Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 202250 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 650 PM EDT Tue Sep 20 2016 .SYNOPSIS... High pressure building across the region will result in cool nights and warm afternoons through Thursday. A cold front sweeps south from Canada Friday with a risk of showers, followed by cool weather with a taste of fall for the weekend. Another cold front may cross New England either Monday or Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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650 pm update... Low clouds over the Cape and Islands are slowly moving off to southeast. Meanwhile cirrus cover lingers across much of region. Expect the cirrus to linger early tonight, then move off. This update makes minor adjustments to temps/dewpts/sky cover, but otherwise the forecast is unchanged. Previous discussion... ***Areas of fog expected to develop late tonight into early Wednesday morning some of which may be locally dense*** Weak high pressure will build across the region tonight. This should allow skies to range from mostly clear to partly cloudy across most of southern New England. The exception will be portions of the far southeast New England coast, where light/moist onshore flow may keep low clouds in place across the Cape/Islands. The biggest concern overnight will be for areas of fog developing late tonight with light winds/relatively high dewpoints. Some of the fog may be locally dense in the typically prone locations. Will go ahead and issue a special weather statement to highlight this potential late tonight into early Wednesday morning. Low temps will mainly be in the 50s, to the lower to middle 60s in the urban heat islands of downtown Boston/Providence as well as the Cape and Islands.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... High pressure in control will result in a beautiful late September afternoon. High temps should recover to between 80 and 85 Wednesday afternoon along with plenty of sunshine/light winds along with low humidity. Wednesday night... High pressure will remain anchored over the region Wednesday night. The dry airmass in place will set the stage for an ideal night of radiational cooling. Low temps will bottom out in the upper 40s to the lower 50s in the normally cooler outlying locations, to the upper 50s and lower 60s in the urban heat islands of downtown Boston/Providence. && .LONG TERM /THURSDAY THROUGH TUESDAY/... 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. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...Moderate to high confidence. Low clouds on Cape Cod and Islands will move off to the east early tonight. The rest of the region should see mainly VFR conditions. Light winds and moist air will allow patchy ground fog to develop in the typically prone locations resulting in localized IFR/LIFR conditions. Most favored should be the interior river valleys and the South Coast/Islands. 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... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight through Wednesday...High confidence. High pressure building in will result in a weak pressure gradient. This will keep winds and seas well below small craft advisory thresholds and nice late season boating weather through Wednesday. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Frank NEAR TERM...WTB/Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Frank is the U.S. government's official web portal to all federal, state and local government web resources and services.