Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210552 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 152 AM EDT THU MAY 21 2015 .SYNOPSIS... CHILLY TEMPERATURES THIS MORNING WILL BE FOLLOWED BY A SEASONABLY MILD AFTERNOON. FAST MOVING LOW PRESSURE WILL LIKELY BRING A PERIOD OF RAIN TO RI AND SOUTHEAST MASSACHUSETTS TONIGHT INTO FRI MORNING. THEN A COLD FRONT MAY TRIGGER SCATTERED SHOWERS LATE FRI AFTERNOON INTO THE EARLY EVENING. BEHIND THE FRONT FROST AND MAYBE EVEN RECORD BREAKING COLD IS POSSIBLE FRI NIGHT INTO SATURDAY MORNING. DRY AND SEASONABLE BENEATH HIGH PRESSURE FOR THE WEEKEND. POSSIBLE WET- WEATHER EARLY NEXT WEEK IS NOT OUT OF THE QUESTION. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 2 AM UPDATE... DRY BUT CHILLY WEATHER DURING THE PREDAWN HOURS WITH TEMPS FALLING INTO THE 40S ACROSS MOST OF THE REGION. WHERE WINDS GO CALM INLAND TEMPS WILL BRIEFLY DIP INTO THE MU30S. FROST ADVISORY CONTINUES FOR NORTHWEST MA WHERE FROST WILL BE MOST WIDESPREAD. WINDS WILL REMAIN LIGHT TO CALM ALONG WITH SOME HIGH THIN CIRRUS CLOUDS FROM TIME TO TIME. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... THURSDAY... PARTLY TO MOSTLY SUNNY SKIES DESPITE SOME MID/HIGH LEVEL CLOUDINESS. SHOULD SEE HIGH TEMPS RECOVER TO THE UPPER 60S TO AROUND 70 IN THE AFTERNOON...WITH A FEW LOCATIONS POSSIBLY REACHING INTO THE LOWER 70S. IT WILL FEEL MORE COMFORTABLE THAN TODAY GIVEN LESS WIND. THURSDAY NIGHT... NO MAJOR ADJUSTMENTS WITH THIS FORECAST UPDATE. GUIDANCE CONTINUES TO STRUGGLE WITH TEMPERATURES. AT ISSUE ALSO ARE THE MINS TONIGHT AND THE FROST POTENTIAL. ALTHOUGH TEMPS ON GUIDANCE ARE RUNNING HIGH...SOME CLOUDS MAY LIMIT DIURNAL COOLING TONIGHT. NOT ENOUGH CONFIDENCE IN THIS UPDATE TO MAKE ANY CHANGES TO THE FROST HEADLINES AS SOME SHELTERED VALLEYS MAY STILL RADIATE EVEN WITH SOME CI. WEAK WAVE OF LOW PRESSURE TRACKS SOUTHEAST OF THE BENCHMARK THURSDAY NIGHT. ITS AN OPEN WAVE AND WILL BE MOVING FAIRLY QUICKLY...SO WILL NOT HAVE MUCH ATLANTIC INFLOW AND THE NORTHWEST EXTENT OF THE PRECIPITATION SHIELD WILL BE LIMITED. THERE IS UNCERTAINTY IN REGARDS TO HOW FAR NORTHWEST THE RAIN SHOWERS MAKE IT...SO LATER SHIFTS MAY CERTAINLY NEED TO MAKE ADJUSTMENTS. CURRENTLY IT LOOKS LIKE MOST OF THE MEASURABLE RAINFALL WILL BE CONFINED TO SOUTHEAST NEW ENGLAND...BUT CAN/T COMPLETELY RULE OUT THAT IT GETS AS FAR NORTH AS THE MA TURNPIKE. HIGHEST RISK FOR RAINFALL IS ACROSS THE CAPE/NANTUCKET...BUT PROBABLY LESS THAN ONE HALF INCH. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ HIGHLIGHTS... - CHANCES FOR WET WEATHER INTO FRIDAY - TURNING COLD INTO SATURDAY MORNING - HIGH PRESSURE AND MODERATE TEMPERATURES OVER THE WEEKEND - POTENTIAL FOR WET WEATHER INTO EARLY NEXT WEEK OVERVIEW AND MODEL PREFERENCES... THERE IS STILL MODERATE AGREEMENT THROUGH THE WEEKEND IN THE SYNOPTIC PATTERN. HOWEVER A RECENT ADJUSTMENT IN THE LONGWAVE TROF AS A ROBUST VORTEX MOVES INTO IT/S BASE FOR FRI HAS LED TO A CHANGE IN THINKING FROM YESTERDAY...REGARDING SENSIBLE WX. THIS VORTEX WILL LEAD TO A TILT-SHIFT IN THE TROF FROM POSITIVE TO NEGATIVE...WHICH