Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 250617 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 217 AM EDT Fri May 25 2018 .SYNOPSIS... High pressure moving offshore, bringing warmer weather on gusty southwest winds for today. A cold front drops south across the region late Saturday into Saturday evening, bringing 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 /THROUGH TODAY/... 2 am update... */ Highlights... - Low clouds / dense fog continues to hug SE-coastal New England - Area of lower conditions eroding, beginning to push E with increasing W flow, out to sea by daybreak - Notably warm temperatures today, some locations reaching 90 */ Discussion... Remainder of tonight... GOES-16 nighttime microphysics continues to show the fog bank along SE-coastal New England, starting to advect E with shifting low-level mean-layer flow out of the W. Some temporary lower visibility down to half a mile, SPECIAL WEATHER STATEMENT continues. Should see lower conditions push out to sea by daybreak. Otherwise clear conditions, light S winds, temperatures continue to drop down to lows around the low to mid 50s, spot upper 40s. Today... Exceptionally warm for late May. High pressure SE, return S/SW flow, deep layer mixing at a time when the thermal gradient aloft is tightening lending to a faster low-level wind profile in addition to warm air advection. Abundant sunshine, highs into the upper 80s with spot 90s more likely in Hartford-Springfield and Boston metro areas. Subsequent deep-layer mixing, the mix- down of drier air and faster momentum. Expecting gusty winds upwards of around 30 mph with dewpoints mixing down around 50 degrees. Hot, yet comfortable. && .SHORT TERM /TONIGHT THROUGH 6 PM SATURDAY/... Tonight... Diminishing winds and quiet, dry weather. S/SW winds prevailing, breezy along the S/SE-coast, ushering higher dewpoint air into the region. This will limit temperatures to drop overnight to their cross-over threshold (the dewpoint that which was observed at time of max daytime heating). Lows around the lower 60s. Saturday... Chance of late-day scattered shower and thunderstorm activity. Yet signals of an unremarkable convective-supporting environment. Rising heights through the column, pockets of dry air, somewhat mid-level confluence, perhaps upper-level speed diffluence. Only thing going, another N-stream vortlobe, Canadian high pressure following, presses a sagged cold front S/W as a back-door across S New England, a much cooler airmass in tow. Certain low-level lift on a mild, somewhat humid boundary layer airmass, with upper 50s to low 60s dewpoints. High-res guidance spitting out some scattered activity, convective with weak mid-level instability, pockets of negative showalters, total-totals above 50, and K-indices exceeding 35. While noting precipitable waters upwards of 1.5 inches and indications of mid- level subsidence, can`t rule out scattered shower and thunderstorm activity along the back-door cold front, yet feel what goes up will not sustain and come right back down given the absence of shear / weak hodograph profile. More likely in regions with surface to H7 lapse rates of 7-8 C/km. Falling cores while noting inverted-V model forecast soundings, in addition to lightning can`t rule out gusty, downdraft winds, especially given downdraft CAPE upwards of 2000 J/kg. Airmass thunderstorms overall, perhaps more focused atop high terrain with any upsloping anabatic flow. Chance PoPs continue but future forecasts may want to include threats of heavy rain and gusty winds into the forecast grids. Difficult to nail down where across the interior, away from the NE coast at least, but as to when, the back door cold front pushing into afternoon during peak interior heating per model forecast consensus, a late-day show seems likely continuing into evening. && .LONG TERM /SATURDAY NIGHT 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 /06Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. 6z update... Though this morning... IFR-VLIFR FG bank over the immediate RI and MA S-coast, gradually push E with increasing W flow towards daybreak. Expect towards 12z only ACK may see some lingering lower CIGs / VSBYs. Today... Increasing SW winds immediately with sunrise. Gusts up to 25-30 kts through much of the day, sustained around 15 kts. SCT mid- decks. Tonight... SW winds taper but remain brisk over S/SE coast with sustained flow around 10 kts and gusts up to 20 kts. Saturday... SCT SHRA/TSRA activity towards late-day. TEMPO CIG / VSBY impacts with RA/+RA. Can`t rule out brief, gusty winds upwards of 30+ kts with any activity, otherwise NE wind-shift late. KBOS Terminal...High confidence in TAF. SW winds becoming brisk around 15z. Can`t rule out gusts as high as 30 kts. KBDL Terminal...High confidence in TAF. SW winds becoming brisk around 15z. Can`t rule out gusts as high as 30 kts. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA. 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: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. 2 pm update... Low clouds and/or fog remain an issue for the waters near the immediate SE-coast. Increasing SW winds, will see the associated fog bank push E, exiting the waters close to 8 am. Today into Tonight... SMALL CRAFT ADVISORIES for all waters, especially S/SE, with increasing westerly winds, busts up wards around 25 kts, thus allowing waves to build at or above 5 feet. Winds dampening overnight, advisories allowed to drop off. Saturday... SMALL CRAFT ADVISORIES may be needed. Increasing chances of late day showers and thunderstorms ahead of a NE wind- shift associated with a back door cold front. Gusty winds immediately behind the front upwards of around 25 kts building seas upwards of 5 feet. Greatest threat of SCA conditions over the E waters. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory from 10 AM this morning to 2 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 11 AM this morning to 8 PM EDT this evening for ANZ230-236-251. Small Craft Advisory from 11 AM this morning to 2 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 8 PM EDT this evening for ANZ250. Small Craft Advisory from 8 AM this morning to 8 PM EDT this evening for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...Sipprell LONG TERM...JWD AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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