Area Forecast Discussion
Issued by NWS Boston, MA

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403 FXUS61 KBOX 250805 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 405 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 /UNTIL 6 PM THIS EVENING/...
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4 am update... */ Highlights... - Low clouds / dense fog moving out, pushing E with increasing W winds, should be out to sea by daybreak - Notably warm temperatures today, some locations reaching 90 */ Discussion... Into this morning.... Last of the low clouds / dense fog peeling away from the SE coast. Only lingering impacts for the next hour or two over S-coastal Cape Cod and neighboring islands. Will allow the SPECIAL WEATHER STATEMENT to drop. Temperatures bottoming out in the low to mid 50s with a few spots in the upper 40s. With sunrise it`ll quickly warm up and winds will begin to blow out of the SW as we begin to mix out the boundary layer. 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.
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&& .SHORT TERM /6 PM THIS EVENING 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/... Big Picture... A southern jet moves over Mexico and the Southern USA. This jet forms a closed low over the Gulf of Mexico with upper high pressure over Northern Mexico and the Southwest Atlantic. This southern pattern remains static through the middle of next week and allows tropical moisture to feed north into areas south of the Ohio Valley and south of Mason-Dixon. The question for New England is whether any northern stream disturbances during the coming week can reach down and pull this moisture farther north into our area. Zonal flow is aloft over Canada and parts of the Northern USA. One shortwave trough moving through this flow crosses the Northeast USA Sunday and Monday. Closed low over the Western USA this weekend ejects into the northern stream by midweek and crosses New England late in the week. Both of these systems have the potential to pull the tropical moisture north as they moves through our area, one on Sunday and the other late in the week. Both will need to be monitored. Model consensus shows 500 mb contours starting around 576 dm and building to 582 dm during mid week. This is higher/warmer than normal during the long term. The surface layer flow favors a cool period over the weekend but then trending warmer than normal again Tuesday through Thursday. Model mass fields are similar over New England through Wednesday night, but diverge over the Western USA Tuesday night. The differences then spread to New England by late week. Our thermal fields also show similarity through Tuesday but then start diverging mid to late week. This generates moderate confidence in the Saturday night-to-Monday forecast, then moderate to high confidence midweek, diminishing a little on Thursday. Concerns... Saturday night-Sunday... The cold front stalls south of our area overnight. High pressure building over the Maritimes brings increasing east winds trailing the front and a cool moist marine flow to Southern New England. Model 1000 mb winds show 20-30 knot easterlies raking the Massachusetts east coast with strongest winds on Cape Ann and Outer Cape Cod. Ocean temps are in the mid 50s, except upper 40s and low 50s around the Outer Cape and Nantucket. This would suggest Saturday night min temps in the 50s and max temps Sunday mid to upper 50s east and mid 60s west, farthest away from the ocean. The upper shortwave approaching from the Great Lakes Sunday and its supporting 70-knot upper jet may induce a wave along the stalled front. This should keep a chance of showers with us Sunday, with enough confidence to toss in likely pops. Monday through Thursday... Shortwave trough moves east of us by Monday morning. Cross sections for relative humidity show moisture below 700 mb lingering but diminishing in magnitude. This suggests clouds will linger, although some breaks are possible. Also can`t rule out a slight chance of showers, but the lack of appreciable lift puts a damper on that possibility. High pressure builds down from Northern Ontario Tuesday and Wednesday, then moves offshore Thursday. This should be a period of sunny weather. Mixing is projected to reach 825 mb Tuesday and Wednesday, and 900 mb Thursday. Temperatures at those levels support max sfc temps in the low to mid 80s, while dew points suggest min temps in the 50s. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/...
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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 Saturday/...High confidence. 8z update... Though this morning... IFR-VLIFR FG bank peeling away, impacts confined to S-coastal Cape Cod and adjacent islands, likely through as late as 10z. 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 VFR, with areas IFR possible. Breezy. Chance SHRA, chance TSRA. Sunday: MVFR/IFR conditions possible. Breezy. Chance SHRA, slight chance TSRA. Sunday Night: Mainly IFR, with areas VFR possible. Breezy. Slight chance SHRA, isolated TSRA, patchy BR. Memorial Day: Mainly VFR, with local IFR possible. Slight chance SHRA. Monday Night: Mainly VFR, with areas IFR possible. Patchy BR. Tuesday: VFR.
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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. Short Term /through Saturday/...High confidence. 4 am update... Low clouds and fog peeling away from SE-coastal New England. Will only be a bother right on up through sunrise over the waters, exiting as late as 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: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of thunderstorms. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers, chance of thunderstorms. Sunday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, isolated thunderstorms. Memorial Day: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Local visibility 1 to 3 nm. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
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&& .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 11 AM this morning to 2 AM EDT Saturday for ANZ232>234-250. Small Craft Advisory from 11 AM this morning to 8 PM EDT this evening for ANZ230-231-236-251. Small Craft Advisory from 11 AM this morning to 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory from 11 AM this morning to 8 PM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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