Area Forecast Discussion
Issued by NWS Boston, MA

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143 FXUS61 KBOX 220249 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 1049 PM EDT THU MAY 21 2015 .SYNOPSIS... FAST-MOVING LOW PRESSURE WILL BRING A PERIOD OF LIGHT RAIN TO RHODE ISLAND AND SOUTHEAST MASSACHUSETTS TONIGHT INTO FRIDAY MORNING. THEN A COLD FRONT MAY TRIGGER AN ISOLATED SHOWER LATE FRIDAY AFTERNOON INTO EARLY EVENING. BEHIND THIS FRONT...FROST AND MAYBE EVEN RECORD-BREAKING COLD...IS POSSIBLE FRIDAY NIGHT INTO SATURDAY MORNING. HIGH PRESSURE WILL BRING ABOUT DRY WEATHER THROUGH MOST OF MONDAY. WARMER BUT MORE UNSETTLED WEATHER IS EXPECTED NEXT WEEK. && .NEAR TERM /THROUGH FRIDAY/... 10 PM UPDATE... DESPITE RADAR RETURNS OVER BID/MVY AND ACK LOW LEVEL DRY AIR AS SEEN ON THE 00Z OKX SOUNDING CONTINUES TO EVAPORATE ANY RAINFALL BEFORE REACHING THE SURFACE ACROSS THIS AREA. EVENTUALLY BOUNDARY LAYER WILL SATURATE AND RAIN WILL REACH THE GROUND ALONG THE SOUTH COAST INCLUDING CAPE COD AND THE ISLANDS THE SECOND HALF OF THE NIGHT INTO EARLY FRI MORNING. AS FOR MODEL GUIDANCE...12Z ECMWF/18Z RUNS OF GFS AND NAM/21Z SREF AND 12Z NMM AND ARW ALL BRING NORTHWEST PORTION OF RAIN SHIELD INTO THE BOSTON-PROVIDENCE 06Z-12Z...ALBEIT VERY LIGHT RAIN WITH STEADIER RAIN CONFINED TO IMMEDIATE SOUTH COAST INCLUDING CAPE COD AND ISLANDS. AT 00Z RADAR AND SATELLITE TRENDS SUPPORT THIS SCENARIO. LATEST RUNS OF THE HRRR AND RAP ARE FARTHER SOUTHEAST. NONETHELESS CURRENT FORECAST CAPTURES THIS THEME WITH BULK OF RAIN ALONG THE SOUTH COAST WITH MUCH LIGHTER RAIN NORTHWARD INTO PROVIDENCE- BOSTON CORRIDOR. ALL OTHER FORECAST PARAMETERS ON TRACK AS WELL...THUS LITTLE IF ANY CHANGES WITH THIS FORECAST UPDATE. EARLIER DISCUSSION BELOW. ================================================================== TONIGHT... NOT MUCH CHANGE FROM PREVIOUS THINKING. NORTHERN STREAM SHORTWAVE ENERGY WILL ARRIVE LATER TONIGHT. THIS FEATURE WILL SPAWN A LATE SEASON COASTAL LOW WAVE OVER THE SOUTHEAST STATES...THAT WILL TRACK NEAR OR JUST SOUTHEAST OF THE 40N/70W BENCHMARK BY FRIDAY MORNING. 