Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 270717 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 317 AM EDT Sun May 27 2018 .SYNOPSIS... Scattered showers continue overnight as a back door cold front continues to push S. Much cooler weather with cloudy conditions and areas of rain today as low pressure along the front passes south of the region. Scattered showers or areas of drizzle over- night prior to possible improvement into Monday. High pressure brings mainly dry and warmer weather Tuesday through Thursday. An approaching weather system brings showers Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 2 am update... Scattered shower activity persisting rearward of the back door cold front which has pushed S of New England as high pressure continues to build over SE Canada. Stretched mid-level short- wave energy NW-SE across NE New England up against energy ejecting out of the OH River Valley along with a continental- tropical airmass, observing mid-level lift and isentropic ascent along the cold frontal slope over S NH and NE MA, as thunder- storms have begun to erupt across the Mid-Atlantic an offshore, consequential of tightening gradients lending to a H3 jet max aloft of S New England, the right entrance region providing favorable venting above mid-level energy ejecting out of the OH River Valley. Likely PoPs for N/NE MA continue with chance PoPs elsewhere. A bigger challenge overnight has become low clouds and fog. Some confidence gained that with cooler, drier air pushing in from N/E beneath warmer air aloft, that beneath the inversion we should see cooling towards condensation overnight lending to low clouds, a bit less on fog. The further depth of cool, dry air S/W, the higher the cloud base. Figuring most locations will be at or below 1000 feet. Patchy fog issues retained for areas of rain and along the S/SE-coast. Preference to the NSSL WRF member within HREF members which has been doing well with respect to model reflectivity output. Other- wise, no specific forecast model has been outperforming the rest of the others with respect to winds, ceilings, and visibility. A preference to consensus of short-term, high-res guidance along with the NWS NBM. Today... Rain forecast. S-stream energy caught up in Pacific-vortmax, weak mid-level trof axis out of the OH River Valley with an associated continental-tropical airmass. Up against a cool, dry airmass brought S/W behind a N-stream vortmax / surface high pressure over SE Canada. Expecting isentropic upslope along 295-300K surfaces with SW H9-7 flow above undercutting surface-H9 E/NE winds. Ascent along the front up against the surface ridge over Canada, at its peak during the morning, lingering impacts late as ejecting energy / weak surface wave low along the now stalled frontal boundary sweeps E. However continued onshore E flow with high pressure NE, the low slipping off S/E, weak, onshore upslope component, expecting a transition from widespread rain to drizzle over much of S New England. Low clouds hanging through much of the day. Likely PoPs for locations along and S of the MA-pike. Prevailing with rain. Holding ceilings at or below 1000 feet. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... A mix of scattered showers OR areas of drizzle. Abundance of low-level moisture, however crux of mid-level energy and trof axis is offshore. Beginning to see subtle height rises with drier air starting to work in within mid-levels. However, continued onshore NE flow as temperatures remain cool, steady around the low 50s. Onshore, upslope ascent beneath cyclonic flow aloft, some mid-level energy lingering, and stout dry-inversion pressing down through the column, leaned with scattered showers OR areas of drizzle. Undoubtedly continued low clouds, perhaps some fog issues for the high terrain with lower clouds, maybe along the SE coast. Monday... Clearing out? Weak ridging ahead of another potent N-stream vort- max over S Canada, kicking earlier synoptics E, out to sea, surface winds begin to revert S. Some improvement is possible. However it remains to be seen whether the combination of daytime heating and boundary layer mixing is able to meet with the suppressing dry-inversion such that the cloud layer between is mixed out. Am somewhat hesitant on forecast model clearing given such weak advective patterns, leaning that clouds hold in tight. However, if dry-air subsidence prevails, lowering down through the column, mechanical mixing could lead to low cloud erosion. Low confidence forecast at this time, but kept with earlier forecast trends of milder conditions W while cooler E remaining socked in longer. