Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230659 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 259 AM EDT WED JUL 23 2014 .SYNOPSIS... WARM AND HUMID WEATHER IS EXPECTED THROUGH WEDNESDAY NIGHT AHEAD OF AN APPROACHING COLD FRONT. THE FRONT WILL TRIGGER SCATTERED SHOWERS AND THUNDERSTORMS WEDNESDAY AFTERNOON AND EVENING. A FEW STRONG STORMS ARE POSSIBLE ESPECIALLY INLAND. SHOWERS MAY LINGER INTO THURSDAY ALONG THE SOUTH COAST HOWEVER THE TREND WILL BE FOR DRIER AND LESS HUMID WEATHER THURSDAY NIGHT INTO SATURDAY. THE WEATHER PATTERN THEN BECOMES UNSETTLED AND MORE HUMID BEGINNING SUNDAY AND ESPECIALLY EARLY NEXT WEEK. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1 AM UPDATE... FORECAST REMAINS ON TRACK OVERNIGHT. OTHER THAN A PATCH OF MID CLOUDS PASSING OVER THE GREATER BOSTON AREA...SKIES ARE MOSTLY CLEAR OVER SOUTHERN NEW ENGLAND AND SHOULD REMAIN SO OVERNIGHT. PATCHY FOG AND STRATUS BOTH ALONG THE SOUTH COAST AND INTERIOR VALLEYS REMAIN A POSSIBILITY BUT PROBABLY NOT A MAJOR FACTOR. THE DEWPOINT DEPRESSIONS ARE NARROWING BUT FOR MOST LOCATIONS THERE IS ENOUGH SURFACE GRADIENT FOR SOME MIXING AND SUNRISE WILL LIKELY OCCUR BEFORE THE LOWER LAYERS BECOME TOO SATURATED. DID NOT MAKE ANY CHANGES TO THE TEMPERATURE FORECAST FOR THE REST OF THE NIGHT. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... * A FEW STRONG TO SEVERE STORMS POSSIBLE LATE THIS AFTERNOON INTO THE EVENING HOURS * * SHOWERS WILL LIKELY LINGER ACROSS THE CAPE...ISLANDS AND ADJACENT SE MASSACHUSETTS DURING MUCH OF THURSDAY TODAY AND TONIGHT... WE CONTINUE TO ANTICIPATE STRONG THUNDERSTORMS TO DEVELOP LATE THIS AFTERNOON IN THE WEST AND MOVE/REDEVELOP FURTHER EAST THIS EVENING. LOOKS LIKE MARGINALLY STRONG ENOUGH WIND SHEAR AND SUFFICIENTLY ROBUST INSTABILITY FOR A FEW THUNDERSTORMS TO REACH SEVERE LIMITS...MOST LIKELY IN THE FORM OF POCKETS OF DAMAGING WINDS...AS A COLD FRONT APPROACHES NEW ENGLAND LATE TODAY. LATEST THINKING IS THAT THE SEVERE RISK WILL BE MAINLY ALONG AND WEST OF I495 AND ALONG AND NORTH OF THE MASS PIKE AND I84. THIS IS CLOSE TO ALTHOUGH JUST A TAD BROADER THAN THE AREA OUTLOOKED BY SPC. MODEL FORECAST SOUNDINGS...INDICES AND CAPE ALL POINT TO SUFFICIENT INSTABILITY FOR STRONG THUNDERSTORMS LATE THIS AFTERNOON AND EVENING. SBCAPES OF 1200 TO 1800 J/KG INDICATED WITH THE 00Z GFS AND 2500 TO 3000+ J/KG PER THE NAM...WHICH HAS A TENDENCY TO OVERDO THE CAPE. NEVERTHELESS...SHOWATERS DROP TO AROUND -3 AND K INDICES REACH INTO THE MID TO UPPER 30S AND TOTAL TOTALS APPROACH 50. ANTICIPATE FAIRLY STRONG LOW LEVEL LAPSE RATES AND MODERATE MID LEVEL LAPSE RATES AROUND 6.5C/KM LATE THIS AFTERNOON/EARLY EVENING. THERE IS A MID LEVEL CAP IN PLACE AROUND 11K FT AT THE START OF THE AFTERNOON BUT LOWERING 500 MB HEIGHTS AND THE APPROACHING COLD FRONT HAS THE CAP ERODING BY MID AFTERNOON IN THE WEST AND LATE AFTERNOON/EARLY