SHIFTS THE MAIN LEFT EXIT REGION OF THE UPPER JET IN BETTER POSITION TO SUPPORT LIFT IN THE NRN DEFORMATION BANDING OF THE COASTAL LOW PRES MOVING OFF THE MID ATLANTIC. THE APPROACHING COLD FRONT COULD STILL INITIATE SOME -SHRA ACTIVITY TO THE N...BUT IT APPEARS THAT MAINLY THE S COAST/CAPE/ISLANDS WILL HAVE THE BEST CHANCE AT SEEING SOME WETTING PRECIPITATION DUE TO THIS SHIFT. OTHERWISE...THE BREAKDOWN OF THE ROCKY MOUNTAIN RIDGE WILL LEAD TO SOME RIDGING ACROSS NEW ENGLAND FOR THE WEEKEND INTO EARLY NEXT WEEK. HOWEVER DISCREPANCIES IN THE ULTIMATE BREAKDOWN OF THIS RIDGE REMAINS A BIT OF QUESTION. ESPECIALLY THE APPROACH OF A WARM FRONT BY TUE. GIVEN THE TIMING DIFFERENCES ARE MOST EVIDENT BETWEEN THE OPERATIONAL GFS AND ECMWF...A BLEND OF THE ENSEMBLE GUIDANCE WILL YIELD A GOOD MIDDLE GROUND. DETAILS... FRI... TWO AREAS OF LIFT FOR PRECIP POTENTIAL. THE FIRST IS WITHIN THE NE AND NW QUAD OF LOW PRES MOVING S OF THE ISLANDS. THERE IS ENOUGH LIFT WITH AN UPPER LVL JET EXIT REGION AND TILTING TROF TO YIELD SOME RAINFALL MAINLY ALONG THE S COAST OF RI AND SE MA...UNTIL THE SYSTEM MOVES OUT TO SEA BY MID DAY. THE OTHER IS A STRONG COLD FRONT CROSSING THE REGION FROM THE NW. THE BETTER PWATS /NEAR AN INCH/ ARE ASSOCIATED WITH THE LOW PRES TO THE S...WHILE FURTHER N...THE TO P OUT AROUND 0.7 INCHES. THEREFORE...THIS FRONTAL PASSAGE...ALTHOUGH WITH DECENT F-GEN SUPPORT MAY BE ENTERING TOO DRY AN AIRMASS TO YIELD WIDESPREAD -SHRA. FEEL THE SLIGHT CHANCE POPS ARE STILL BEST GIVEN SEVERAL LOCATIONS ARE LIKELY TO REMAIN DRY. SAT THROUGH MON... HIGH PRES WILL REGAIN CONTROL BUT NOT WITHOUT SOME CONSEQUENCES INITIALLY. COLD ADVECTION BEHIND THE STRONG FRONT WILL ALLOW H85 TEMPS TO DROP TO -2C OR -4C...ALMOST 3 STD DEVIATIONS BELOW NORMAL. MODEL 2M TEMP ANOMALIES FOR FRI NIGHT AND SAT ARE ON THE ORDER OF -10F. THEREFORE...COOL WX WILL PREVAIL PARTICULARLY FRI NIGHT INTO SAT. MAY NEED TO WATCH FOR FROST...AS 2M TEMPS DIP WELL INTO THE MID 30S. THE ONE ISSUE REGARDING FROST PRODUCTION IS THE POSSIBILITY DECENT ISALLOBARIC COUPLET...WHICH COULD YIELD ENOUGH OF A NOCTURNAL BREEZE TO MITIGATE FROST DEVELOPMENT IN SPITE OF THE COLD TEMPERATURES...SHELTERED VALLEYS ARE MOST AT RISK. GRADUAL MODERATION IS EXPECTED SUN INTO MON...ALTHOUGH WITH A WEAKER PRES GRADIENT SUN NIGHT...ANOTHER /ALTHOUGH LOWER/ RISK FOR FROST WILL HAVE TO BE WATCHED. TEMPS NEAR TO SLIGHTLY ABOVE SEASONAL VALUES BY MON. TUE AND WED... ALTHOUGH THERE CONTINUE TO BE SOME ISSUES HERE REGARDING THE TIMING OF A WARM FRONTAL PASSAGE THROUGH THIS PERIOD. THE TRANSITION TO A MORE UNSETTLED PATTERN IS EXPECTED. INCREASING MOISTURE AND OVERALL WARM ADVECTION SUGGESTS MORE CLOUDS/SHRA CHANCES THROUGHOUT. GIVEN THIS IS A WARM FRONTAL APPROACH...IT/S FINAL LOCATION WILL MATTER SIGNIFICANTLY IN REGARDS TO TEMPS AND PRECIP CHANCES. ALSO...THIS LOCATION PROVIDES STEERING FLOW FOR UPSTREAM MCS. SOMETHING TO CONSIDER. