Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212323 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 723 PM EDT THU MAY 21 2015 .SYNOPSIS... FAST-MOVING LOW PRESSURE WILL BRING A PERIOD OF RAIN TO RHODE ISLAND AND SOUTHEAST MASSACHUSETTS TONIGHT INTO FRIDAY MORNING. THEN A COLD FRONT MAY TRIGGER SCATTERED SHOWERS 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/... 7PM UPDATE... ADJUSTED RAIN SHIELD A BIT BASED ON CURRENT WSR-88D OUTPUT AND OBS ALONG LONG ISLAND. NOTED THAT HRRR/SREF SEEMED TO BE DOING THE BEST WITH TIMING/LOCATION SO POPS WERE ADJUSTED TOWARD THIS THINKING. THEREFORE...FEEL THAT THE IJD-BOS LINE AND POINTS SE STILL HAVE THE BEST CHANCE OF SEEING ANY WETTING RAIN BY THE AM. HOWEVER AS THE UPPER TROF MOVES OVER COULD SEE SOME SLIGHT BLOSSOMING OF THE SHIELD A BIT FURTHER W. MOST OTHER FORECAST PARAMETERS ARE ON TRACK THIS EVENING. 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/... HIGHLIGHTS... * RAIN SAT WITH WARM FRONT * BACK DOOR COLD FRONT WILL BRING COOLER TEMPERATURES SUN AND MON * COLD FRONT MAY BRING ANOTHER ROUND OF SHOWERS TOWARDS MIDWEEK OVERVIEW AND MODEL PREFERENCES... 14.12Z MODEL GUIDANCE IS IN RATHER GOOD AGREEMENT IN SPITE OF THE FACT THERE IS A TREND FROM A VERY AMPLIFIED WRN CONUS TO A RATHER AMPLIFIED WRN ATLANTIC THROUGH THE LONG TERM. IN ESSENCE A RIDGE WILL BE BUILDING ACROSS THE AREA INTO THE WEEKEND. THIS RIDGE THEN BECOMES QUITE AMPLIFIED BUT ALSO RATHER ACUTE AS AN UPPER LVL SHORTWAVE PHASES WITH CUTOFF LOW PRES WELL S OF NOVA SCOTIA. THIS CREATES A BLOCKED FLOW REGIME WITH THE RIDGE REMAINING THE DOMINANT FEATURE FOR NEW ENGLAND UNTIL A COLDER CANADIAN WAVE FLATTENS THE RIDGE TOWARD THE MIDDLE PERIOD OF NEXT WEEK. GIVEN THERE IS FAIR AGREEMENT HERE...FEEL THAT A BLEND OF LONG TERM OPERATIONAL GUIDANCE WILL WORK AS A GOOD BASELINE FOR THIS FORECAST. DETAILS... SAT AND SAT NIGHT...THIS PERIOD IS MARKED BY THE SLOW PASSAGE OF A SFC WARM FRONT FROM THE SW. AS THE WAVE ASSOCIATED WITH IT PHASES WITH THE CUTOFF TO THE E...EXPECT THE FRONT TO SLOW SOMEWHERE INVOF SRN NEW ENGLAND TO JUST OFFSHORE IN THE GULF OF MAINE. TYPICAL SETUP HERE...WITH ISENTROPIC LIFT ON THE LEADING EDGE OF THIS FRONT ALONG WITH INCREASED MOISTURE CONVERGENCE YIELDING PWATS NEAR 1.5 INCHES OR NEARLY 2 STD DEVIATIONS ABOVE MAY NORMALS. ABOVE THIS WARM FRONT...REASONABLY STEEP LAPSE RATES OF ABOUT 6.0+ C/KM ALSO YIELD NEARLY 500-1000 J/KG ELEVATED CAPE PER LATEST BUFKIT PROFILES. THEREFORE...DECENT SETUP FOR WARM FRONTAL INSTABILITY SAT INTO SAT EVENING. MODEST SHEAR PROFILES AS WELL...TYPICAL IN THESE WARM FRONTAL SETUPS. THEREFORE...WILL CONTINUE TO INCLUDE THREAT FOR T- STORMS ALONG