EVENING IN THE EAST. AT THE VERY LEAST THE INSTABILITY ALONE IS ENOUGH TO SUPPORT STRONG PULSE THUNDERSTORMS ACROSS THE NW INTERIOR. THE 0 TO 6 KM BULK SHEAR MAGNITUDE...HOWEVER...IS MARGINAL AT BEST...25 TO 30 KT AND EVEN WEAKER TO THE SOUTHEAST OF THE WORCESTER AREA. ANOTHER FACTOR TO CONSIDER IS THE RIGHT ENTRANCE REGION OF THE UPPER JET WHICH APPROACHES THIS EVENING BUT NOT IDEALLY SITUATED OVER THE AREA UNTIL THU MORNING WHEN THE SURFACE COLD FRONT REACHES THE SE COASTAL WATERS. A MORE GENERAL SYNOPTIC RECOGNITION OF THIS EVENT SUGGESTS THE TIMING OF THE SURFACE COLD FRONT SHOULD SUPPORT CONVECTION IN AT LEAST WESTERN AND CENTRAL SECTIONS AND PROBABLY IN A WEAKENED STATE IN THE EAST LATER INTO THE EVENING. THE AXIS OF INSTABILITY IS INITIALLY ACROSS WESTERN MA AND WESTERN CT LATE THIS AFTERNOON AND THEN WORKS ACROSS SW NH AND CENTRAL MA DURING THE EARLY EVENING AND MORE INTO NE MA LATER INTO THE EVENING. OVERNIGHT THE AXIS OF INSTABILITY SHIFTS TO THE SOUTHEAST ZONES BUT WEAKENS. IT CURRENTLY SEEMS DOUBTFUL WHETHER DEEP CONVECTION WILL SURVIVE PAST THE BOS TO PVD CORRIDOR. PUTTING THIS ALL TOGETHER...WE PERCEIVE A RISK OF SEVERE THUNDERSTORMS WITH DAMAGING WINDS THE GREATEST THREAT OVER A SIZABLE CHUNK OF THE NW INTERIOR BOUNDED TO THE EAST BY I495 AND TO THE SOUTH BY THE MASS PIKE AND I84. THE TIME FRAME LOOKS TO BE PRIMARILY 4 PM TO 11 PM...STARTING IN THE WEST. THUNDERSTORMS MAY REACH METRO BOSTON LATER IN THE EVENING BUT LIKELY IN A WEAKENED STATE. WITH PWATS NEAR 2 INCHES AHEAD OF THE COLD FRONT...THE POTENTIAL ALSO EXISTS FOR INTENSE RAINFALL AND LOCALIZED FLASH FLOODING. THIS CONCERN IS HEIGHTENED WITH THE SLOWING OF THE FRONTAL BOUNDARY AS IT MOVES ACROSS SOUTHERN NEW ENGLAND TONIGHT. HOWEVER...WE DO NOT SEE A STRONG TROPICAL PLUME CONNECTION AND SO CONTINUE TO BE INCLINED TO HOLD OFF ON PUTTING A LARGE PART OF THE FORECAST AREA UNDER A FLASH FLOOD WATCH AT THIS TIME. NEVERTHELESS...WE WILL STILL NEED TO BE VIGILANT FOR THE POSSIBILITY OF LOCALIZED FLASH FLOODING...A THREAT THAT MAY PERSIST TONIGHT AFTER THE DAMAGING WIND THREAT HAS FADED. THURSDAY... THE FRONT DEACCELERATES AS IT APPROACHES SOUTHEAST NEW ENGLAND AND THEN A WAVE MOVES ALONG THE FRONT DURING THE DAY THURSDAY. THUS...EXPECT SHOWERS TO LINGER THURSDAY MORNING ACROSS THE SOUTHEAST HALF OF THE AREA AND FOR MOST OF THE DAY ACROSS CAPE COD...THE ISLANDS AND ADJACENT SOUTHEAST MA. WILL BE MUCH COOLER THURSDAY BEHIND THE FRONT...AND SOUTHEAST OF THE BOSTON TO PROVIDENCE CORRIDOR CLOUDS WILL ALSO ACT TO CAP THE TEMPERATURE. FOR MOST LOCATION...EXPECT HIGHS IN THE 70S ON THURSDAY. THE ATMOSPHERE MAY BE MARGINALLY UNSTABLE ENOUGH TO SUPPORT A FEW EMBEDDED THUNDERSTORMS OVER THE CAPE AND ISLANDS AS WELL AS SOUTHEAST COASTAL WATERS ON THURSDAY. