NEARBY WILL RESULT IN A WEAK PRESSURE GRADIENT KEEPING WINDS/SEAS BELOW SCA THRESHOLDS. TUESDAY AFTERNOON INTO WEDNESDAY...MODERATE CONFIDENCE. SWELLS BEGIN TO BUILD TO BETWEEN 5 AND 7FT BEHIND THE DEPARTING LOW PRESSURE SYSTEM AND COLD FRONT. THIS IS ABOVE SCA LEVELS AND THEREFORE A SCA MAY BE NEED. THURSDAY...AS NEXT HIGH PRESSURE SYSTEM APPROACHES WITH WEAKER PRESSURE GRADIENT...EXPECT SWELLS TO DIE DOWN DURING THE DAY TO BELOW SCA THRESHOLD. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. NH...NONE. RI...NONE. MARINE...SMALL CRAFT ADVISORY FROM 6 AM TO 11 AM EDT SUNDAY FOR ANZ231- 232-251-255. SMALL CRAFT ADVISORY FROM 6 AM TO 4 PM EDT SUNDAY FOR ANZ250- 254. && $$ SYNOPSIS...GAF/ROLLER NEAR TERM...FRANK/NOCERA/GAF SHORT TERM...GAF LONG TERM...GAF/ROLLER AVIATION...NOCERA/GAF/ROLLER MARINE...GAF/ROLLER

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