21/12Z GUIDANCE CONTINUES ITS EXCELLENT AGREEMENT ON STEADY RAINFALL WORKING INTO SOUTHEAST MA AND PORTIONS OF RI OVERNIGHT. THERE IS STILL SOME QUESTION HOW FAR WEST THE BACK EDGE OF THE PRECIPITATION SHIELD WILL MAKE IT. THE 09Z SREF IS THE FARTHEST WEST...WITH THE 12Z NAM THE FARTHEST EAST. STAYED WITH A MORE MODERATE APPROACH...AS EVIDENCED BY THE GFS AND ECMWF SOLUTIONS. SINCE THIS WILL BE A DEVELOPING COASTAL LOW...TIMING OF THE DYNAMIC COMPONENTS COMING TOGETHER WILL BE CRUCIAL. WILL NEED TO WATCH THIS ASPECT OF THIS STORM CLOSELY. REGARDING CONVECTIVE POTENTIAL...MODEL SOUNDINGS ARE NOT FAVORABLE FOR ANY SORT OF CONVECTION OVERNIGHT. && .SHORT TERM /FRIDAY NIGHT/... FRIDAY... A 3-PART DAY WITH MORNING RAINS LIKELY OVER RI AND SOUTHEAST MA...POSSIBLY NORTHWARD TO BOSTON. DRIER WEATHER...ALONG WITH MORE SUNSHINE...BRIEFLY DEVELOPS MID TO LATE MORNING AS COASTAL LOW PRESSURE MOVES FARTHER OFFSHORE. ANOTHER MID-LEVEL SHORTWAVE IS STILL EXPECTED TO MOVE THROUGH SOUTHERN NEW ENGLAND FRIDAY AFTERNOON. COMBINED WITH AN APPROACHING COLD FRONT...THERE SHOULD BE ENOUGH OF A TRIGGER TO GENERATE SCATTERED SHOWERS ACROSS SOUTHERN NEW ENGLAND...MAINLY ACROSS EASTERN MA. UNSEASONABLY STRONG COLD POOL ALOFT WILL ALSO MEAN FAIRLY STEEP LAPSE RATES. THE BIG QUESTION WHICH REMAINS TO BE ANSWERED WILL BE THE AMOUNT OF AVAILABLE MOISTURE. JUST ABOUT ALL OF THE GUIDANCE REMAINS VERY DRY ABOVE 10 KFT. THUS CANNOT COMPLETELY RULE OUT THE POSSIBILITY OF ISOLATED THUNDERSTORMS. SHOULD ANY THUNDERSTORMS DEVELOP...SMALL HAIL WOULD BE A RISK IN THE STRONGER CELLS. FRIDAY NIGHT... CONFIDENT WITH THE COLD FRONT PUSHING OFFSHORE THAT COLD AIR ADVECTION WILL BE IN FULL SWING INTO SATURDAY MORNING. PROBABLY LOOKING AT TEMPERATURES COLD ENOUGH TO WARRANT FROST ADVISORIES ACROSS PORTIONS OF WESTERN MA...AGAIN. AS PREVIOUSLY MENTIONED... THERE IS STILL AN ISSUE WITH WINDS. STILL EXPECTING A STEADY NORTHWEST WIND...WHICH WOULD BE ENOUGH TO KEEP THE SURFACE LAYER MIXED...AND PRECLUDE WIDESPREAD FROST DEVELOPMENT. WILL NEED TO KEEP AN EYE ON THIS POTENTIAL. WILL BE CLOSE TO RECORD LOWS FOR A FEW LOCATIONS /SEE THE CLIMATE SECTION BELOW/. && .LONG TERM /SATURDAY THROUGH THURSDAY/...