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... HIGHLIGHTS... * MUCH COOLER THU ALONG WITH RAIN LINGERING SOUTH COAST * DRY...LESS HUMID AND SEASONABLE THU NGT THRU SAT * PATTERN BECOMES UNSETTLED AND MORE HUMID SUN INTO NEXT WEEK SYNOPTIC OVERVIEW AND MODEL PREFERENCES.... NORTHERN STREAM MID LEVEL TROUGH MOVES ACROSS THE AREA THU WITH ITS SURFACE COLD FRONT SLOWLY EXITING THE SOUTH COAST. POST FRONTAL AIRMASS OVERSPREADS THE REGION FRI. THE MID LEVEL MEAN TROUGH AXIS THEN MOVES OFFSHORE SAT WITH SUBTLE HEIGHT RISES AND ANTICYCLONIC FLOW OVERSPREADING NEW ENGLAND. THIS WILL MAINTAIN DRY WEATHER OVER THE AREA THRU SAT. BY SUNDAY AND ESPECIALLY EARLY NEXT WEEK ALL MODEL GUIDANCE INCLUDING THE GEFS AND ECENS ENSEMBLES ARE IN GOOD AGREEMENT ON A HIGH AMPLITUDE /+2 SD/ LONG WAVE TROUGH SETTING UP OVER SOUTHEAST CANADA SOUTHWARD INTO THE OH VLLY. BOTH DETERMINISTIC AND ENSEMBLE GUIDANCE ARE IN VERY GOOD AGREEMENT ON THE LARGE SCALE FLOW ACROSS NORTH AMERICA THIS PERIOD. THEREFORE WILL BASE THIS PORTION OF THE FORECAST ON A MODEL BLEND. DAILY DETAILS... THURSDAY... MID LEVEL TROUGH AXIS STILL WELL WEST OF NEW ENGLAND AT 12Z THU. THUS COLD FRONT WILL BE SLOW TO EXIT THE SOUTH COAST WHICH WILL INCREASE THE RISK FOR SHOWERS AND POSSIBLE A TSTM TO LINGER ACROSS THIS REGION. THE HIGH RES DETERMINISTIC GUID /GFS AND NAM/ ARE SLOWER WITH THE FROPA AS THESE MODELS FORM A STRONGER FRONTAL WAVE THAN THE ENSEMBLES. THIS IS POSSIBLE GIVEN THE HIER RES MODELS WOULD PERFORM BETTER THAN THE COURSE ENSEMBLES SIMULATING A SMALL SCALE FRONTAL WAVE. THEREFORE WILL GO WITH LIKELY POPS THU MORNING ACROSS RI AND SOUTHEAST MA AND TRENDING TO CHANCE AS THE RAIN SHIELD MOVES SLOWLY SOUTHEAST AND EVENTUALLY OFFSHORE LATER THU. DEPARTURE OF RAIN WILL BE CRITICAL TO TEMP FORECAST AS THE WETTER GFS 2 METER TEMPS NEVER CLIMB OUT OF THE 60S! EVEN THE WARMER ECMWF AND NAM 2 METER TEMPS ARE ONLY INTO THE L70S. THUS A COOLER DAY ON TAP THU AND IF THE GFS VERIFIES OUR TEMPS WILL BE TOO WARM. ELSEWHERE ACROSS INTERIOR CT/MA INTO SOUTHWEST NH MAINLY DRY CONDITIONS SHOULD PREVAIL WITH LOTS OF MID AND HIGH CLOUDS ALONG WITH N-NE WINDS YIELDING A COOLER DAY. THURSDAY NIGHT THRU SATURDAY... POST FRONTAL AIRMASS OVERSPREADS THE AREA THU NIGHT AND FRI WITH A 850 TEMP ANOMALIES AROUND 1 STANDARD DEVIATION COOLER THAN CLIMO... ABOUT +10C/+11C. HENCE REFRESHING AIRMASS WITH DEW PTS IN THE 50S. THE CORE OF THE DRY/COOLER AIRMASS WILL BE OVER SOUTHERN NEW ENGLAND AROUND 12Z FRI. THIS WILL YIELD A VERY COMFORTABLE AND COOL TEMPS THU NIGHT/EARLY FRI AM. SUNDAY THRU TUESDAY... PATTERN BECOMES UNSETTLED AS LONG WAVE TROUGH SETS UP OVER SOUTHEAST CANADA SOUTHWARD INTO THE OH VALLEY. THIS PROVIDES DEEP CYCLONIC FLOW ACROSS THE NORTHEAST ALONG WITH A BROAD SURFACE LOW PRES OVER NY AND NEW ENGLAND. LOW LEVEL FLOW WILL BE SSW OFF THE ATLANTIC INTO NEW ENGLAND SO EXPECT INCREASING HUMIDITY ALONG WITH THE RISK OF SCATTERED SHOWERS AND THUNDERSTORMS. && .