Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201852 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 252 PM EDT Tue Sep 20 2016 .SYNOPSIS...
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Partial clearing will gradually overspread most areas through early evening, except for the Cape and Islands where clouds will hang tough. High pressure builds into the area tonight and Wednesday. High pressure brings dry weather Thursday. A cold front sweeps south from Canada Friday with showers, followed by cooler weather for the weekend. A cold front may cross New England either Monday or Tuesday.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 155 pm update... Trapped low level moisture and only a very slow push of drier air into the region has allowed clouds to hang tough across much of the region into mid afternoon. Finally, starting to see skies become partly to mostly sunny north of the MA turnpike and should see this clearing trend slowly push southeast into portions of RI/southeast MA by late afternoon. However, the immediate south coast, Cape, and Islands will likely remain in the clouds through early evening. A few showers may even graze portions of the Cape and Islands, with the best shot across Nantucket. Much of the region still had temps in the 70s at mid afternoon, but late afternoon/early evening high temps will likely reach between 80 and 85 inland from the coast once some sun emerges. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Tonight ... weak high pressure builds into the region. However with dew pts slow to fall...light winds/mostly clear skies and dew pts higher than expected low temps...anticipate widespread fog. Remaining warm with lows 60-65...few upper 50s western MA. Wednesday ... drier/less humid airmass overspreads the region with dew pts falling into the 40s! Still warm with highs in the upper 70s low lower 80s. && .LONG TERM /THURSDAY THROUGH TUESDAY/...
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Big Picture... Longwave ridge remains in place over the West Atlantic midweek transitions late week as a ridge builds over the Plains and Western USA while a Hudson Bay trough digs over the East Coast and Maritimes. Shortwave pattern shows flat west-northwest flow midweek along the Northern Tier of states while subtropical ridge extends from the West Atlantic to Texas and low pressure sits along the Southeast Coast. Upper low digs over the Western USA with high pressure slipping over the top into Western Canada. This creates a brief Rex Block configuration that shifts the flow of energy into Northern Canada,which in turn dislodges closed upper low from the Hudson Bay area and nudges it southeast over the Maritimes and Northeast USA. Expect above normal heights through Friday when the trough moves down from Canada. With the trough overhead expect below normal heights over the weekend into Tuesday. So a warm midweek followed by much cooler temps starting this weekend. Details... Thursday... High pressure moves offshore but still provides dry weather for Southern New England. Meanwhile increasing south flow over the Great Lakes and Northern NY forms a front near the Canadian border. best chance of showers will be along and north of this front, near the most favorable jet dynamics. Fair weather with a light pressure gradient, which would allow sea breezes along the coastlines. Dew points in the upper 40s and 50s will mean comfortable air. Temps at 850 mb will be 13-14C, which fully mixed would support max sfc temps 80-85F. Friday... Approaching trough from Hudsons Bay will push the northern cold front south through our area, mainly during the afternoon/evening. Models hint at 500-1000 Joules of SBCAPE but placed in different parts of the region, with the GFS across CT-RI and the remaining models across Western/Central MA. Other parameters are similarly distributed with totals near 50 and LI 0 to -2. Available moisture increases during the day with PW max values around 1.5 inches. Based on this, expect a typical convective scenario with periods of sunshine as well as a cloudy period as the showers move through. Convective parameters are sufficient to include a chance of thunder. As for precipitation amounts, the available moisture is elevated but not excessive. Based on this and the fact we are in a drought, expectation is for a modest amount of rainfall. Even so, that modest rain could come in a brief heavy downpour. Due to the clouds the mixing may not be as high, but the temps that are mixed should support max sfc temps either side of 80. Winds turn from north behind the front, possibly northeast. The pressure gradient looks sufficient to maintain sustained winds through Friday night. Saturday-Sunday... Upper trough digs over the Maritimes with a northwest upper flow and colder temps aloft across New England. Temps at 500 mb will reach -20C, colder in Eastern MA. This run of the models is showing additional cloud level moisture in the cold pool. Temps at 850 mb will be 2-4C Saturday, and 0C Sunday. This will mean max sfc temps in the upper 50s and 60s during this period. Dew points will fall into the mid and 30s. As winds diminish under approaching high pressure, probably Sunday night, temperatures may fall close to dew point values and allow patchy frost in the sheltered parts of the interior. Winds in the mixed layer will be up to 25 knots, most of that able to reach the surface in daytime wind gusts. Meanwhile ocean temps will be 18-20C, bringing a 16-20C temperature differential over the waters. Chance of ocean effect clouds and showers along the Eastern MA coast, especially Cape Cod. Monday-Tuesday... High pressure builds over the region. Meanwhile a shortwave moves across central Canada and the USA Northern Tier. The GFS and ECMWF show different timing with these features. The GFS brings the trough through New England on Monday, the ECMWF late Tuesday. This means a chance of showers on one of these two days. At this stage we favor the slower ECMWF but with low confidence.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Rest of this afternoon....Moderate to high confidence. VFR conditions north of the Pike will slowly push south through early evening, but expect MVFR-IFR conditions to hang tough on the south coast, Cape and Islands. Tonight...Moderate to high confidence. IFR conditions in areas of low clouds and fog are likely across the Cape/Islands with moist light southwest flow. The rest of the region should see mainly VFR conditions, although patchy ground fog should develop late in the typically prone locations resulting in localized MVFR-IFR conditions. Wednesday and Wednesday night...High confidence. VFR conditions expected after any patchy ground fog burns off by mid morning Wednesday. KBOS TAF...High confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Mainly VFR but may have some localized MVFR-IFR conditions in patchy ground fog late tonight into early Wednesday morning. Outlook /Thursday through Sunday/... Moderate confidence. Mainly VFR through the period. Areas of MVFR cigs and vsbys in showers and thunderstorms Friday afternoon and night. Winds increasing from the north or northeast on Saturday.
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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. Today ... showers across the southern waters...local downpours possible. Fog will limit vsby at times. Light winds. Tonight ... west to southwest winds improve vsby. Any evening showers move offshore overnight. High pres builds over the waters. Wednesday ... High pres over the waters yielding tranquil boating weather. Outlook /Thursday through Sunday/... Moderate confidence Thursday... High pressure builds offshore. Winds below 10 knots, seas below 4 feet. Sea breezes likely along the coast. Friday... Cold front drops south across the waters Friday evening and night. Showers and isolated thunder possible late Friday and Friday night with brief poor vsbys. Southwest winds Friday 15 knots or less, becoming North to Northeast Friday night with gusts to 20 knots. Seas 4 feet or less Friday. Seas 5 to 6 feet on the exposed waters Friday night. Saturday-Sunday... North to Northeast winds with gusts 20 to 25 knots. Seas up to 4 feet on the nearshore waters and 5 to 6 feet on the exposed waters. Ocean effect clouds/showers possible on the waters around Cape Cod.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Frank/Nocera NEAR TERM...Frank SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Nocera

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