660
FXUS61 KBOX 122246
AFDBOX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
646 PM EDT SAT SEP 12 2026
WHAT HAS CHANGED
EXPANDED THE FLOOD WATCH TO INCLUDE WINDHAM, SOUTHERN WORCESTER,
SUFFOLK, AND SUFFOLK COUNTIES. MONITORING A VERY LOW CHANCE FOR
WATERSPOUTS AND A WEAK TORNADO ACROSS THE SOUTH COAST. THE RISK FOR
ANY SEVERE THUNDERSTORMS IS DWARFED BY THE HYDROLOGIC RISK, HOWEVER.
KEY MESSAGES
- CONFIDENCE CONTINUES FOR A PERIOD OF TORRENTIAL AND A RISK OF FLOODING
ACROSS MUCH OF RHODE ISLAND AND SOUTHEAST MASSACHUSETTS.
- HIGH PRESSURE IN CONTROL NEXT WEEK BRINGS GENERALLY QUIET
WEATHER. BRIEF BOUT OF SUMMER WARMTH AND HUMIDITY EXPECTED
WED-FRI.
DISCUSSION
KEY MESSAGE 1...CONFIDENCE CONTINUES FOR A PERIOD OF TORRENTIAL AND
A RISK OF FLOODING ACROSS MUCH OF RHODE ISLAND AND SOUTHEAST
MASSACHUSETTS.
MAIN FORECAST FOCUS FOR THE NEXT 48 HOURS CONTINUES TO BE ON A
SHORTWAVE AND WARM FRONT LATER TONIGHT THROUGH MUCH OF THE DAY ON
SUNDAY. ALOFT, THE AREA WILL FIND ITSELF IN THE RIGHT ENTRANCE
REGION OF A 80-90 KT JET BY 12Z SUNDAY MORNING. SO, SYNOPTICALLY,
THERE ARE PLENTY OF JET AND FRONTAL DYNAMICS AT PLAY TO ALLOW FOR A
HEAVY RAIN EVENT. THE THERMODYNAMIC ENVIRONMENT CERTAINLY ISN'T
LACKING EITHER, WITH MODEL SOUNDINGS SHOWING PWATS CLIMBING TO >2"
WHICH IS NEAR THE 99TH PERCENTILE OF CLIMATOLOGY FOR SEPTEMBER 13TH.
THESE HIGHLY ANOMALOUS PWATS PAIR WITH A NEARLY SATURATED
ATMOSPHERIC COLUMN AND WARM CLOUD HEIGHTS OF 12-14KFT AND ALLOW FOR
EFFICIENT RAINFALL PROCESSES AND AREAS OF TORRENTIAL RAIN. TAKING
THESE CONDITIONS INTO ACCOUNT IT MAKES SENSE THAT THE 12Z HREF SHOWS
A 40-55% CHANCE FOR 3 HOUR RAINFALL AMOUNTS TO EXCEED FLASH FLOOD
GUIDANCE COVERING AN AREA FROM EASTERN CONNECTICUT TO RHODE ISLAND
AND SE MASSACHUSETTS. ADDITIONALLY, THERE IS STILL A STRONG (55-70%)
CHANCE FOR 6-24 HR PRECIPITATION VALUES TO EXCEED THE 5 YEAR ARI AND
A MODERATE (25-40%) CHANCE OF EXCEEDING THE 10 YEAR ARI. THE
GREATEST CONCERN FOR AREAS OF FLASH FLOODING IS ACROSS THE URBAN
CORRIDOR FROM PROVIDENCE TO INTERIOR SE MA. STILL SOME UNCERTAINTY
WITH REGARDS TO THE EXACT PLACEMENT OF THE HEAVIEST RAIN IT WILL BE
CONTINGENT UPON THE PLACEMENT OF A MESOSCALE LOW. SUBTLE
DIFFERENCES IN DETERMINISTIC GUIDANCE HAVE THIS BAND AS FAR SOUTH AS
THE SOUTH COAST AND AS FAR NORTH AS NORTHERN MASSACHUSETTS. THINK
THE HREF AND RRFS HANDLE THESE DIFFERENCES BEST BY CENTERING
THE HIGHEST RAINFALL TOTALS ALONG THE I-95 CORRIDOR.
