Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180235 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 935 PM EST Fri Nov 17 2017 .SYNOPSIS... High pressure crests across our area tonight, providing dry but cold weather. Strengthening low pressure will push across the St. Lawrence River Valley this weekend with gusty southwest winds developing Saturday afternoon and night. Strong to damaging wind threats continue behind this departing system Sunday, potentially lingering into Monday. The ebb and flow pattern of southerly warm air surges followed by northerly cold air drainage continues through the week into the Thanksgiving weekend. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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940 pm update... Winds beginning to decouple as ridge of high pressure over NY state continues to advect eastward into New England. Where winds are calm or 5 kt or less temps already in the 20s. This includes OWD (27), BED (28) and ORE at 24 degs. Forecast mins appears on track with most locations bottoming out in the 20s before sunrise Saturday. A few upper teens possible in the CT river valley of western MA. Around 30 in Boston with low 30s Cape Cod and Islands where winds will take the longest to decouple.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... An intensifying low pressure moving through the Great Lakes into the Saint Lawrence River valley Saturday into Saturday night will mean an increasing southwest wind for southern New England. These southwest winds will also mean higher temperatures, likely slowly rising Saturday night instead of falling. Lots of dry air to overcome in the lower levels. Thinking the NAM might be a bit too fast bringing precipitation into our region. Stayed with the slower timing presented by the GFS, ECMWF and CMC solutions. This would bring a chance of showers to much of our region during the afternoon. The brunt of the light rainfall should be Saturday night across our region. Will need to contemplate a Wind Advisory for the south coast of RI and MA late Saturday afternoon into Saturday night. Will mention in the Hazardous Weather Outlook, and let the next shift get a closer look at the details. && .LONG TERM /SUNDAY THROUGH FRIDAY/... */ Highlights... - Widespread strong to damaging W/NW winds Sunday into Sunday night - Winds will linger into Monday with perhaps less of an impact - Ebb and flow pattern of S warm air surges followed by N cold air continues */ Overview... Ebb and flow continues. Without greater amplification and subsequent transfer between the equator and the poles, the flow remains mainly zonal. The MJO largely subdued as the present -WPO/-EPO/-PNA all trend positive into December per ensemble means, counter-productive, lending seemingly to a signal of persistent W to E flow. The only consistencies it seems from analysis of potential vorticity and isentropic surfaces is a persistence of poleward transfer in the N Pacific and the NE Atlantic that lends to S shearing of energy into the Gulf of Alaska and S across Europe, respectively. Otherwise it`s zonal in-between. Any energy across the CONUS is quick moving with little residency such that forces acting to invoke greater curvature and storm development reach their full potential E of our region across E/SE Canada into the NW Atlantic. With racing storm systems over our region, warm-surge in advance followed by cool shots, both with attributes of gusty winds, a chance of wet-weather in-between along associated frontal boundaries, cold fronts with more emphasis. Attendant threats and impacts highlighted in the discussion below. */ Discussion... Sunday into Monday... Winds shifting NW ushering colder air, remaining blustery. Consensus of forecast guidance, cold front sweeping 7-10a Sunday, undercutting colder air, lapse rates mixing out to H85, dry adiabatic, the top of the mix layer averaging W/NW 45 to 50 mph (40 to 45 kts). Widespread WIND ADVISORY likely, more likely daytime Sunday with boundary layer heating and contributions from quick pressure rises. Ensemble means, strongest winds likely across the high terrain and along the coast adjacent to 50-degree ocean waters, also N/E closer to the storm center, so N/E portions of MA at greatest threat. CIPS analogs with a high probability of at least 35 mph widespread gusts, lower for 40 to 45 mph, however considering local climatology, believe there is the potential to see 40+ mph gusts. Headlines may drop overnight with the potential of a shallow surface inversion, however model guidance consensus signals continued cold air advection right into Monday with additional energy rotating through the cyclonic flow across the region lending to an enhancement of the wind profile. Something will have to watch closely. Will highlight the wind threat in the HAZARDOUS