Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 291320 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 920 AM EDT SUN MAY 29 2016 .SYNOPSIS... A backdoor cold front will bring much cooler temperatures today along the eastern MA coast, but another hot afternoon is in store farther inland where hit and miss showers and thunderstorms are expected. Mainly dry weather is expected this evening, but bands of torrential rain showers along with isolated thunderstorms are expected very late tonight and Memorial Day. Dry conditions with warm afternoons follow Tuesday into Wednesday. Unsettled weather may return by the end of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 920 am Update... * Wide range in temperatures this afternoon * Showers/storms expected across interior Forecast is in good shape and only making minor changes for the afternoon. First concern is persistent deck of low clouds which were covering much of region this morning...roughly as far south and west as North Adams, Willimantic, Providence, Taunton, and Plymouth which is associated with the back door cold front. High-res models suggest front has made it about as far inland as it will get today. The southern edge of the low cloud deck has shown some erosion over the past hour as the sun goes to work, and due to it being fairly shallow (according to GOES-E experimental cloud thickness and aircraft soundings), it should continue to erode on its southern fringes through the afternoon with breaks developing closer to the coast. This will result in a wide range in temperatures this afternoon, from the lower 70s along the immediate coast to around 90 across parts of the Connecticut River Valley. We anticipate having to make some further tweaks as even high-res guidance will have difficulty pinpointing temperatures through the afternoon. Our other concern is with potential for showers and thunderstorms across the interior. Already seeing some activity break out across central NY where airmass is more unstable. Greater instability in SNE (CAPE 1000-1500 j/kg) is expected to be located across western and central MA into northern CT, especially closer to the Berkshires. High-res models and ensembles (SPC SREF and SSEO) suggest showers and storms will develop in these areas early this afternoon before dissipating toward sunset. Potential for organized severe weather is low given very marginal 0-6km shear of 20-25kt and meager mid level lapse rates. Main hazards are brief downpours and lightning, and perhaps one or two storms that produce 40 mph gusts. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... ***Bands of torrential rain showers with isolated thunderstorms develop very late tonight and continue through Memorial Day*** ***Potential for localized flash flooding*** Tonight and Memorial Day... Convection across the interior will quickly dissipate early this evening with the loss of daytime heating/instability. Otherwise, much of the evening should be dry with perhaps just a spot shower or two. May see some patchy fog develop especially near the south coast. Things then become interesting very late tonight and Memorial Day. Tropical Storm Bonnie off the South Carolina coast will remain well to the south of our region. While Bonnie will not directly impact us, its plume of tropical moisture will get pulled northward out ahead of an approaching cold front. PWATs will approach or even exceed 2 inches, which is near record high levels for this time of year based on historical climatological upper air data. These extremely high PWATs will combine with some elevated instability coupled with a low level jet of 30 to 40 knots. The result will be bands of rain showers with torrential rainfall along with isolated thunderstorms. This is one of those scenarios that has the potential to result in very localized 2+ inch rainfall amounts in less than one hour. This will bring the potential for localized flash flooding, particularly if the heaviest bands move across our more vulnerable urban centers. The problem is that this will come down to a lot of mesoscale processes and its uncertain where the highest risk for this will occur. We actually may see two main areas with the heaviest rainfall. The first may occur with the shortwave that moves across interior southern New England overnight into Monday morning. There is also the potential for a second area across RI/southeastern MA on the nose of the low level jet. Depending on how the timing works out, may see bulk of heavy rain threat shift to the coastal plain by mid to late Monday afternoon. If this occurs there may be a small window for destabilization and a few surface based thunderstorms to move into far western MA early Monday evening. Instability/shear do not look to be enough for severe weather, but low risk for an isolated strong storm. So to sum up, main concern will be bands of torrential rain showers/isolated thunderstorms with a localized flash flood threat, particularly if torrential rain bands setup across our more vulnerable urban centers. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Lingering showers & thunderstorms SE MA & RI Mon Night * Mainly dry with above normal temperatures Tue/Wed * Unsettled weather may return toward next weekend Medium range models are in good agreement on the overall synoptic pattern. There are points of uncertainty, including how Tropical Storm Bonnie and her remnants interact with the pattern at large as well as the usual uncertainties further out in time. A mid-level ridge will move over southern New England Tuesday and then move offshore Wednesday. Mid-level troughing then moves into Quebec towards the end of the work week and starts influencing our weather with several shortwaves working around the base of the trough. This will result in more unsettled weather towards the end of the week. Monday night...Showers and isolated thunderstorms will linger across southeastern Massachusetts and Rhode Island through the evening hours before they diminish or move offshore. Clearing conditions are expected through the remainder of the night. Tuesday and Wednesday...Mid-level ridge moves over southern New England. This will bring dry weather to much of the area. A dry cold front will move through southern New England Tuesday, resulting in a wind shift to the north. This is followed by a backdoor cold front on Wednesday. Temperatures will be above normal through both days. Thursday through Saturday...Mid-level trough moves into Quebec allowing several shortwaves rotating through the base of the trough to move through southern New England. This will result in periods of showers throughout this period. Thus have continued with chance PoPs through this period. Temperatures will be around or just below normal for much of the time. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...Moderate Confidence. IFR ceilings extending from E MA coast into much of central MA and northern RI will slowly lift to MVFR through early afternoon, with VFR ceilings away from coast. Showers and thunderstorms are expected to develop across western MA and possibly northern CT from 17z to 23z. Storms may bring IFR visibilities for a brief time and possibly gusts to 35kt. Tonight and Monday...While a period of VFR conditions may occur for a few hours this evening, low clouds and fog patches should result in low end MVFR to IFR conditions developing across most locations during the late evening/overnight hours. Bands of heavy rain showers/isolated thunderstorms will then overspread the region very late tonight into Memorial Day. KBOS TAF...Moderate confidence. Ceilings should lift to MVFR around midday, possibly to VFR later this afternoon and evening. KBDL TAF...High confidence. Main time frame for any TS is 19-21z. Outlook /Monday Night through Thursday/... Monday night...Moderate confidence. MVFR/IFR conditions possible early in SCT -SHRA/TSRA especially across SE MA and RI. Improving conditions after midnight as any lingering -SHRA/TSRA moves offshore. Tuesday and Wednesday...Moderate confidence. VFR. Thursday...Moderate confidence. Mainly VFR conditions expected. Increasing clouds with diminishing conditions through the night, particularly along the south coast. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Winds and seas should generally remain below small craft advisory thresholds through Monday, but seas may approach 5 feet across our far outer-waters by Monday afternoon. Main concern though for mariners will be for areas of fog, particularly tonight into Monday across our southern waters. In addition, heavy showers and isolated thunderstorms will result in poor visibility. Outlook /Monday Night through Thursday/... Monday night and 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. Visibilities may be limited at times Monday night in both fog and showers and thunderstorms. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/RLG NEAR TERM...JWD SHORT TERM...Frank LONG TERM...RLG AVIATION...Frank/RLG/JWD MARINE...Frank/RLG

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