Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210757 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 357 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. AN ACTIVE WET- WEATHER PATTERN BEGINS TO EMERGE MEMORIAL DAY AND MAY CONTINUE THRU THE REMAINDER OF THE WEEK. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... TODAY... CHILLY MORNING IN PROGRESS WITH PREDAWN TEMPS MAINLY IN THE 40S. INTERESTINGLY MESONETS INDICATE COOLEST TEMPS ARE LOCATED IN NORTHEAST MA WHERE TEMPS HAVE DIPPED INTO THE U30S! AS HIGH PRES BUILDS SOUTH OF NEW ENGLAND TODAY RETURN SSW WIND FLOW WILL MODIFY CHILLY AIRMASS OVER THE REGION ALONG WITH SUNSHINE THRU HIGH CLOUDS. THIS WILL BOOST TEMPS TO 65 TO 70. IT WILL FEEL MILDER THAN YESTERDAY GIVEN LESS WIND. ALTHOUGH A MODEST SSW WILL DEVELOP OVER SOUTHEAST MA THIS AFTERNOON WITH SPEEDS 15 TO 20 MPH. LIGHTER WINDS ELSEWHERE. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... TONIGHT... NORTHERN STREAM SHORT WAVE ENERGY OVER THE HIGH PLAINS EARLY THIS MORNING WILL TRACK EASTWARD TODAY AND CAPTURE SOME SOUTHERN STREAM MOISTURE AS IT APPROACHES THE EAST COAST. THIS FEATURE WILL SPAWN A LATE SEASON COASTAL LOW/BAROCLINIC WAVE OVER THE SOUTHEAST STATES THAT WILL TRACK NEAR OR JUST SOUTHEAST OF THE 40N/70W BENCHMARK BY FRI MORNING. MORE IMPORTANTLY ALL GUIDANCE HAS MODEST MID LEVEL FGEN OVER EASTERN CT/RI AND SOUTHEAST MA. MODELS VARY ON THE EXACT LOCATION OF MID LEVEL FGEN BUT ALL MODELS HAVE BEEN TRENDING FARTHER NORTH. HI RES ARW AND NMM ARE FARTHEST NORTH WITH MODEST MID LEVEL RAIN BANDS OVER EASTERN CT/RI TO NEAR OR JUST SOUTH OF BOSTON. DIDN/T WANT TO GO FULL THROTTLE ON THIS MOST NORTHERN SOLUTION BUT DID INCORPORATE SOME OF THIS HI RES GUIDANCE INTO THE FORECAST TO TREND THE FORECAST FARTHER NORTH WITH RAIN SHIELD. GIVEN MODEST FORCING AND SIGNS OF SOME INSTABILITY /NEG EPV/ COULD SEE A FEW MODERATE TO PERHAPS BRIEFLY HEAVY RAIN BANDS 06Z-12Z FRI OVER EASTERN CT/RI AND SOUTHEAST MA...POSSIBLE NORTHWARD TO THE MASS PIKE. FRIDAY...A 3 PART DAY WITH MORNING RAINS LIKELY OVER RI AND SOUTHEAST MA...POSSIBLY NORTHWARD TO BOSTON. THEN AS MID LEVEL TROUGH AND ATTENDING COASTAL LOW EXIT DRY WEATHER ALONG WITH SUNSHINE DEVELOP MID TO LATE MORNING IN RESPONSE TO SHORT WAVE RIDGING. HOWEVER THIS WILL BE SHORT LIVED AS VORTEX OVER QUEBEC DROPS SOUTHWARD ALONG WITH ATTENDING COLD FRONT. THIS SHOULD BE SUFFICIENT TO KICK OFF WIDELY SCATTERED SHOWERS MID TO LATE AFTERNOON AND EARLY EVENING. VERY COLD TEMPS ALOFT ADVECT OVER THE NORTHEAST WITH 500 MB TEMPS FALLING TO -24C BY DAYS END AT 00Z. THIS PROVIDES MID LEVEL LAPSE RATES OF 6.5-7.0C/KM ALONG WITH TT/S CLIMBING TO 50. THESE COLD TEMPS ALOFT MAY BE SUFFICIENT FOR CHANCE OF THUNDER AND LOW RISK OF SMALL HAIL. OTHERWISE TEMPS WILL CLIMB INTO THE L70S FRI AFTERNOON. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ 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. && .AVIATION /08Z 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... FRIDAY NIGHT INTO SUNDAY NIGHT...HIGH CONFIDENCE. VFR. BREEZY NW-WINDS THROUGH SATURDAY VEERING SW INTO SUNDAY. MONDAY INTO MONDAY NIGHT...MODERATE CONFIDENCE. LOW-END VFR TOWARDS THE LATER-HALF WITH INCREASING -SHRA CHANCES. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH FRIDAY/... TODAY... WNW WINDS BECOME SSW BY MIDDAY. DRY WEATHER AND GOOD VSBY. TONIGHT... RAIN OVERSPREADS THE SOUTHERN WATERS AS LOW PRES TRACKS NEAR OR JUST SOUTH OF 40N/70W BENCHMARK. SOME UNCERTAINTY ON NORTHERN EXTENT OF RAIN SHIELD. FRI... RAIN SHIELD OVER SOUTHEAST MA EXITS BY MID TO LATE MORNING. THEN A PERIOD OF DRY WEATHER FOLLOWED BY ISOLATED TO WIDELY SCATTERED SHOWERS DURING MID TO LATE AFTERNOON INTO THE EVENING AS A COLD FRONT APPROACHES FROM THE WEST. OUTLOOK...FRIDAY NIGHT THROUGH MONDAY... BREEZY NW-WINDS WITH GUSTS OVER 25 KTS THROUGH SATURDAY VEERING SW INTO SUNDAY AND REMAINING BRISK WITH GUSTS UP TO 20 KTS. LOOKING AT WAVES BUILDING IN EXCESS OF 5 FEET ON THE OUTER WATERS MONDAY INTO MONDAY NIGHT...MODERATE CONFIDENCE. CONTINUED BREEZY SW-WINDS WITH GUSTS UP TO 20 KTS. WAVES IN EXCESS OF 5-FEET MAY REMAIN ON THE S/SE-WATERS. && .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...NOCERA/SIPPRELL NEAR TERM...NOCERA SHORT TERM...NOCERA LONG TERM...SIPPRELL AVIATION...NOCERA/SIPPRELL MARINE...NOCERA/SIPPRELL

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