Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221123 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 723 AM EDT SUN MAY 22 2016 .SYNOPSIS... Coastal low pressure brings wet weather today as it passes well offshore of Nantucket. The low continues on to Nova Scotia tonight. We get a break in the wet weather tonight and Monday. A second low pressure then moves up the coast and brings more scattered showers Monday night and Tuesday. A significant warmup will occur Wednesday...continuing into next weekend. Scattered showers or thunderstorms are possible from Thursday into the weekend, especially in western sections. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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720 AM update... Steady rain has exited Cape Cod and the Islands. Elsewhere light rain in southwest CT is nearly stationary. Models show that it will pivot northwestward during the day. In between, we may begin to even see a few breaks in the overcast cloudiness. Have adjusted PoPs downward significantly for this morning over southeast MA, now that the main rain area has moved off the coast. Also, made slight adjustments to wind directions, which have become more N to NNE over inland areas, behind the departing low pressure. Previous discussion... Low pressure was centered about 200 miles south of Providence overnight and moving northeast. Radar showed the rain shield from this weather system over Cape Cod and Islands and also trending northeast. Meanwhile showers were moving across Connecticut and Western/Central MA. These are tied in with the upper trough and cold pool which hangs back across New York and PA. The coastal low will pass about 125 miles southeast of Nantucket around midday. Expect light rain to continue over the Cape and Islands until this passage. Also potential for drizzle or sprinkles in Eastern MA through the morning. The potential then diminishes this afternoon as the low moves off. Showers from the NY trough will remain a concern in Western sections this morning. Lapse rates linger around 6.2C/KM this afternoon but stability parameters remain mediocre at best. So the chance of showers in the west should diminish this afternoon. Northeast surface flow off the water will slowly turn north this afternoon but should be enough to keep eastern areas in the 60s. Farther west the light flow and breaks of sun with 6-7C temps aloft supporting surface temps around 70. There is a high risk of rip currents today into this evening along the eastern portions of Cape Cod and Nantucket, as well as along the south coast, from Bristol County, MA to Westerly, RI.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Upper closed low settles in to place centered over Virginia with the closed circulation extending up the coast to New England. This circulation along with plently of moisture between 850 mb and 700 mb should maintain at least partly cloudy skies Sunday night and Monday. Dew points in the 40s should keep surface temps in the mid 40s to lower 50s. The sunshine that does occur on Monday should bring surface temps into the upper 60s and lower to mid 70s inland. A developing northeast wind will move ashore on the Cape and Islands and East Coastal Massachusetts and keep max surface temps in the upper 50s and 60s there. Upper jet moving around the closed low will move the associated surface low north on Monday. The favorable left exit region of this jet, which will support lift and shower development, will also move north. Current model information holds this feature south of New England through much of Monday. It may come close enough to bring a few showers to the South Coast late in the day. Most if not all of the showers should wait until Monday night. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Good chance of showers Tuesday * Significantly warmer Wednesday into next weekend * Scattered showers and thunderstorms Thursday into the weekend, especially in western sections. Overview and Model Preferences... Upper level low pressure passes northeast from the Mid-Atlantic coast, reaching the Gulf of Maine Tuesday night. By Thursday and Friday, and continuing into next weekend, a strong upper level ridge develops over the Mid- Atlantic states...extending northward into western New England. In general, models are in good agreement at the upper level features but differ somewhat on timing and location of possible convection later in the week. All agree on a major warmup to summertime levels. There will also be increasing moisture. A warm frontal boundary could provide the focus for scattered thunderstorms Thursday as it moves northward. Scattered showers or thunderstorms will be possible during the afternoons and evenings into the weekend. Details... Monday night into Tuesday... Core of the upper level cold pool moves over southern New England Mon night into Tue...with -18C at 500 mb...which is cold but not that extreme. Models are now in better agreement on the track of surface low pressure, which moves between outer Cape Cod and the benchmark of 40N 70W on Tue. The ECMWF, which had it moving into RI, is thus a bit less unstable than it was. We will need to keep an eye on the possibility of some nocturnal thunderstorm activity for southeastern MA and RI Mon night. All models show a rapid influx of unstable air into southeastern sections between midnight and daybreak Tue. Total Totals Indices reach 55 and there is an indication of very unstable lapse rates of 7C from 700-500 mb. 0-6 km bulk shear values increase to 45-55 kts from the south-southeast at that