Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 250159 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 959 PM EDT Thu May 24 2018 .SYNOPSIS... High pressure moves offshore tonight and Friday, bringing warmer weather on gusty southwest winds. A cold front drops south across the region late Saturday into Saturday evening and will bring scattered showers and thunderstorms followed by much cooler and unsettled weather Sunday with some improvement by Monday. Mainly dry and warmer weather returns for Tuesday through Thursday as high pressure moves in. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 10 pm update... */ Highlights... - Areas of dense fog for S/SE coastal RI and MA - Visibility as low as a quarter of a mile - Associated fog bank, expanding across the region towards midnight, pushing E and offshore by daybreak */ Discussion... GOES-16 nighttime microphysics tells the story, as does the 0z Chatham sounding. An area of pooled moisture trapped beneath a stout inversion just off the deck is lending to low clouds and/or fog. Moving with the mean wind flow, this deck pushed through late last night with NE winds pushing out towards Long Island as high pressure built across the Gulf of Maine. Now with the high S/E, seeing the start of return S flow and the mean wind shifting out of the W. Progressing towards morning, will see the associated fog bank expand and then begin to push off to the E towards daybreak. SPECIAL WEATHER STATEMENT for lower visibility, but given residency time and visibility holding for the most part above a quarter of a mile, will hold off on any dense fog advisories. A combination of high-res guidance including the NBM and HRRR has modeled the fog bank quite well, the basis for the going forecast. Otherwise, clear conditions expected tonight with light S/SW winds. Airmass remains dry enough to preclude valley fog. Lows should drop back into upper 40s to mid 50s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... * Some locations make a run at 90 degrees Friday * High pressure moves offshore Friday and Friday night with prevailing W/SW flow. Forecast soundings show deep mixing with decent 30-40kt low level jet, strongest near Cape Cod and Islands where strong inversion will be present. Thus, we should see 25-35 mph gusts across much of SNE with perhaps a few 40kt gusts across SE MA. Model 2-meter temperatures suggest highs easily reach into 80s inland, if not 90 degrees in lower CT valley, Merrimack Valley, and greater Boston, especially given gusty W/SW winds. Dewpoints drop into 40s (or even upper 30s) during the afternoon so it will just be a "dry heat" this time. Certainly not humid. Winds diminish Friday night but airmass remains warm in advance of frontal boundary still well to our north. Lows should drop back into 60s along with some mid and high clouds. Less confident on low clouds/fog returning to South Coast so this may need to be added in later forecasts. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Very warm and humid Sat with sct afternoon showers/t-storms * Much cooler with occasional showers Sun * Some improvement Mon but below normal temps * Mainly dry and warmer weather returns Tue through Thu Saturday into Saturday night... Very warm airmass in place Sat ahead of approaching backdoor cold front. 925 mb temps 22-23C supports highs upper 80s to perhaps some lower 90s interior, but cooler near the south coast due to SW flow. Dewpoints will be climbing into the 60s so it will be increasingly humid. This front will move south into SNE during mid/late afternoon reaching the south coast toward evening. Much cooler air will follow with ENE winds and sharply falling temps. Locations in NE MA may drop into the 60s by late afternoon and there could be some gusty winds here as well Sat evening as the cooler air rushes in. Environment destabilizes ahead of the front with MLCAPES approaching 1000 J/kg with moderate deep layer shear. However, mid level lapse rates are unfavorable and will likely by a limiting factor to widespread convection. Still expect sct showers/t-storms developing in the afternoon and lingering into Sat evening. Sunday into Monday... Much cooler airmass Sunday as high pres builds south from the Maritimes with NE flow. Models are also indicating a wave developing on the frontal boundary south of New Eng which will likely bring some rain at times with cool NE winds and temps potentially remaining in the 50s. Some improvement expected Monday with mainly dry weather but still cool with lots of clouds and onshore winds. Tuesday through Thursday... Mainly dry and warmer weather returns as mid level ridge builds into the region with rising heights. Weak fropa Tue but it should come through dry then high pres builds into the region Wed/Thu. Temps should warm back well into the 70s and some 80s. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Tonight... IFR-VLIFR FG bank over SE RI and MA, expanding towards 6z, will gradually push E with increasing W flow towards daybreak. Expect towards 12z only ACK may see some lingering lower VSBYs. Friday... Increasing SW winds immediately with sunrise. Gusts up to 25-30 kts through much of the day, sustained around 15 kts. SCT mid- decks. Friday night... SW winds taper but remain brisk over S/SE coast with sustained flow around 10 kts and gusts up to 20 kts. KBOS Terminal...High confidence in TAF. Will hold the fog bank just S of the terminal. It is possible it may edge in tonight close to 04-05z. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Tuesday/...Moderate confidence. Sunday: Mainly MVFR, with areas IFR possible. Breezy. Chance SHRA. Sunday Night: Mainly MVFR, with areas IFR possible. Slight chance SHRA. Memorial Day: Mainly MVFR, with local IFR possible. Slight chance SHRA. Monday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday through Tuesday Night: VFR. Wednesday: && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. 10 pm update... Low clouds and/or fog will remain an issue for the S/SE waters overnight. Expanding towards midnight, the associated fog bank will push E towards daybreak with increasing, steering W flow, the last traces over the SE outer waters by 8 am, exiting. Previous discussion... Small Craft Advisories posted for all waters for increasing SW winds tonight (NE MA waters) and Friday (remainder of MA/RI waters). S/SE flow this afternoon with local sea breezes. Should see some 15-20kt gusts along south coastal near shore waters with locally choppy seas due to incoming high tide. Fog and low clouds linger on waters south of islands. High pressure moving off the coast will bring increasing SW winds tonight and Friday. Enough of a low level jet tonight to bring marginal SCA conditions to NE MA waters, otherwise winds increase during day Friday across all waters with strongest gusts (30kt) near shore. Winds and seas gradually subside Friday night. Outlook /Saturday through Tuesday/...Moderate confidence. Sunday through Sunday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. Memorial Day: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night through Wednesday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...Air Quality Alert from 11 AM to 11 PM EDT Friday for RIZ001>008. MARINE...Small Craft Advisory from 10 AM Friday to 2 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 11 AM to 8 PM EDT Friday for ANZ230- 236-251. Small Craft Advisory from 11 AM Friday to 2 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 8 PM EDT Friday for ANZ250. Small Craft Advisory from 8 AM to 8 PM EDT Friday for ANZ254>256. && $$ SYNOPSIS...KJC/JWD NEAR TERM...KJC/Sipprell/JWD SHORT TERM...JWD LONG TERM...JWD AVIATION...KJC/Sipprell/JWD MARINE...KJC/Sipprell/JWD is the U.S. government's official web portal to all federal, state and local government web resources and services.