Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 292009 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 406 PM EDT WED JUL 29 2015 .SYNOPSIS... ANOTHER VERY WARM AND HUMID DAY IS IN STORE FOR THURSDAY. AN APPROACHING COLD FRONT WILL TRIGGER SHOWERS AND EMBEDDED THUNDERSTORMS THURSDAY AFTERNOON AND EVENING WITH LOCALLY HEAVY RAINFALL...ESPECIALLY ACROSS THE INTERIOR. NOT AS WARM AND LESS HUMID WEATHER IS EXPECTED FRIDAY INTO THE WEEKEND AND PROBABLY INTO EARLY NEXT WEEK. WHILE FRIDAY THROUGH EARLY NEXT WEEK WILL BE MAINLY DRY...CAN/T RULE OUT AN AFTERNOON AND EVENING THUNDERSTORM EACH DAY. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... AN UPPER LEVEL RIDGE OF HIGH PRESSURE WILL KEEP THE WEATHER DRY TONIGHT. DEWPOINTS WILL BE ON THE INCREASE...SO EXPECT IT TO FEEL RATHER MUGGY. LOW TEMPS FOR MOST LOCATIONS WILL ONLY DROP TO BETWEEN 70 AND 75 WITH A FEW UPPER 60S POSSIBLE IN NORTHWEST MA. GIVEN HIGH DEWPOINTS AND LIGHT WINDS...WILL ONCE AGAIN SEE SOME PATCHY FOG DEVELOP IN THE TYPICALLY PRONE LOCATIONS. LOW CLOUDS MAY DEVELOP ACROSS THE FAR SOUTHEAST NEW ENGLAND COAST...BUT WITH SOUTHWEST FLOW SHOULD NOT ADVANCE MUCH FURTHER NORTHWEST THAN THAT. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... ***SHOWERS AND EMBEDDED THUNDERSTORMS WILL BRING HEAVY RAIN AND THE POTENTIAL FOR LOCALIZED STREET FLOODING THURSDAY AFTERNOON AND EVENING ESPECIALLY ACROSS INTERIOR SOUTHERN NEW ENGLAND*** ***A SECONDARY CONCERN WILL BE FOR A FEW STRONG TO MARGINALLY SEVERE THUNDERSTORMS LATE THURSDAY AFTERNOON/EARLY EVENING ACROSS WESTERN MA AND NORTHERN CT*** THURSDAY AND THURSDAY NIGHT... SOUTHWEST FLOW ALOFT AND AT THE SURFACE WILL TRANSPORT DEEP MOISTURE INTO SOUTHERN NEW ENGLAND. BOTH THE GFS/NAM SHOW PWATS EXCEEDING 2+ INCHES AND A SOUTHWEST LOW LEVEL JUST OF 25 TO 30 KNOTS. WE MAY SEE SOME PEEKS OF SUNSHINE IN THE MORNING...BUT SHOULD SEE CLOUDS BEGIN TO DOMINATE IN THE WARM AIR ADVECTION MOISTURE. GIVEN THE DEEP MOISTURE IN PLACE...SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS COULD BREAK OUT ANYWHERE ACROSS THE REGION BY THU AFTERNOON AND PERHAPS EVEN EARLIER. LOCALLY HEAVY RAINFALL WILL BE POSSIBLE WITH THIS ACTIVITY. THE MAIN SHOW WILL ARRIVE LATE THURSDAY AFTERNOON AND EVENING IN ASSOCIATION WITH A PRE-FRONTAL TROUGH AND ASSOCIATED COLD FRONT. POOR MID LEVEL LAPSE RATES AND CLOUD COVER WILL LIMIT INSTABILITY TO SOME DEGREE. NONETHELESS...WE SHOULD BE ABLE TO DEVELOP AROUND 1000 MLCAPES ACROSS THE INTERIOR GIVEN 70+ DEWPOINTS IN PLACE. 