Area Forecast Discussion
Issued by NWS Boston, MA

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352 FXUS61 KBOX 201106 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 706 AM EDT Sun Aug 20 2017 .SYNOPSIS... A cold front will move offshore this morning, bringing less humid air into the region today and Monday. High pressure offshore brings a warm southwest flow on Tuesday, along with dry weather. The returning warm air triggers showers Tuesday night, with showers and thunder Wednesday as a cold front sweeps through New England. High pressure brings dry weather Thursday through Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Drier air is moving in from the west. Dew points still around 70 on the Cape and Islands, but around 60 in the Connecticut Valley. Expect this trend to continue. Plenty of sunshine expected today as upper flow flattens out and column remains fairly dry. Forecast soundings show rather deep mixing this afternoon, which will bring some 20-25 mph gusts over land and also transport drier air down, as dewpoints fall into 50s. Given prevailing W/NW flow, went a little higher than guidance with highs in mid to upper 80s for much of area, even Cape Cod.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Clear skies and diminishing winds will allow temperatures to drop off quickly after sunset. Stayed closer to NAM/GFS MOS for lows tonight which tend to perform best, which gives lows in 50s and lower 60s. High pressure moves offshore Monday, resulting in S/SW flow developing during afternoon. Gradient may remain weak enough for a few hours of sea breezes along E MA coast, but even if that occurs it should erode by mid afternoon. Once again column remains fairly dry, aside from some high clouds, so those looking to get a view of solar eclipse shouldn`t have any issues (just remember to wear safe glasses!) Airmass remains similar to today so expect highs once again to reach mid to upper 80s, but it will be a little cooler near South Coast, Cape, and Islands with more of an onshore flow. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Big Picture... Zonal flow amplifies over the Northern USA and Canada during mid and late week, replacing warm air early in the week with cooler than normal air late week. GFS and ECMWF mass and thermal fields are similar through the period, although with small differences on Saturday. Meanwhile the GGEM develops a tropical system that moves up the USA East Coast mid to late week. Satellite imagery does show a disorganized system north of Puerto Rico moving west, but the GGEM is the only model to develop it into a threat to the East Coast while the other models develop no system. This will be monitored, but for now left out of our long range forecast. Details... Monday night and Tuesday... High pressure shifts offshore with a developing west to southwest flow over Southern New England. Moist layer between 800 and 850 mb suggests some low cloud formation under a cirrus canopy. The lack of deep moisture, including dry air below the moist layer, will work against shower formation. Data does show a developing theta-e ridge along the New York border, and this may provide a focus for a few showers to form late Tuesday in our far west. Precipitable water values rise through the day. The core of high values will remain west of us in New York, but values of 1.5 to 1.75 inches arrive during the afternoon. Expect increasing humidity during the day, with dew points climbing to the upper 60s and near 70. Temps at 850 mb reach 18C, which with full heating would support max sfc temps either side of 90. Tuesday night and Wednesday... Upper shortwave moving up the St Lawrence Valley will swing a cold front across the Northeast USA. Consensus of timing would bring the front across New York State Tuesday night and across Southern New England on Wednesday. Deep moisture along with PW values of 2+ inches will be acted on by favorable dynamics in the right entrance region of the upper jet. This suggests potential for strong thunderstorms along with locally heavy downpours, especially Wednesday just ahead of the front. Thursday through Saturday... High pressure builds in at the surface while an upper trough digs south from Quebec. The trough will carry a cold pool with cold advection aloft on Thursday and Friday. Temperatures at 500-mb will minimize at -16C to -18C early Saturday morning. Moisture fields show a moist layer between 850 and 700 mb during this period, but dry air above and below. This looks like a partly cloudy and dry pattern, but the instability shows some potential for a few showers. The mixed layer reaches to between 850 and 800 mb, with temps at the top of the layer supporting max sfc temps in the 70s. && .AVIATION /12Z 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 /Today through Monday/... VFR through the period with northwest winds today coming around from the west-southwest Monday. Winds this afternoon will gust to 15-20kt near South Coast. Low chance of a sea breeze along East Coast for a few hours early in afternoon, but right now we are more confident in no sea breeze occurring. Light winds tonight with patches of IFR in fog, most likely in the CT Valley. Winds again gusting 15-20 kt near the South Coast Monday. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Generally VFR through the period. Areas of IFR cigs/vsbys in fog late Monday night and early Tuesday. Areas of MVFR cigs/vsbys in showers/thunderstorms Tuesday night and Wednesday. Areas of IFR cigs/vsbys in fog Wednesday night and early Thursday.
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&& .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 below Small Craft Advisory levels. Cold front moving offshore this morning will bring W/NW winds to coastal waters today. Should have some 15-20kt gusts nearshore this afternoon, along with choppy seas, before winds diminish this evening. High pressure becomes centered over region tonight before heading offshore Mon with S/SW flow prevailing. Gradient may weaken enough to allow for sea breeze along East Coast Mon afternoon, but chances seem low right now. Outlook /Monday Night through Thursday/... Monday night-Tuesday...High confidence. High pressure shifts east of the waters. Winds 20 knots or less, and seas 4 feet or less. Tuesday night-Thursday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday or Wednesday night. Showers and scattered thunderstorms will move across the waters Tuesday night and Wednesday. Southwest winds increase with gusts 20 to 25 knots. Winds shift from the northwest Wednesday night and diminish to 15 knots or less. Seas build Tuesday night and Wednesday reaching 5 to 6 foot heights on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. Once the cold front moves through and winds become northwest, seas will subside to 3 feet or less. A Small Craft Advisory may be needed on some of the waters Tuesday night and Wednesday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/JWD NEAR TERM...WTB/JWD SHORT TERM...JWD LONG TERM...WTB AVIATION...WTB/JWD MARINE...WTB/JWD is the U.S. government's official web portal to all federal, state and local government web resources and services.