Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170229 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1029 PM EDT Wed May 16 2018 .SYNOPSIS... An area of light rain moves up the coast and across Southern New England tonight. A weak cold front sweeps south from Canada on Thursday, pushing any leftover rain offshore. Friday will feature cooler weather with showers moving in by the later half of the day. Expect widespread showers Friday night into Saturday night, with locally heavy rainfall at times. Warmer temperatures move in Sunday into early next week, though scattered showers will linger. Drier conditions may return next Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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10PM UPDATE... Lowering clouds and spotty rain showers will continue to move into the region overnight. Fog will likely overspread the region overnight due to light southerly-southeasterly winds, reaching as far north as Westfield and Boston. Hi-Res guidance show that visibilities could drop as low as 5 miles inland, and down to 1/2 mile across the Cape and Islands. Fog will burn off mid morning as skies clear north to south. Rain will continue into tomorrow, mainly across the southern coast, and move offshore by the afternoon. There is a chance that showers could linger over the Cape and islands as the stationary front remains to our southeast. Previous forecast remains on track. PREVIOUS DISCUSSION... The broader scale features a northern jet stream across Canada and a southern jet stream over the southern tier of the USA. The northern stream cold front that moved through Southern New England Tuesday night has stalled south of the region. A weak shortwave in the northern stream will swing through either late tonight or early Thursday. This will generate a wave along the front. Meanwhile a broad upper trough covers the Southeast USA and brings an upper flow up the coast from the Gulf of Mexico. This southerly flow will bring a new surge of moisture up the coast and overrunning the stalled front. This overrunning along with favorable jet dynamics from the northern stream will generate sufficient lift to generate a shield of light rain that moves across Southern New England. The rain will initially fall into dry surface air and evaporate. But eventually it will reach the surface. The best chance for measurable rain will be along the South Coast, with lower chances farther north. Dew points are 45-55, and precipitation will be falling. Expect min temps overnight in the upper 40s to mid 50s.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Another northern stream cold front, also associated with the northern stream shortwave, will enter Northern New England early Thursday and sweep south across Southern New England during the day. As it does so and winds shift out of the north, we would expect this to sweep any lingering light rain/showers offshore. High pressure builds south behind the cold front and brings partial clearing to our area. Best chance for clearing will be north of the Mass Pike. Temps at 850 mb are expected to be 12-13C. With full mixing this would support max sfc temps in the lower 80s, but any clouds would keep temps a little lower than this. We started with guidance values and added a couple of degrees. Clouds linger Thursday night, especially in CT-RI-SE Mass, so we will go with min temps similar to tonight. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Widespread, locally heavy rainfall Friday through Saturday night * Scattered showers and milder temps Sunday into Tuesday * Dry weather may return next Wednesday Details... Friday through Sunday...Moderate confidence. Subtropical ridging continues across the western Atlantic, with another high from Quebec into central/N Maine. A stationary front will sit S of the region Friday, then will slowly shift N Friday night through the weekend as the high slowly moves E and low pressure pushes NE across the Ohio Valley to the Great Lakes. A continued SW flow at the surface and aloft will keep a steady moisture feed moving up the eastern seaboard. A PWAT moisture plume, which is forecasted to be 2 to 3 SD above normal, shifts NE. Will likely see moisture training with showers that may be heavy at times. Also noting good instability moving across with passing weak short waves in the mid level flow. Can not rule out isolated thunder during the daytime hours both Saturday and Sunday. With cool E-NE winds in place as the front lies S of the region, expect temps to run 5-10 degrees below seasonal levels Friday and Saturday. As the boundary lifts N, as well as H5 heights which could up to 582 or so dm, should see temps end up a few degrees warmer than normal. Monday through Wednesday...Low to moderate confidence. Confidence lowers during this timeframe as the mid level steering flow across the northern stream becomes nearly zonal once again. The the steady westerly wind flow aloft, this will keep weak waves moving along, which will lend to lower confidence in the timing and track of individual systems. Went with a blend of available guidance to keep some continuity in place. May see some scattered showers from time to time Monday and Tuesday, then could see drying conditions and seasonable temperatures Wednesday. && .AVIATION /03Z THURSDAY THROUGH MONDAY/...
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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 Thursday Night/...Moderate confidence. Tonight...VFR conditions this evening/early tonight in light rain. Cigs/Vsbys then lower to MVFR late tonight. South Coast areas may lower to IFR, primarily due to cigs but possibly affected by areas of fog along the coast. Light wind. Thursday...Light rain tapers off early in the morning with conditions improving to VFR during the morning hours. Light winds in the morning turn from the north-northwest by afternoon. Thursday night... VFR. Clouds linger, especially along the South Coast. Winds turn from the northeast with gusts near 20 knots along the coast after midnight. KBOS Terminal...Moderate confidence in TAF. Main uncertainty is the timing of ceilings lowering to 2500 feet. Second concern is whether light rain will be able to affect visibility; for now we forecast that it will not. KBDL Terminal..Moderate confidence in TAF. Main uncertainty is the timing of ceilings lowering to 2500 feet. Second concern is whether light rain will be able to affect visibility; for now we forecast that it will not. Outlook /Friday through Monday/...Moderate confidence. Friday: VFR. Breezy. Chance SHRA. Friday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA. Saturday: Mainly MVFR, with areas IFR possible. Breezy. SHRA likely, isolated TSRA. Saturday Night: MVFR/IFR conditions possible. SHRA likely, isolated TSRA, patchy BR. Sunday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, isolated TSRA. Sunday Night: VFR. Slight chance SHRA. Monday: Slight chance SHRA.
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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 Thursday Night/...Moderate confidence. Winds below 20 knots and seas less than 5 feet tonight and Thursday morning. Winds increase Thursday night with gusts to 25 knots north of Provincetown. Seas slowly build, with 5 foot values expected on the outer waters Thursday night. Light rain tonight and Thursday morning, especially on the southern waters. Areas of fog should reduce vsbys below 3 miles overnight. Dry weather Thursday afternoon and much of Thursday night. Outlook /Friday through Monday/...Moderate confidence. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Local visibility 1 to 3 nm. Saturday: Winds less than 25 kt. Local rough seas. Rain showers likely, isolated thunderstorms. Saturday Night: Winds less than 25 kt. Local rough seas. Rain showers likely, isolated thunderstorms, patchy fog. Sunday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, isolated thunderstorms. Sunday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain showers. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Belk/Correia SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/Belk/Correia MARINE...WTB/EVT

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