Area Forecast Discussion
Issued by NWS Boston, MA

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262 FXUS61 KBOX 292334 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 734 PM EDT SUN MAY 29 2016 .SYNOPSIS... Bands of heavy rainfall and isolated thunderstorms are expected overnight into Memorial Day, especially closer to the South Coast. Dry conditions with warm afternoons follow Tuesday into Wednesday. Unsettled weather may return by the end of next week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... * Patchy dense fog tonight, especially near South Coast * Bands of heavy rain arrive late tonight Rainfall chances have diminished across southern New England this evening. Stratus deck associated with the backdoor cold front has eroded slightly along the western edge. Main changes to the forecast this evening were to bring the forecast back in line with observed trends. Low clouds will begin to fill back in tonight as flow becomes more southerly. Increasing low level moisture should also bring patchy dense fog, especially near South Coast. We can already see low clouds/fog south of islands which was slowly heading north. Our attention turns to the south later tonight as moisture from what was once Bonnie gets drawn northward as a cold front approaches from the west. Just about all guidance brings an area of showers and isolated thunderstorms into SNE after midnight from combination of an approaching short wave and on the nose of a low level jet. We should see these showers and isolated storms arrive roughly between 2 and 5 am. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... * Localized flash flooding Memorial Day, esp near S Coast * Few storms possible Monday afternoon, mainly in W MA Although confidence in the exact location remains low, pattern favors locally heavy rainfall in parts of SNE Memorial Day due to remnants of tropical system combined with approaching front and strong low level jet offshore. Precipitable water values will climb to around 2 inches, which as noted previously is near record high levels for this time of year. This scenario could bring rainfall rates as high as 2-3 inches per hour in a few locations, possibly resulting in rapid urban flooding. 12z models and ensembles are showing greater potential for this to occur near the South Coast. However, it is likely that model resolution is not resolving these small-scale features very well at this time frame, so the location projected by the models may not be accurate. It is possible that axis of heavier rainfall sets up closer to northeast CT, Providence, and Boston or even ends up farther offshore south of Cape Cod and the Islands. We probably won`t know for sure until later tonight when we get within 12 hours of the heaviest rain. As a result, we will hold off on issuing any Flash Flood Watches at this time, since confidence is low. It does appear that the showers will end up focusing across southeast New England Monday afternoon, meaning there may be some drying (and brief clearing) across the interior. Should that occur, it`s possible that a few thunderstorms could reach western MA by evening. However, the threat for severe weather looks minimal given weak instability and shear expected to be in place. Cold front will move through the region Monday night, pushing any remaining showers offshore and ushering some drier air into SNE. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Mainly dry, with above normal temperatures Tue/Wed * Unsettled weather may return toward next weekend 29/12z guidance remains in good overall agreement over this portion of the forecast. There are typical timing differences of smaller scale features. Will go with a consensus approach, as predictability of these smaller details is often low. Tuesday and Wednesday...Mid-level ridge moves over southern New England. This will bring dry weather to much of the area. A weak cold front should continue to move offshore Tuesday, with perhaps some lingering showers toward the south coast, Cape and islands. Another backdoor cold front tries to move into our region Wednesday. Temperatures will be above normal through both days, especially across the interior. More seasonable temperatures are expected Wednesday along the east coast of MA due to the onshore flow. Thursday through Sunday...A mid-level trough moves into Quebec, allowing several weak shortwaves to pass by to our north. This will result in periods of showers throughout this portion of the forecast. Not expecting a complete washout. Kept a broad period of chance PoPs, but not expecting it to be raining the entire time. Greatest risk for showers will be ahead of a passing cold front sometime Friday into Saturday. && .AVIATION /23Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...Moderate Confidence. Tonight through Monday Night...IFR and even LIFR becomes widespread as bands of showers/isolated storms approach from the south. Activity should be more numerous 09z-18z Mon, especially closer to the South Coast, before it begins to push offshore later Mon. It is also possible that a few showers or storms reach western New England from eastern NY late Mon afternoon or evening, but confidence is low. KBOS TAF...Moderate confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Tuesday through Friday/... Tuesday and Wednesday...Moderate confidence. VFR. Low probability for IFR conditions Wednesday toward the east coast, as a backdoor cold front tries to move inland. Thursday...Moderate confidence. Mainly VFR conditions expected. Increasing clouds with diminishing conditions through the night, particularly along the south coast. Friday...Moderate confidence. Mainly VFR conditions expected. Scattered MVFR in showers. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. Winds and seas should generally remain below small craft advisory thresholds through Monday, but seas may approach 5 ft on waters east of Cape Cod Monday afternoon. Main threat will be patchy dense fog this evening into Monday, mainly on the south coastal waters. Showers with heavy rain will reach all waters late tonight and last into Monday. We should see building seas to 5-7 ft Monday night on the south coastal waters, as swells from what was once Bonnie arrive in our area. Small craft advisories may be needed Outlook /Tuesday through Friday/... Tuesday...Moderate confidence. Some uncertainty on wave heights with swell from Tropical Storm Bonnie potentially moving up into the coastal waters. At this point there is potential for 5 foot seas, especially on the outer waters. Southwesterly winds remain below 20 kts, shifting to the north Tuesday night. Wednesday...Moderate confidence. Northeasterly winds and seas will remain below Small Craft Advisory criteria. Thursday...Moderate confidence. Easterly winds and seas increase. Seas may rise above 5 feet, especially on the outer waters. Friday...Moderate confidence. Persistent easterly winds less than 20 kt will likely keep rough seas across the outer coastal waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/JWD NEAR TERM...Belk/JWD SHORT TERM...JWD LONG TERM...Belk AVIATION...Belk/JWD MARINE...Belk/JWD is the U.S. government's official web portal to all federal, state and local government web resources and services.