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*/ HIGHLIGHTS... - UNSEASONABLY COOL SATURDAY - FROST / FREEZE HEADLINES LIKELY FOR SATURDAY MORNING - SEASONABLE AND DRY FOR SUNDAY - BEGINNING MEMORIAL DAY...A WET-WEATHER PATTERN BEGINS TO EMERGE - COULD REMAIN SEASONABLE AND WET THROUGH THE REMAINDER OF THE WEEK */ DISCUSSION... CONFIDENT WITH THE COLD FRONT PUSHING OFFSHORE FRIDAY NIGHT THAT COLD AIR ADVECTION WILL BE IN FULL SWING INTO SATURDAY MORNING. BREEZY NW FLOW REARWARD OF THE PIVOTING VORTEX THRU N NEW ENGLAND WILL USHER AN ANOMALOUSLY COLD AIRMASS S ALOFT. EXPECT WIDESPREAD FROST/FREEZE HEADLINES ACROSS THE INTERIOR WITH THE BIGGEST IMPACT OVER N/W PORTIONS OF MA. BUT AGREE WITH PREVIOUS FORECASTER THINKING THAT THE BREEZY NW-FLOW COULD MITIGATE FROST DEVELOPMENT DESPITE COLD TEMPERATURES. WILL BE CLOSE TO RECORD LOWS /SEE THE CLIMATE SECTION BELOW/. COLD AIRMASS LINGERS AS HIGH PRESSURE BUILDS S OF NEW ENGLAND. COOL AND DRY FOR SATURDAY. MORE SEASONABLE SUNDAY WITH RETURN S-FLOW. BEGINNING MEMORIAL DAY AM LESS CONFIDENT WITH THE FORECAST. PACIFIC- ORIGIN IMPULSES ROTATING THROUGH THE W-HALF OF THE CONUS INVOKE DOWN- STREAM RIDGING INTO NEW ENGLAND. BERMUDA HIGH REAFFIRMS ROUND THE W- PERIPHERY WHICH SW-FLOW PREVAILS. THE WARM-FRONTAL LEADING EDGE OF WARM-MOIST AIR WILL BE A FOCUS FOR WET-WEATHER AS PACIFIC IMPULSES EJECT E THROUGH RIDGE PATTERN. COMBINATION OF ISENTROPIC ASCENT BENEATH DIFFLUENCE ALOFT ALLOWING FOR BROADSCALE LIFT...CAN NOT RULE OUT SHOWERS AROUND MONDAY - TUESDAY. FEEL BETTER CHANCES ARE N/W AWAY FROM THE ANTICYCLONIC FLOW / DRY AIR AND TOWARDS MORE FAVORABLE DYNAMICS. FOR THE REST OF THE WEEK ENSEMBLE AND DETERMINISTIC GUIDANCE SUGGEST A FLAT-FLOW REGIME ACROSS THE CONUS THROUGH WHICH PACIFIC-ORIGIN IMPULSES PREVAIL. CONSIDERING THE OFFSHORE BERMUDA HIGH AND A VORTEX PREVAILING ACROSS CANADA...MAY BE DEALING WITH A MORE ACTIVE PATTERN AS THE WARM-MOIST FLOW IS STRETCHED ACROSS THE NE CONUS UNDERGOING LIFT AND ENHANCEMENT AS MID-LEVEL IMPULSES TRANSLATE THROUGH THE REGION. UNCERTAINTY AS ANY WOBBLE IN THE FLOW PATTERN BETWEEN THE VORTEX N AND THE BERMUDA HIGH S WOULD MODIFY THE CONVERGENT FLOW. GFS SUGGESTS A +1 SD OF H925 TEMPERATURES FOR NEXT WEEK. IF +2 IT WOULD BE MORE LIKELY THAT THE CONVERGENT FLOW WOULD BE N AND W WITH S NEW ENGLAND WARMER AND DRY BENEATH THE RIDGE. WITH A CONSENSUS BLEND...WILL KEEP IT SEASONABLE WITH SLIGHT CHANCE TO CHANCE POPS. ECHOING THE POINT MADE BY THE PREVIOUS FORECASTER...COULD FIND OURSELVES WITHIN THE STEERING FLOW OF UPSTREAM MCS/S. CONTINUED SIGNALS OF A CUTOFF LOW WOBBLING ACROSS THE NW-ATLANTIC. WILL HAVE TO KEEP AN EYE ON THAT.