GIVEN THE UNCERTAINTY AND THE N JOG IN GUIDANCE, OPTED TO EXPAND THE
FLOOD WATCH TO INCLUDE WINDHAM COUNTY IN EASTERN CONNECTICUT,
SOUTHERN WORCESTER COUNTY, NORFOLK COUNTY, AND SUFFOLK COUNTY
INCLUDING BOSTON. ELSEWHERE, A WIDESPREAD 0.50" TO 1.50" EXPECTED
ACROSS MUCH OF OUR REGION WITHIN THE WATCH THERE WILL LIKELY BE
LOCALIZED NARROW BANDS OF 2"-4" OF RAIN POSSIBLE INSIDE 3 HOURS. THE
NARROW BUT INTENSE AREA OF RAINFALL LIKELY COMES TO AN END FROM WEST
TO EAST BETWEEN 2 AND 5 PM.
ON A FINAL NOTE, WHILE THE MAIN THREAT IS AREAS OF FLASH FLOODING,
THERE IS A LOW ALBEIT NON-ZERO CHANCE FOR ISOLATED SEVERE WEATHER
ACROSS COASTAL RI AND MA. CAMS SHOW A NARROW AREA OF MLCAPE
APPROACHING 300-500 J/KG ACROSS COASTAL AREAS. PAIRED WITH 0-1KM
SHEAR OF ~20KTS AND CURVED HODOGRAPHS THERE IS A VERY LOW CHANCE FOR
A SPIN UP OR A WATERSPOUT AS THE MESOLOW PASSES LATE TOMORROW
MORNING INTO THE AFTERNOON. WHAT'S LACKING IS ADEQUATE INSTABILITY
OR LAPSE RATES FOR DEEPER CONVECTION BUT WORTH KEEPING AN EYE ON.
KEY MESSAGE 1...HIGH PRESSURE IN CONTROL NEXT WEEK BRINGS GENERALLY
QUIET WEATHER. BRIEF BOUT OF SUMMER WARMTH AND HUMIDITY EXPECTED WED-
FRI.
THE START OF THE NEW WORK WEEK STARTS WITH SOME LINGERING SHOWERS AS
THE COLD FRONT MAKES ITS WAY OFFSHORE. RAIN COMES TO AN END AFTER
SUNRISE FOR WESTERN MA/CT WHILE SHOWERS LINGER INTO THE EARLY-MID
AFTERNOON ACROSS EASTERN MA/RI. EVENTUALLY DRY NW POST FRONTAL FLOW
ADVECTS 40-50F DEWPOINTS OVERHEAD WHICH COMBINED WITH HIGHS IN THE
LOW TO MID 70S MON AND TUE WILL MAKE FOR A COMFORTABLE AND
SEASONABLE FIRST HALF OF THE WEEK. THIS, AS HIGH PRESSURE IS IN
CONTROL AT THE SURFACE THROUGH MUCH OF THE WEEK KEEPING US DRY. AS
THE HIGH SHIFTS OFFSHORE WEDNESDAY FLOW TURNS SOUTHERLY BRINGING THE
RETURN OF MORE SUMMER-LIKE TEMPERATURES AND DEWPOINTS IN THE UPPER
70S AND MID 60S RESPECTIVELY. UNCERTAINTY INCREASES WITH REGARD TO
THE FORECAST FOR LATE WEEK NEXT WEEKEND. ZONAL MID LEVEL FLOW
CONTINUES INTO THE WEEKEND WITH POTENTIAL FOR A LOW TO CROSS
SOMEWHERE IN THE VICINITY OF SOUTHERN NEW ENGLAND; WHETHER THAT
STAYS TO OUR NORTH OR GOES OVERHEAD IS UNCERTAIN, BUT GENERALLY
LOOKING AT A BETTER SHOT OF UNSETTLED WEATHER LATE THURSDAY
ONWARD.