0 TO 6 KM SHEAR IS RATHER MARGINAL...BUT DOES INCREASE TO BETWEEN 30 AND 35 KNOTS ACROSS OUR NORTHWEST ZONES TOWARDS 00Z. A LOT OF THE MESOSCALE MODELS SHOW A LINE OF SHOWERS AND EMBEDDED THUNDERSTORMS WORKING INTO WESTERN MA/NORTHERN CT SOMETIME LATE THU AFTERNOON/EVENING. THAT ACTIVITY WEAKENS AS IT MOVES EAST INTO AN ENVIRONMENT WITH LESS INSTABILITY WHICH IS TYPICAL WITH SOUTHWEST FLOW ALOFT. BULK OF SHOWERS AND EMBEDDED STORMS SHOULD EXIT THE COASTAL PLAIN BY MIDNIGHT...BUT LINGERING ACTIVITY IS POSSIBLE ACROSS THE CAPE AND ISLANDS. SO TO SUM UP...HEAVY RAIN WITH THE POTENTIAL FOR LOCALIZED STREET FLOODING ACROSS THE INTERIOR WILL BE THE MAIN THREAT WITH THESE STORMS. HOWEVER...A FEW STRONG TO PERHAPS MARGINALLY SEVERE THUNDERSTORMS ARE POSSIBLE MAINLY ACROSS WESTERN MA AND NORTHERN CT. THE MAIN CONCERN WOULD BE LOCALIZED STRONG TO DAMAGING WIND GUSTS. THE HAIL THREAT IS RATHER LOW GIVEN 500 MB TEMPS OF AROUND -6C AND POOR MID LEVEL LAPSE RATES. ONE LITTLE NUANCE WE WANTED TO MENTION WAS THE LOW RISK FOR A BRIEF WEAK TORNADO ACROSS WESTERN MA AND NORTHERN CT. THERE IS ABOUT 25 TO 30 KNOTS OF WIND OFF THE DECK WITH RGEM/NAM SHOWING 0 TO 1 KM HELICITY VALUES NEAR OR OVER 100. GIVEN 70+ DEWPOINTS AND LOW LCL/S THERE IS A LOW RISK FOR THAT. HIGH TEMPS THU SHOULD MAINLY BE IN THE UPPER 80S TO NEAR 90...BUT A FEW PLACES ACROSS EASTERN NEW ENGLAND MAY END UP A FEW DEGREES WARMER IF THEY SEE A LONGER PERIOD OF SUNSHINE. EITHER WAY IT WILL BE VERY HUMID WITH 70+ DEWPOINTS. LOW TEMPS THU NIGHT WILL BE IN THE 60S TO NEAR 70 AND AREAS OF FOG WILL LIKELY DEVELOP IN THE TYPICALLY PRONE LOCATIONS LATE. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... HIGHLIGHTS... * PATTERN CHANGE TO MORE SEASONABLE TEMPS IS ACCOMPANIED BY SHOWERS AND THUNDERSTORMS THU NIGHT * NOT AS WARM AND HUMID FRIDAY AND BEYOND * MAINLY DRY WEATHER PREVAILS FRIDAY THRU TUESDAY SYNOPTIC OVERVIEW AND MODEL PREFERENCES... IMPRESSIVE AND ANOMALOUS VORTEX CURRENTLY OVER SOUTHERN MANITOBA AS SEEN ON THE LATEST WATER VAPOR IMAGERY WILL BECOME THE MAIN WEATHER FEATURE THIS PERIOD...AS IT EVOLVES INTO A MUCH BROADER CIRCULATION OVER JAMES AND HUDSON BAY THIS WEEKEND AND NEXT WEEK. THIS RESULTS IN A BROAD MEAN LONG WAVE TROUGH SETTING UP ACROSS THE GREAT LAKES AND THE NORTHEAST. THIS WILL SUPPRESS EXCESSIVE HEAT AND HUMIDITY AIRMASS CURRENTLY OVER THE AREA