Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230739 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 339 AM EDT WED JUL 23 2014 .SYNOPSIS...
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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. HIGH PRESSURE BUILDS WITH DRIER WEATHER FOR FRIDAY AND SATURDAY. AN APPROACHING WARM FRONT SUNDAY WILL BRING MORE HUMID UNSETTLED WEATHER FOR THE EARLY PART OF NEXT WEEK.
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&& .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/...
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BIG PICTURE... FOUR FEATURES IN THE LARGE SCALE FLOW. TROUGH ALONG THE PACIFIC NORTHWEST COAST...RIDGE EXTENDING FROM THE ROCKIES AND PLAINS TO NORTHERN CANADA...CLOSED LOW OVER NORTHERN QUEBEC WITH TROUGH AXIS ACROSS THE GREAT LAKES...BUILDING RIDGE OVER THE WESTERN ATLANTIC. THE QUEBEC TROUGH HANGS OVER NORTHEAST NORTH AMERICA...THE TROUGH AXIS AND COLDEST TEMPS ALOFT SWING ACROSS NEW ENGLAND THURSDAY NIGHT/FRIDAY. THE PACIFIC NORTHWEST TROUGH RUNS OVER THE TOP OF THE ROCKIES RIDGE...THEN MERGES WITH THE QUEBEC LOW OVER THE WEEKEND. THIS DRAWS THE CLOSED LOW SOUTH OVER THE GREAT LAKES/NORTHEAST EARLY NEXT WEEK. THE ATLANTIC HIGH PRESSURE MEANWHILE BLOCKS OFF THE EASTWARD EXIT OF THIS MERGER. THE COMBINATION OF UPPER LOW PRESSURE OVER THE LAKES AND HIGH PRESSURE OVER THE ATLANTIC CREATES A SOUTHWEST FLOW UP THE EAST COAST FROM THE DEEP SOUTH FOR THE FIRST HALF OF NEXT WEEK. MODELS HAVE BEEN SHOWING CONSISTENCY BOTH RUN TO RUN AND WITH EACH OTHER. THIS INCREASES CONFIDENCE IN THE OVERALL SOLUTION...BUT WITH THE USUAL UNCERTAINTY IN TIMING SHORTWAVES MOVING THROUGH THE FLOW. THIS IS ESPECIALLY TRUE FOR MONDAY AND TUESDAY. THE DAILIES... THURSDAY NIGHT...COLD FRONT LINGERS OFFSHORE WHILE THE UPPER JET RIGHT ENTRANCE REGION MOVES OVERHEAD. THIS MAY SUPPORT A WAVE ALONG THE FRONT DURING THE FIRST PART OF THE NIGHT...BUT THE JET SHIFTS EAST AFTER MIDNIGHT AS SHOULD THE SUPPORTING DYNAMICS. WE WILL KEEP CHANCE POPS ON THE CAPE AND ISLANDS EARLY IN THE NIGHT. IF THE JET POSITION AT 00Z IS AS ADVERTISED...WE MAY BE A LITTLE CONSERVATIVE ON EVENING POPS. OTHERWISE THE MOVEMENT OF THE UPPER JET SHOULD TREND SOUTHERN NEW ENGLAND TO END ANY REMAINING PCPN AND BRING CLEARING SKIES. FRIDAY-SATURDAY... SURFACE HIGH PRESSURE BUILDS OVER THE REGION WITH DRY WEATHER AND DEWPOINTS IN THE 50S. TEMPS AT THE TOP OF THE MIXED LAYER WILL SUPPORT UPPER 70S TO MID 80S BOTH DAYS...A LITTLE WARMER SATURDAY. ONE CONCERN WILL BE SEA BREEZE DEVELOPMENT ON FRIDAY WHICH MAY HOLD DOWN AFTERNOON TEMPS ON THE EASTERN COAST. OTHER CONCERN WOULD BE THE PASSAGE OF THE UPPER TROUGH AND COLDER TEMPS ALOFT. THIS LATTER CONCERN COULD DESTABLIZE THE AIRMASS AND PROMOTE CUMULUS