Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241357 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 957 AM EDT Fri Sep 24 2021 .SYNOPSIS... A slow-moving front will bring isolated strong to severe thunderstorms with localized flooding possible this morning. A secondary wave of low pressure will likely bring another round of rain sometime Saturday into Saturday night across eastern Massachusetts and Rhode Island. Otherwise, Canadian high pressure will bring early fall-like weather with cool nights and very pleasant days for the first half of next week. Mainly dry weather expected too with the best chance for a round of showers sometime late Monday into Monday night with a cold front. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 am update... Heavy rain and embedded thunderstorms continue to slowly creep eastward this morning. The back edge of the rain has started to make its way into our western MA counties. Given that the heaviest rain has pushed out of this region, our Flash Flood watch was cancelled for Hampshire, Hampden and Franklin counties in MA and Hartford county in Connecticut. A surge in surface based CAPE, greater than 1000 J/kg, is expected across eastern MA during the late morning and early afternoon hours, fueled by breaks of sunshine, but instability will be battling weak low and mid level lapse rates, with mid level lapse rates struggling to get to 5.5-6 C/km. Rotation is possible given healthy shear, but a lull in 0-3 km helicity during the same period that CAPE surges likely limits spin ups. Strong storms located south of Long Island have begun to weaken as they ingest cooler marine air, but still deserve a close eye as they make their way into southern RI. 7 am update... Very slow going still with regard to the east to west motion of these showers. Mesoanalysis shows favorable instability and shear parameters but the severe threat has struggled to materialize with poor lapse rates and shallow storm cores. As we continue through the morning and the cold front progresses east instability will increase which may help things to get going, so can`t rule out a few stronger, rotating storms. Previous Discussion... * Isolated strong to severe thunderstorms with localized flooding possible between 4 AM and noon today * Flash Flood Watch in effect for our western zones. * Elevated rip current risk through tomorrow afternoon for ocean-exposed portions of the coast. Not much change to the overall synoptic pattern. Still dealing with a mid level cutoff over the Great Lakes and a modest ridge over the North Atlantic. Not expecting rapid changes through today, despite a potent southerly low level jet. Storm motion this morning as estimated by regional radars was 30-50 kt. The issue with this motion is its south to north, with very little east to west component. Thus, training cells could lead to some flooding issues. This far, the brunt of any rainfall has been along and west of the spine of the Berkshires. Not seeing much rainfall so far across the east slopes of the Berkshires. This will be something which will need to be watched, especially if stronger convection gets underway. Speaking of convection, it has been rather limited thus far. Instability and lapse rates were marginal. Recent SPC mesoanalysis continued to show model shear, but not much else to support strong convection. Stronger thunderstorms remain possible, especially after sunrise. Some insolation may be just the thing to steepen low level lapse rates. In any case, not thinking widespread severe weather will be the result. Precipitation expected to diminish from west to east late this afternoon across the western half of southern New England.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... With a front lingering across the eastern half of southern New England, showers are expected to continue into Saturday. Still quite muggy towards the east coast, with much drier air towards the Berkshires. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Drier, less humid air moves in over the weekend and through next week * Rain showers come to an end in eastern MA overnight leaving a mostly dry extended forecast with cooler temperatures. Next chance of rain showers comes late Monday into Tuesday. Details... Saturday night and Sunday... The stalled cold front will finally move east of the region sometime Sunday night but not before a surface front lifts north along it bringing renewed rain chances overnight. The NAM is most bullish with the low and low level jet, bringing rain as far west as western MA. Other guidance would keep western zones drier like much of the day Saturday. For now going with a blend of guidance, keeping a slight chance of rain as far west as Springfield, MA. Fortunately it lifts north quickly and much of the daytime Sunday should be drier. Temperatures remain about the same as Saturday, topping out in the low to mid 70s. Much drier air moves in behind the front with dewpoints down into the 40s on Sunday for much of the region, low 50s on the extreme east coasts. Monday through Thursday... Broad troughing pattern remains over the eastern U.S. through much of the week with a ridge building the western Great Lakes region. This trough allows for an intrusion of cooler air and return to more fall-like conditions (cooler and less humid). 850 mb temps around 5 to 6 C on Monday will drop as low at 1 to 2 C by Wednesday/Thursday. This will put us in the position to finally see a sub-70 degree day in Boston for the first time this season (a record late occurrence!). As far as precipitation, a trough and surface frontal system may bring rain chances late Monday and Tuesday, but overall much of next week looks mostly dry. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z update... Today...Mix of MVFR/VFR as a cold front crosses the region. Widespread showers and isolated thunderstorms. South winds 10-15 kt with gusts up to 25 kt across Cape terminals. Tonight...VFR conditions for western terminals. MVFR improving to VFR across eastern terminals after midnight except for Cape terminals, where MVFR ceilings could linger through daybreak. South winds 5-10 kt except gusts up to 20 kt across Cape terminals early in the evening. Saturday...Mainly VFR across the interior, with MVFR more likely towards southeast MA, including the Cape and islands. Scattered showers developing as another low pressure traverses a stalled front. KBOS Terminal...High confidence in trends. Moderate confidence in timing. KBDL Terminal...High confidence in trends. Moderate confidence in timing. Outlook /Saturday Night through Tuesday/... Saturday: Mainly VFR, with local MVFR possible. Chance SHRA. Saturday Night: VFR. Chance SHRA. Sunday: VFR. Breezy. Slight chance SHRA, isolated TSRA. Sunday Night: VFR. Breezy. Monday: VFR. Breezy. Slight chance SHRA. Monday Night: VFR. Breezy. Chance SHRA, isolated TSRA. Tuesday: VFR. Slight chance SHRA, isolated TSRA.
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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. Persistent southerly flow will maintain rough seas on the outer coastal waters. Still looking like a period of gusts up to 25 kt later today ahead of a cold front as it crawls east across southern New England and the waters. This front is not likely to pass completely east of the waters through Saturday. Winds diminish some tonight, which will permit seas to subside overnight. Tweaked the timing of Small Craft Advisories. Outlook /Saturday Night through Tuesday/... Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, isolated thunderstorms. Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. Sunday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Sunday Night through Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, isolated thunderstorms.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Rip Current Risk until 8 PM EDT this evening for MAZ007- 020-022>024. RI...High Rip Current Risk until 8 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ231- 232-251. Small Craft Advisory until 8 PM EDT this evening for ANZ233>235-237. Small Craft Advisory until 1 PM EDT this afternoon for ANZ230- 236. Small Craft Advisory until 2 AM EDT Saturday for ANZ250-255- 256. Small Craft Advisory until 3 AM EDT Saturday for ANZ254.
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&& $$ SYNOPSIS...Belk/BW NEAR TERM...Belk/BW/KS SHORT TERM...Belk LONG TERM...BW AVIATION...Belk/BW/KS MARINE...Belk/BW

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