SOUTHWARD BEGINNING FRIDAY AND LASTING THRU THE WEEKEND INTO EARLY NEXT WEEK. GIVEN GOOD MULTI MODEL AGREEMENT AMONG DETERMINISTIC AND ENSEMBLE GUIDANCE REGARDING THE LARGE SCALE FLOW THIS PERIOD...FORECAST CONFIDENCE IS ABOVE AVERAGE. DAILY SENSIBLE WEATHER DETAILS... THURSDAY NIGHT... NORTHERN STREAM SHORT WAVE AND ATTENDING COLD FRONT MOVE ACROSS THE AREA. MODEST SYNOPTIC SCALE FORCING ASSOCIATED WITH THIS FEATURE. WHILE INSTABILITY AND SHEAR IS MARGINAL IN THE PREFRONTAL ENVIRONMENT...PWATS CLIMB TO 2+ INCHES /+2 STD DEVIATIONS ABOVE CLIMO/. THUS GREATER RISK OF HEAVY DOWNPOURS AND LOCALIZED FLOODING. WARM AND MUGGY CONDITIONS PREVAIL AS POST FRONTAL AIRMASS DOESN/T ARRIVE UNTIL FRIDAY. FRIDAY THRU TUESDAY... TREND TOWARD MORE SEASONABLE TEMPERATURES AND LOWER HUMIDITY ARRIVES FRIDAY BEHIND DEPARTING COLD FRONT WITH HIGHS IN THE 80S AND LOWS IN THE 60S. TOO DIFFICULT AT THIS TIME RANGE TO TIME INDIVIDUAL SHORT WAVES EMBEDDED IN THE CYCLONIC FLOW. SO BEST WORDING WOULD BE MANY HOURS OF DRY WEATHER THIS PERIOD BUT CYCLONIC FLOW AND COOLER TEMPS ALOFT WILL RESULT IN THE RISK OF A THUNDERSTORM DURING PEAK HEATING HOURS EACH DAY. && .AVIATION /20Z WEDNESDAY 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/... TONIGHT...MODERATE TO HIGH CONFIDENCE. MAINLY VFR CONDITIONS EXCEPT FOR PATCHY FOG DEVELOPING LATE IN THE TYPICALLY PRONE LOCATIONS. IN ADDITION...LOW CLOUDS MAY WORK ONTO THE ISLANDS AND CAPE COD...BUT SHOULD NOT MAKE IT MUCH FURTHER NORTHWEST THAN THAT WITH SOUTHWEST FLOW. THURSDAY...MODERATE CONFIDENCE. PROBABLY VFR CONDITIONS DOMINATE INTO THE MID AFTERNOON...EXCEPT THE SOUTH COAST/CAPE AND ISLANDS WHERE LOW CLOUDS/FOG PATCHES WILL BE AT LEAST NEARBY. SCATTERED SHOWERS/ISOLATED THUNDERSTORMS ARE POSSIBLE IN THE WARM AIR ADVECTION PATTERN DURING THE DAY. HOWEVER...THE MAIN FOCUS WILL BE A LINE OF SHOWERS AND EMBEDDED THUNDERSTORMS THAT WILL LIKELY WORK INTO WESTERN MA/NORTHERN CT BY LATE AFTERNOON/EARLY EVENING. A FEW OF THESE STORMS MAY CONTAIN LOCALIZED STRONG WIND GUSTS AND TORRENTIAL RAINFALL. IN ADDITION...SOUTHWEST WIND GUSTS OF 25 TO 30 KNOTS ARE EXPECTED TO DEVELOP ALONG THE COASTAL PLAIN DURING THE AFTERNOON. THURSDAY NIGHT...MODERATE CONFIDENCE. IFR CIGS/VSBYS LIKELY ACROSS SOUTHEAST NEW ENGLAND COAST WITH MAINLY BROKEN MVFR TO VFR CIGS ELSEWHERE. SHOWERS AND EMBEDDED THUNDERSTORMS WILL WEAKEN AS THEY PUSH INTO EASTERN MA AND RHODE ISLAND THU EVENING...BUT SOME LOCALLY