AVIATION /00Z SUNDAY THROUGH THURSDAY/
FORECASTER CONFIDENCE LEVELS...
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
00Z TAF UPDATE...
TONIGHT...HIGH CONFIDENCE.
VFR. S WINDS. CEILINGS REMAIN VFR MOST OF THE NIGHT, THOUGH
AREA GRADUALLY LOWERING TO LOW- END VFR BETWEEN 08-12Z.
SUNDAY...HIGH CONFIDENCE IN TRENDS, MODERATE CONFIDENCE IN TIMING.
WIDESPREAD SHOWERS AND POTENTIALLY A FEW EMBEDDED THUNDERSTORMS WILL
OVERSPREAD THE REGION FROM WEST TO EAST ROUGHLY 09Z-14Z SUNDAY
MORNING. THIS WILL RESULT IN LOW-END MVFR TO IFR CONDITIONS
DEVELOPING WITH EVEN BRIEF LIFR CONDITIONS IN NARROW BANDS OF
TORRENTIAL RAINFALL. THE BULK OF THE HEAVIEST SHOWERS SHOULD BE
EXITING THE SOUTHEAST NEW ENGLAND COAST BY 22Z/23Z. WINDS WILL BE
LIGHT FROM A S-SE DIRECTION. WITH EASTERN COASTAL MA GOING E. THERE
ALSO MAY BE A PERIOD OF LLWS.
SUNDAY NIGHT...HIGH CONFIDENCE.
LOW-END VFR TO MVFR CEILINGS. WIND SHIFT TO THE NW BEHIND THE FRONT.
KBOS TERMINAL...HIGH CONFIDENCE IN TAF.
KBDL TERMINAL...HIGH CONFIDENCE IN TAF.
OUTLOOK /MONDAY THROUGH THURSDAY/...
MONDAY THROUGH MONDAY NIGHT: VFR. BREEZY.
TUESDAY THROUGH THURSDAY: VFR.
MARINE
FORECASTER CONFIDENCE LEVELS...
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
TODAY THROUGH SUNDAY...HIGH CONFIDENCE.
A WARM FRONT THAN APPROACHES AND CROSSES THE WATERS ON SUNDAY.
SURFACE GRADIENT SHOULD REMAIN WEAK ENOUGH TO KEEP WINDS/SEAS
BELOW SMALL CRAFT ADVISORY THRESHOLDS THROUGH SUNDAY.
HOWEVER...THE MAIN CONCERN FOR MARINERS WILL BE HEAVY SHOWERS
AND POTENTIALLY A FEW EMBEDDED THUNDERSTORMS ON SUNDAY WITH
REDUCED VISIBILITY. THERE IS A VERY LOW CHANCE FOR A WATER SPOUT
OR TWO MAINLY ACROSS THE NEAR-SHORE WATERS ALONG THE S COAST.
OUTLOOK /MONDAY THROUGH THURSDAY/...
MONDAY: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING 5 FT.
CHANCE OF RAIN SHOWERS.
MONDAY NIGHT: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH
GUSTS UP TO 25 KT. SEAS UP TO 5 FT.
TUESDAY THROUGH TUESDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF
SEAS APPROACHING 5 FT.
WEDNESDAY THROUGH WEDNESDAY NIGHT: WINDS LESS THAN 25 KT. SEAS
UP TO 5 FT.
THURSDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
BOX WATCHES/WARNINGS/ADVISORIES
CT...FLOOD WATCH FROM 5 AM EDT SUNDAY THROUGH SUNDAY AFTERNOON FOR
CTZ004.
MA...FLOOD WATCH FROM 5 AM EDT SUNDAY THROUGH SUNDAY AFTERNOON FOR
MAZ012-013-015>024.
RI...FLOOD WATCH FROM 5 AM EDT SUNDAY THROUGH SUNDAY AFTERNOON FOR
RIZ001>008.
MARINE...NONE.
DISCUSSION...BW/FT
AVIATION...BW/DOOLEY
MARINE...FT
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