time, too. However, with cool northeast surface flow, there is no Convective Available Potential Energy (CAPE). This is not the usual situation to expect a chance of thunderstorms, but we have veering of wind with height (WAA) and increasing speeds with height... so, could see some organized, but scattered and elevated thunderstorms, especially RI and southeast MA late Mon night. Scattered showers or an isolated thunderstorm will gradually move away by Tue evening. Highs Tue will hold in the 65 to 70 range due to cloud cover and scattered showers. Wednesday... Brief ridge of high pressure to our south takes over, leading to downslope air flow, drier conditions, and more summer-like temperatures with highs in the lower 80s away from the coast. There could be sea breezes, but it appears the synoptic flow could be just strong enough to prevent lower temperatures along parts of the MA east coast. Cape Cod and the islands, as well as Cape Ann, however, will likely hold in the upper 60s to lower 70s. Thursday through Saturday... On Thursday, 925 temps warm to +21C, so am forecasting mid 80s in the CT valley and 80-85 elsewhere, cooler at the coast. A warm front extending eastward from low pressure in western NY may provide the focus for scattered showers and thunderstorms Thu afternoon and evening. The GFS shows an axis of 36 K Index, indicating a lot of moisture and instability, extending eastward along the MA/CT/RI border by 00z. ECMWF, however, keeps it all to our west. So will go with chance PoPs for now. Warm weather with highs in the 80s are expected Friday and Saturday with a front mainly north of the area. Diurnally-driven showers and thunderstorms are possible both days, especially across the western half of southern New England. && .AVIATION /11Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Today... Moderate Confidence. 720 AM update... Still some lower ceilings over Cape Cod and the Islands and portions of southeast MA. Otherwise a majority of the region is covered with mainly mid level clouds...VFR. Expecting improvement to VFR by afternoon. Gusty winds over Cape Cod and Islands may reach 25-30 knots for a time this afternoon/evening. Tonight... Moderate Confidence. Mainly VFR. Potential for MVFR from low ceilings on Cape Cod and Islands. Monday... Moderate Confidence. VFR with potential patches of MVFR in showers. North flow turns from the Northeast during the day especially along the East Massachusetts coast and across Cape Cod and Islands. This will bring a low chance of MVFR cigs/vsbys in light showers. A more substantial area of showers will move up from the south during the day, reaching the South Coast toward evening. KBOS TAF...Moderate confidence in TAF. KBDL TAF...High confidence in TAF. Outlook...Monday night through Thursday Monday night into Tuesday...Moderate Confidence. Although not a complete washout, scattered showers will continue across the area with northeasterly flow. An isolated thunderstorm is also possible. This also suggests low clouds. Therefore, periods of MVFR/IFR are likely, but it`s possible that occasional VFR will occur, especially across the interior. Wednesday...High Confidence. Mainly VFR. Thursday...Moderate Confidence. Mainly VFR. Scattered MVFR in scattered showers and thunderstorms, mainly west of KLWM-KORH-KIJD.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today... Moderate confidence. Low pressure passes about 125 miles southeast of Nantucket early afternoon. This will bring light rain to the southern waters this morning. Winds will increase from the northeast toward midday and afternoon with gusts 25 to 30 knots. Seas will build today with 5 to 9 feet on the outer waters and the RI waters. Small Craft advisory continues for most of our waters through the day. Tonight... Moderate confidence. Low pressure moves off across Nova Scotia. This will turn winds from the north with diminishing speeds. But seas will linger at 5 to 9 feet, mainly across the outer waters. Small Craft advisory continues on most waters due to seas. Monday... High confidence. North winds diminish in the morning. Lingering 5 to 7 foot seas on the Eastern Outer Waters. Low pressure moves up from the south during the late afternoon and spreads showers across the Southern waters with briefly low vsbys. This will also turn winds from the northeast, but speeds will remain at or below 20 knots. Small Craft Advisory will continue to be needed on the Eastern Outer Waters. Outlook /Monday Night through Thursday/... Monday night and Tuesday...Moderate Confidence. Winds still remain below 25 kt, but a second approaching low pressure will allow for building southerly swell. Seas may reach 7 to 8 feet south of Nantucket. Small craft advisories may need to continue for some waters. Wednesday into Thursday...High Confidence. High pressure briefly builds over the waters. Mainly quiet boating weather is expected with winds and seas below Small Craft Advisory levels. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Rip Current Risk through this evening for MAZ020-022-024. RI...High Rip Current Risk through this evening for RIZ006>008. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ231>234. Small Craft Advisory until 5 AM EDT Monday for ANZ235-237. Small Craft Advisory until 6 PM EDT Monday for ANZ250-254>256. Small Craft Advisory until 6 PM EDT Monday for ANZ251.
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&& $$ SYNOPSIS...WTB/GAF NEAR TERM...WTB/GAF SHORT TERM...WTB LONG TERM...GAF AVIATION...WTB/GAF MARINE...WTB/GAF

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