HEAVY RAIN IS STILL POSSIBLE. BULK OF ACTIVITY OVER AFTER MIDNIGHT EXCEPT PERHAPS CAPE AND ISLANDS...BUT PATCHY FOG AND LOW CLOUDS MAY DEVELOP. KBOS TERMINAL...HIGH CONFIDENCE THROUGH TONIGHT. SEA BREEZE SHOULD COME TO AN END BETWEEN 21Z AND 23Z THIS EVENING. KBDL TERMINAL...HIGH CONFIDENCE THROUGH TONIGHT. PATCHY FOG MAY AFFECT THE TERMINAL FOR A FEW HOURS BEFORE DAYBREAK THURSDAY. OUTLOOK /FRIDAY THROUGH SUNDAY/... THURSDAY NIGHT...MVFR AND IFR IN SCATTERED SHOWERS/T-STORMS AND FOG. MODERATE TO HIGH FORECAST CONFIDENCE. FRIDAY AND SATURDAY...MODERATE TO HIGH CONFIDENCE IN VFR CONDITIONS. VERY LOW RISK FOR AN AFTERNOON T-STORM SATURDAY. SUNDAY...MODERATE CONFIDENCE FOR MAINLY VFR. && .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. WEAK PRESSURE GRADIENT SHOULD KEEP WINDS AND SEAS BELOW SMALL CRAFT ADVISORY THRESHOLDS. MAY SEE SOME AREAS OF FOG DEVELOP LATE ACROSS THE SOUTHERN WATERS. THURSDAY AND THURSDAY NIGHT...MODERATE TO HIGH CONFIDENCE. DEVELOPING LOW LEVEL JET WILL RESULT IN NEAR SHORE SOUTHWEST WIND GUSTS OF 25 TO 30 KNOTS DEVELOPING THU AFTERNOON. IN ADDITION...LONG SOUTHWEST FETCH WILL ALLOW FOR SEAS TO BUILD TO BETWEEN 3 TO 5 FEET ACROSS MOST OPEN WATERS. SCA HEADLINES GO INTO EFFECT FOR ALL WATERS THU AFTERNOON. WINDS DIMINISH THU EVENING...BUT WILL NEED TO KEEP HEADLINES UP THROUGH THU NIGHT FOR THE OUTER-WATERS THROUGH THU NIGHT FROM THE LEFT OVER SWELL. OUTLOOK /FRIDAY THROUGH SATURDAY/... FRIDAY/SATURDAY AND SUNDAY...COLD FRONT SLOWLY MOVES OFFSHORE WITH WIND SHIFT FROM SSW TO WSW. DRY WEATHER AND GOOD VSBY. HIGH FORECAST CONFIDENCE. LEFTOVER SOUTHERLY SWELLS ON SOUTH OCEAN WATERS FROM THU NIGHT LINGER FRI BUT SLOWLY DIMINISH. && .CLIMATE... RECORD HIGHS FOR 7/29: BOSTON97 ON 1933 & 1931 PROVIDENCE95 ON 2002 & 1949 BRADLEY96 ON 1933 WORCESTER94 ON 1892 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...AIR QUALITY ALERT UNTIL 11 PM EDT THIS EVENING FOR MAZ017>024. RI...AIR QUALITY ALERT UNTIL 11 PM EDT THIS EVENING FOR RIZ005>008. MARINE...SMALL CRAFT ADVISORY FROM NOON TO 7 PM EDT THURSDAY FOR ANZ230>234-236-251. SMALL CRAFT ADVISORY FROM NOON TO 11 PM EDT THURSDAY FOR ANZ235-237. SMALL CRAFT ADVISORY FROM NOON THURSDAY TO 8 AM EDT FRIDAY FOR ANZ250-254>256. && $$ SYNOPSIS...FRANK/NOCERA NEAR TERM...FRANK SHORT TERM...FRANK LONG TERM...NOCERA AVIATION...FRANK/NOCERA MARINE...FRANK/NOCERA CLIMATE...STAFF

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