WITH SHOWERS UNTIL DRY HIGH PRES ASSOCIATED WITH THE RIDGE FORCES THE F-GEN AXES OFFSHORE. ONE STICKING POINT WOULD BE THE POSSIBILITY OF A REMNANT MCS MOVING THROUGH IN THE MORNING...WHICH COULD PUT A DAMPER ON AFTERNOON INSTABILITY IF THINGS DOWN SCOUR OUT QUICKLY ENOUGH. SOMETHING TO MONITOR. TEMPS WILL BE DEPENDENT ON SUNSHINE...BUT WITH ANY BREAKS COULD SEE HIGHS APPROACHING THE 80S. SUN INTO MON...JUST IN CASE YOU FORGOT THAT WE LIVE IN NEW ENGLAND AND IT/S STILL SPRING...WE WILL GET A STARK REMINDER IN THE FORM OF YET ANOTHER BACKDOOR COLD FRONT WHICH WILL BEGIN TO SLIDE OUT OF THE GULF OF MAINE SUN. THERE ARE STILL A FEW QUESTIONS AS TO EXACTLY HOW FAR TO THE SW THIS FRONT ENDS UP MAKING IT...BUT ITS LIKELY AT LEAST MOST OF ERN MA/RI AND PORTIONS OF E CT WILL ONCE AGAIN SEE FALLING TEMPS AND SOME MARINE STRATUS. DRY AIR ALOFT DOES SUGGEST THE POSSIBILITY OF DRIZZLE...BUT THERE IS SOME QUESTION OF HOW MUCH LOW LVL MOISTURE WILL REMAIN GIVEN THE DRY PUNCH THAT COMES IN AHEAD OF THE BACKDOOR FRONT. MODELS LOOK A LITTLE TOO WARM...BUT THERE WILL LIKELY AT LEAST BE SOME AREAS ON SUN THAT MAKE A RUN INTO THE 80S AHEAD OF THE FRONT. BUT WILL LIKELY BE ADJUSTING THE REMAINING TEMPS FOR THE LATE AFTERNOON SUN AND MON HIGHS DOWN. TUE...ROBUST THETA-E RIDGE COINCIDENT WITH PWATS 3 STD DEVIATIONS ABOVE NORMAL WILL SLIDE OVER NEW ENGLAND IN ADVANCE OF A FAIRLY STRONG COLD FRONT. THE BIG QUESTION FOR INSTABILITY WILL BE HOW MUCH WARM AIR WILL BE ALLOWED BACK INTO THE LOW LVLS AS THE BACKDOOR COLD FRONT WASHES OUT. THERE IS REASONABLY GOOD AGREEMENT THAT THE TIMING OF THIS FRONT IS PERFECT...RIGHT DURING PEAK HEATING. AVAILABLE MODEL SOUNDINGS ARE SHOWING THIS UNCERTAINTY AS WELL...KEEPING A MODEST INVERSION IN PLACE ACROSS MUCH OF THE REGION WHICH IS KEEPING INSTABILITY PARAMETERS LOW. THERE IS SHEAR THOUGH...SO WILL JUST HAVE TO KEEP AN EYE ON THIS FEATURE AS WE GET CLOSER. IN ANY CASE...THE ROBUST MOISTURE AND LIFT SHOULD YIELD SOME FAIRLY GOOD...ALBEIT BRIEF RAINFALL. WED...THIS COLD FRONT WILL USHER IN HIGH PRES AND RATHER COOL AIRMASS PER AVAILABLE GUIDANCE. WILL LIKELY SEE A PERIOD WHERE TEMPS ARE ONCE AGAIN NEAR OR EVEN BELOW NORMAL. DO ALSO NOTE A FAIRLY ROBUST PRES GRADIENT ON WED...SO DEPENDING ON RAINFALL...WITH VERY DRY AIRMASS MOVING IN MAY NEED TO MONITOR FOR YET ANOTHER DAY OF ELEVATED FIRE WEATHER. && .AVIATION /00Z 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/... 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/SIPPRELL NEAR TERM...BELK/DOODY SHORT TERM...BELK LONG TERM...SIPPRELL AVIATION...BELK/SIPPRELL MARINE...BELK/SIPPRELL CLIMATE...

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