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH TONIGHT/... GENERALLY MODERATE CONFIDENCE ON FORECAST. THROUGH THIS MORNING...CONDITIONS SHOULD REMAIN VFR FOR MOST OF THIS PERIOD EXCEPT FOR PATCHY FOG AND STRATUS ALONG THE SOUTH COAST AND INTERIOR VALLEYS. THERE IS ENOUGH LOW LEVEL GRADIENT FOR ENOUGH MIXING TO LIMIT ANY VISIBILITIES BELOW MVFR. ANY STRATUS OF PATCHY FOG SHOULD BURN OFF FAIRLY QUICKLY AFTER SUNRISE. LATE THIS AFTERNOON INTO TONIGHT...MVFR CIGS AND VSBYS IN VICINITY OF SCT TSTMS AHEAD OF A COLD FRONT. A FEW TSTMS MAY BRIEFLY LOWER CIGS/VSBYS TO IFR LEVELS. ALSO...A FEW TSTMS MAY PRODUCE STRONG WIND GUSTS. VSBYS MAY BE REDUCED IN PATCHY FOG AHEAD OF FRONTAL PASSAGE AS WELL. SREF PROBABILITIES ARE HIGH FOR LOW CLOUDINESS TO FORM OVER SOUTHEAST MA AND RI MID TO LATE WED AFTERNOON IN THE WARM MUGGY AIRMASS...SO WILL NEED TO WATCH FOR THIS SEPARATE AREA OF CLOUDINESS WELL AHEAD OF ANY STORMS. KBOS TAF...HIGH CONFIDENCE. VFR THROUGH MOST IF NOT ALL OF THE DAY. TSTMS POSSIBLE AFT 00Z OR 8 PM...AFT 22Z TO THE N AND W. KBDL TAF...HIGH CONFIDENCE IN TAF. LOW PROB ON A BRIF PERIOD OF MVFR VSBYS IN FOG OVERNIGHT BUT LOOKING LESS LIKELY. TSTMS POSSIBLE AFT 21Z. ANY TSTMS COULD CONTAIN STRONG WIND GUSTS. OUTLOOK...THURSDAY THROUGH SUNDAY...MODERATE CONFIDENCE MVFR POSSIBLE BOSTON TO PROVIDENCE IN RAIN...POSSIBLY IFR SOUTH COAST WITH VFR NORTHWEST MA INTO SOUTHWEST NH. THEN VFR ALL TERMINALS THU NIGHT INTO SAT. LOW RISK OF MVFR IN SCATTERED SHOWERS/TSTMS SUN AFTN WESTERN NEW ENGLAND. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH WEDNESDAY/...MODERATE CONFIDENCE. WINDS AND SEAS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY LEVELS THROUGH TONIGHT. THE SW FLOW WILL STRENGTHEN AHEAD OF A COLD FRONT ON WED WITH A RISK OF GUSTS APPROACHING 25 KT. AREAS OF FOG MAY DEVELOP ALONG THE SOUTH COASTAL WATERS TONIGHT. OUTLOOK...THURSDAY THROUGH SUNDAY...MODERATE CONFIDENCE. THU...MODEST N-NE WIND ALONG WITH PERIODS OF RAIN ESPECIALLY SOUTH COASTAL WATERS. FRI/SAT...PLEASANT BOATING WEATHER WITH DRY CONDITIONS AND LIGHT WINDS BECOMING A MODEST SSW SAT AFTN. SUN...MODEST SSW WIND ALONG WITH LOW RISK OF A FEW SHOWERS NEAR SHORE. OTHERWISE MAINLY GOOD VSBYS. && .EQUIPMENT... KBOX RADAR MAY BE IN STANDBY MODE WEDNESDAY MORNING WHILE CONSTRUCTION TAKES PLACE NEARBY. THE RADAR WILL RETURN TO SERVICE BY EARLY AFTERNOON PRIOR TO START OF ANY CONVECTION IN OUR FORECAST AREA. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. NH...NONE. RI...NONE. MARINE...SMALL CRAFT ADVISORY FROM 10 AM THIS MORNING TO 7 PM EDT THIS EVENING FOR ANZ230>237-251. SMALL CRAFT ADVISORY UNTIL 3 AM EDT THURSDAY FOR ANZ250-254. && $$ SYNOPSIS...NOCERA/DUNTEN/THOMPSON NEAR TERM...THOMPSON SHORT TERM...THOMPSON LONG TERM...NOCERA AVIATION...NOCERA/DUNTEN/GAF/THOMPSON MARINE...NOCERA/DUNTEN/GAF EQUIPMENT...THOMPSON

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