DEVELOPMENT. BUT MODEL RH FIELDS SHOW A DRY AIRMASS WITH RH VALUES BELOW 700 MB BELOW 50 PERCENT. WILL KEEP SKY COVER LOW FRIDAY...THEN INCREASING CIRRUS ON SATURDAY AFTERNOON. SUNDAY... PACIFIC NORTHWEST SHORTWAVE DIGS AS IT APPROACHES THE GREAT LAKES AND GENERATES A SURFACE LOW. STRONG LOW LEVEL JET IS EXPECTED TO DEVELOP IN THE OHIO VALLEY SATURDAY WITH 35-40 KNOT WINDS AT 850 AND 900 MB. AS THIS MOVES EAST IT WILL BRING DEEPER MOISTURE TO NEW ENGLAND WITH PRECIP WATER VALUES CLIMBING TO 1.5 INCHES BY SUNDAY. THE LIFTING OF THIS AIR OVER THE DEPARTING SURFACE LAYER WOULD GENERATE SOME SHOWERS/SCT TSTMS. WE WILL MENTION CHANCE POPS LATER SATURDAY NIGHT INTO SUNDAY. MONDAY-TUESDAY...THE MERGING LOW OVER THE GREAT LAKES SURFACE AND ALOFT WILL SWING A FRONT THROUGH NEW ENGLAND. TIMING IS IN QUESTION AT THIS POINT WITH THE GFS SHOWING FROPA MONDAY NIGHT WHILE THE ECMWF STALLS THE FRONT OVER US. WITH NO CLEAR TREND ON A DAY 7 FORECAST...WE WILL STAY CLOSE TO BLENDED GUIDENCE SOLUTION OF DIMINISHING POPS TUESDAY. BUT WITH LOW CONFIDENCE.
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&& .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
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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 NIGHT THROUGH SUNDAY...MODERATE CONFIDENCE. THURSDAY NIGHT THROUGH SATURDAY...VFR. LOCAL IFR POSSIBLE IN EARLY MORNING FOG/LOW CLOUDS. POTENTIAL SEA BREEZE FRIDAY AFTERNOON. SUNDAY... VFR BUT WITH AREAS OF MVFR IN SHOWERS/TSTMS/FOG.
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&& .MARINE...
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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 NIGHT THROUGH SUNDAY...MODERATE CONFIDENCE. THURSDAY NIGHT-FRIDAY-SATURDAY... NORTH TO NORTHEAST WIND THURSDAY NIGHT BECOMES LIGHT FRIDAY WITH AFTERNOON SEA BREEZES DEVELOPING. WINDS BECOME SOUTHWEST SATURDAY BUT REMAIN BELOW 25 KNOTS. SEAS 4 FEET OR LESS THROUGH THE PERIOD. SUNDAY...A WARM FRONT APPROACHES THE WATERS WITH WARMER MORE HUMID AIR AND WITH CLOUDS/SHOWERS/TSTMS. INCREASING SOUTH-SOUTHWEST WINDS WITH GUSTS 20-22 KNOTS...APPROACHING SMALL CRAFT THRESHOLD. THESE WINDS WILL ALSO BUILD SEAS ON THE EXPOSED WATERS TO 5 OR 6 FEET. A SMALL CRAFT ADVISORY MAY BE NEEDED.
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&& .EQUIPMENT...
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KBOX RADAR MAY BE IN STANDBY MODE THIS 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.
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&& .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...WTB/DUNTEN/THOMPSON NEAR TERM...THOMPSON SHORT TERM...THOMPSON LONG TERM...WTB AVIATION...WTB/DUNTEN/GAF/THOMPSON MARINE...WTB/DUNTEN/GAF EQUIPMENT...

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