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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Big Picture... Zonal flow remains in place over Canada and the Northern tier of the USA. One sharp shortwave moves through this flow across Maine and the Maritimes Tuesday. A shortwave ridge then moves east from the Great Lakes. Closed low over the Western USA gets kicked northeast as another low moves south from the coast of Alaska. Upper circulation associated with Alberto will be channeled north through the Southern USA and merges midweek with the ejecting Western low somewhere over the Midwest. The combined system then moves east and overcomes the shortwave ridge over New England Thursday night. Model solutions show above normal contour heights through the forecast period, and thermal fields also indicate above normal temperatures. Model solutions remain similar through Thursday. The models then develop differences in detail for Friday and Saturday, while maintaining a similar overall feel for the pattern. This gives us high confidence in the forecast through midweek and moderate confidence late week. Concerns... Monday night through Thursday... Lingering moisture below 700 mb Monday night but with a diminishing trend. This may be enough for partial clearing or a temporary clearing overnight. Meanwhile a sharp northern stream shortwave dives through Maine and the Maritimes and pushes another cold front through New England. This may be enough for a zone of clouds along the front, but instability along the front Tuesday in Srn New England is marginal with mid level lapse rates mostly less than 6C/Km and totals in the mid 40s. The GFS maintains a sub-zero lifted index across CT-RI in the afternoon along with some low level convergence. This is not supported by the other models. Once the front moves through, high pressure from Northern Ontario builds in with strong drying...dew points in the 40s and strong dew point depressions. The high moves overhead Tuesday night and Wednesday, then shifts over the West Atlantic Thursday. This will be a dry period with developing south-southwest winds Thursday. Expect clear/clearing skies and light winds through midweek. Possible sea breeze on the Mass East Coast Tuesday and Wednesday. Temps aloft...or rather the 850 mb equiv...will be in the low to mid teens suggesting max sfc temps in the low to mid 80s each day away from the coasts. Dew points in the mid 40s to mid 50s suggest min temps generally in the 50s. Thursday night through Saturday... The ejecting upper low from the Western USA merges with the remnants of Alberto over the Midwest midweek and then move east into New England toward the end of the week. Precipitable water values climb sharply to 2 inches or higher Thursday night and Friday. This points to a wet period to end the week with high moisture content and potential for heavy downpours.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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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 Monday/...Moderate confidence. 6z update... Remainder of tonight... Lowering MVFR-LIFR CIGs if not so already. NE winds persisting. Sustained around 10 kts, coastal gusts up around 20 kts. Low confidence concerning VSBYs, trending lower for high terrain terminals and along the S-coast. Today... Continued MVFR-LIFR CIGs with -RA/RA, especially along and S of the MA-pike. NE winds persist, strongest along coastal terminals. A threat of LLWS over the Cape and Islands. Tonight... No let up in CIGs nor winds. -RA/RA replaced by SCT SHRA and DZ. Monday... Possible gradual improvement W to E as winds shift S. CIGs lifting MVFR to low-end VFR, lingering longest for SE coastal terminals. KBOS Terminal... Confident in MVFR-IFR CIGs. Possible today that CIGs could lift to low-end VFR mid to late in the day, lowering back into evening with DZ. Majority of -RA/RA S of terminal today. KBDL Terminal... Mainly IFR today into tonight. N winds prevailing through the CT River Valley. Outlook /Monday Night through Thursday/...Moderate confidence. Monday Night: Mainly VFR, with local IFR possible in patchy fog. Tuesday through Thursday: 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 Monday/...High confidence. Through tonight... High pressure situated over SE Canada with low pressure sweeping E along a stalled frontal boundary S of New England, NE winds persist with potential gusts up to 25 kts, especially through tonight. Waves building 6 to 8 feet over the E/SE waters, trickling into the S waters later tonight, beginning to dissipate after midnight tonight. Small craft advisories continue. Potential visibility restrictions today as rain is forecast, especially for the S/SE waters, while drizzle will be an issue overnight for a majority of waters. Patchy fog at times. Monday... Weak high pressure setting in, winds relax and begin to revert S. Will continue to see wave action dissipate, as well as the conclusion of small craft advisories. Outlook /Monday Night through Thursday/...Moderate confidence. Winds less than 25 kt. Seas less than 5 feet. Visibility locally 1 to 3 miles in fog Tuesday night.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 7 AM this morning to midnight EDT tonight for ANZ232>235-237. Small Craft Advisory until 6 PM EDT Monday for ANZ231-250-251- 254-255. Small Craft Advisory from 7 AM this morning to 6 AM EDT Monday for ANZ256.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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