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&& .AVIATION /03Z FRIDAY THROUGH TUESDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH THURSDAY NIGHT/... 10 PM UPDATE...NO CHANGES FROM 00Z TAFS. EARLIER DISCUSSION BELOW. ================================================================== THROUGH TONIGHT...HIGH CONFIDENCE. MAINLY VFR. BEST CHANCE FOR MVFR AT ACK/MVY AND POSSIBLY PERIODS OF MVFR AT HYA/FMH. THIS WILL MAINLY BE IN AN AREA OF RAIN WHICH WILL MOVE IN OFF THE SOUTH COAST BETWEEN 01Z AND 06Z. HEAVIEST ALONG THE ISLANDS. FRIDAY...MORNING -RA EXITS SE MA. DRY LATE MORNING INTO EARLY AFTERNOON. THEN A COLD FRONT WILL LIKELY TRIGGER SCT -SHRA FROM LATE AFTERNOON INTO EARLY EVENING. LOW RISK OF SMALL HAIL IN MORE ROBUST SHRA WITH ISOLATED TSRA. OVERALL VFR. MODERATE CONFIDENCE. FRIDAY NIGHT...HIGH CONFIDENCE VFR. KBOS TERMINAL...HIGH CONFIDENCE IN TAF. KBDL TERMINAL...HIGH CONFIDENCE IN TAF. OUTLOOK...SATURDAY THROUGH TUESDAY... SAT...MODERATE CONFIDENCE. MAINLY VFR. BUT SOME PERIODS OF MVFR POSSIBLE IN OCCASIONAL SHOWERS/FOG. SUN AND MON...MODERATE CONFIDENCE. A BACKDOOR COLD FRONT WILL ALLOW WINDS TO SHIFT TO THE E AT MOST TERMINALS THROUGH THIS PERIOD. CONDITIONS START VFR...BUT SOME MVFR/IFR POSSIBLE SUN NIGHT INTO MON AS STRATUS MOVES OFF THE E COAST. TUE..MODERATE CONFIDENCE. MAINLY VFR...HOWEVER ONCE AGAIN A COLD FRONT MOVES THROUGH WITH SHOWERS AND ISOLATED THUNDERSHOWERS. THIS BRINGS ABOUT THE CHANCE FOR PERIODIC MVFR/IFR. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH FRIDAY NIGHT/... RAIN OVERSPREADS THE SOUTHERN WATERS OVERNIGHT AS LOW PRES TRACKS NEAR OR JUST SOUTH OF 40N/70W BENCHMARK. SOME UNCERTAINTY ON NORTHERN EXTENT OF RAIN SHIELD. STEADY RAINFALL GIVES WAY TO SCATTERED SHOWERS FRIDAY AFTERNOON AND EVENING AS A COLD FRONT APPROACHES FROM THE WEST. GUSTY NORTHWEST WINDS DEVELOP BEHIND THIS FRONT...WITH THE POTENTIAL FOR GUSTS UP TO 30 KT IN PLACES. SMALL CRAFT ADVISORIES MAY BE NEEDED SOMETIME FRIDAY NIGHT INTO SATURDAY...ESPECIALLY ACROSS THE OUTER COASTAL WATERS. OUTLOOK...SATURDAY THROUGH TUESDAY... SAT INTO SUN...HIGH CONFIDENCE. MAINLY QUIET BOATING WEATHER THROUGH THE PERIOD...ALTHOUGH WINDS MAY APPROACH 25 KT OUT OF THE S ON SAT NEAR CAPE COD AND THE ISLANDS. OTHERWISE...SHOWERS AND AN ISOLATED THUNDERSHOWER POSSIBLE. MON AND TUE...HIGH CONFIDENCE. E WINDS ON MON WILL GIVE WAY TO A RETURN TO S WINDS EARLY TUE WITH ANOTHER ROUND OF SHOWERS. LATE TUE WINDS SHIFT TO THE W-NW. CONDITIONS MAINLY BELOW SMALL CRAFT ADVISORY THRESHOLDS. && .CLIMATE... RECORD LOWS FOR MAY 23RD... BOSTON - 40 /1967/ HARTFORD - 37 /1976/ PROVIDENCE - 40 /1950/ WORCESTER - 32 /1929/ && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...BELK/DOODY NEAR TERM...NOCERA SHORT TERM...BELK LONG TERM...SIPPRELL AVIATION...BELK/SIPPRELL MARINE...BELK/SIPPRELL CLIMATE...

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