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH THURSDAY NIGHT/... TODAY...HIGH CONFIDENCE FOR VFR AND DRY CONDITIONS. WNW WINDS BECOMING SSW THIS AFTERNOON AT FAIRLY LIGHT SPEEDS. TONIGHT...RAIN OVERSPREADS MUCH OF RI AND SOUTHEAST MA. STILL SOME UNCERTAINTY REGARDING NORTHWARD EXTENT OF RAIN SHIELD. COULD GET AS FAR NORTH AS IJD-SFZ-BOS. EXPECTED LOW VFR TO MVFR OVER RI AND SOUTHEAST MA WITH VFR ELSEWHERE. A PERIOD OF MODERATE TO PERHAPS BRIEFLY HIGH IS POSSIBLE ACROSS RI AND SOUTHEAST MA INCLUDING CAPE COD AND THE ISLANDS. FORECAST CONFIDENCE IS MODERATE GIVEN UNCERTAINTY IN NORTHERN EDGE OF RAIN SHIELD. FRIDAY...MORNING RAIN EXITS SOUTHEAST MA WITH A PERIOD OF DRY WEATHER FROM LATE MORNING INTO EARLY AFTERNOON. THEN A COLD FRONT WILL LIKELY TRIGGER SCATTERED SHOWERS FROM LATE AFTERNOON INTO EARLY EVENING. LOW RISK OF SMALL HAIL IN MORE ROBUST SHOWERS WITH ISOLATED THUNDER. OVERALL VFR CONDITIONS WILL PREVAIL. MODERATE TO HIGH CONFIDENCE. KBOS TERMINAL...HIGH CONFIDENCE IN TAF. LOW RISK FOR A BRIEF SEA BREEZE FROM ABOUT 14Z TO 17Z. KBDL TERMINAL...HIGH CONFIDENCE IN TAF. OUTLOOK...FRIDAY NIGHT THROUGH MONDAY... FRI NIGHT THROUGH MON...HIGH CONFIDENCE. VFR. GUSTY NW WINDS SAT AROUND 25 KT. SEA BREEZES POSSIBLE SUN. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... TONIGHT...HIGH CONFIDENCE. NEAR SHORE WNW WIND GUSTS OF 25 TO 30 KNOTS ACROSS BOSTON HARBOR/NARR BAY AND PORTIONS OF IMMEDIATE EASTERN MA COAST DIMINISH THIS EVENING. THESE LOCALIZED NEAR SHORE SMALL CRAFT HEADLINES WILL BE ALLOWED TO EXPIRE BY 7 PM. OTHERWISE...WINDS/SEAS BELOW SMALL CRAFT THRESHOLDS TONIGHT. THURSDAY AND THURSDAY NIGHT...HIGH CONFIDENCE. WEAK PRESSURE GRADIENT WILL KEEP WINDS/SEAS BELOW SMALL CRAFT ADVISORY THRESHOLDS. OUTLOOK...FRIDAY THROUGH MONDAY... FRI THROUGH SAT NIGHT...MODERATE CONFIDENCE. WITH A COLD FRONT CROSSING THE WATERS ON FRI EVENING. EXPECT WINDS TO SHIFT TO THE W-NW AND BEGIN GUSTING TO AROUND 25 KT. THIS WILL ALSO ALLOW FOR BUILDING SEAS...POSSIBLY REACHING 5-6 FT ON THE OCEAN WATERS ESPECIALLY. THESE CONDITIONS LINGER INTO THE DAY SAT...WITH WINDS DIMINISHING SAT EVENING AND SEAS DIMINISHING SAT NIGHT. SMALL CRAFT ADVISORIES LOOK LIKE THEY WILL BE NEEDED THROUGH THIS PERIOD. SUN AND MON...HIGH CONFIDENCE. MAINLY QUIET BOATING WEATHER THANKS TO HIGH PRES BUILDING OVER THE WATERS. && .FIRE WEATHER... THE NEXT CHANCE FOR MARGINAL FIRE WEATHER CONDITIONS APPEARS TO BE ON FRI. ALTHOUGH SOME PRECIPITATION IS POSSIBLE ESPECIALLY NEAR THE SOUTH COAST OF RI AND SOUTHEAST MA...ALONG WITH A SPOT SHOWER EVERYWHERE ELSE. MIN RH VALUES ARE EXPECTED TO DIP TO BETWEEN 20 AND 30 PERCENT. WINDS WILL SHIFT FROM W TO NW WITH GUSTS APPROACHING 25 MPH IN THE AFTERNOON. FIRE WEATHER CONDITIONS INCREASE ON SAT. THANKS TO LOW DEWPOINTS...RH VALUES FALL BACK INTO THE TEENS...WITH W-NW WIND GUSTS 25 TO 30 MPH AT TIMES. TEMPS NOT AS WARM...MAINLY IN THE 60S. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...FROST ADVISORY UNTIL 8 AM EDT THIS MORNING FOR MAZ002-003- 008>011. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...FRANK/DOODY NEAR TERM...NOCERA SHORT TERM...FRANK LONG TERM...DOODY AVIATION...NOCERA/DOODY MARINE...FRANK/DOODY FIRE WEATHER...

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