WEATHER OUTLOOK. No headlines at this time associated with W/NW winds. Otherwise, drier air surging in aloft behind the quick sweeping cold front, will see an end to shower activity rather abruptly during the morning hours. With the blustery W/NW winds and cold airmass pouring in, across the warmer Great Lakes, the lake effect machine will be ongoing and could see some of that energy reach far enough E with the mean flow to impact N/W portions of MA and CT with some snow and/or rain shower activity, 2m temperature dependent. If snow, some light accumulations are possible, mainly for the high terrain. All threat diminish into Monday night, and it is during this period that winds may become light enough beneath clear conditions to allow for radiational cooling and night-time lows to drop well down into the 20s, possibly teens. Hard to say right now. If W/NW winds remain brisk, then the mechanical mixing in the environment would limit the radiational cooling potential. Will hold it conservative with wide- spread lows in the 20s. Highs Sunday into the upper 40s to low 50s right before the cold front punches through in the morning, upper 30s to low 40s for Monday. Tuesday... Quiet. High pressure. Return S flow. Turning mild. Still believe that warmest temperatures will be over interior SE New England, that an onshore will develop in response to an approaching storm system for the overnight period into Wednesday. Highs in the upper 40s to low 50s. Tuesday night into Wednesday... Possible showers. Keeping with slight chance PoPs, the EC on again off again on the N and S stream energy beginning to interact with a draw of S-moisture ahead of the N sweeping cold front before both systems push offshore quickly and phase downstream. The GFS keeps the dry-streak going. Given the emphasis on the cold front agreed upon within all available guidance, would expect that the associated lift on available moisture should get something going, especially considering the warmer ocean waters that line the shore. Thanksgiving onward... Thanksgiving looks quiet and seasonable, while into the weekend it appears another quick moving storm system sweeps the region. The ebb and flow pattern continues with milder shots and SW flow ahead of any disturbance followed by colder air and NW winds as the system departs. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
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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 Night/... 0230z... No change from previous TAFs...VFR, dry weather and light winds overnight. Earlier discussion below. =================================================================== Tonight...High confidence. VFR. Dry weather along with diminishing winds. Saturday...High confidence. VFR, light winds and dry weather through the morning. MVFR and rain begin to increase from west to east, with current timing 18/20z-19/00z. Also S-SW winds increase during the afternoon with gusts approaching 35 kt by late in the day along the south coast including Cape Cod and the Islands. Saturday Night...High confidence. Mainly MVFR, with local IFR possible. Strong winds with local gusts to 40 kt. RA, patchy BR. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday: Mainly VFR, with areas MVFR possible. Strong winds with gusts to 40 kt. SHRA likely. Sunday Night: VFR. Windy with gusts to 35 kt. Monday: VFR. Strong winds with local gusts to 40 kt. Monday Night: VFR. Windy with gusts to 35 kt. Tuesday: VFR. Breezy. Tuesday Night: VFR. Windy with local gusts to 30 kt. Slight chance SHRA, slight chance FZRA. Wednesday: VFR. Breezy. 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 Saturday Night/...High confidence. Tonight... Winds will continue to diminish this evening along with seas. Saturday and Saturday Night... Increasing SW winds as a low pressure moves through the Great Lakes into the Saint Lawrence Valley. Showers most likely to arrive over the waters late Saturday afternoon, and continue into Saturday night. Some risk for storm force gusts Saturday night, but have greater confidence in gale force winds. Warnings posted. Outlook /Sunday through Wednesday/... Sunday: Moderate risk for gale force winds with local gusts up to 45 kt. Rough seas up to 14 ft. Rain showers likely. Sunday Night: Strong winds with gusts up to 35 kt. Rough seas up to 14 ft. Monday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Monday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from 5 PM Saturday to 6 AM EST Monday for ANZ232>234. Gale Watch from Sunday morning through Sunday evening for ANZ230. Gale Warning from 1 AM Sunday to 6 AM EST Monday for ANZ231- 251. Gale Warning from 7 PM Saturday to 7 PM EST Sunday for ANZ236. Gale Warning from 3 PM Saturday to 6 AM EST Monday for ANZ235- 237-255-256. Gale Warning from 7 PM Saturday to 6 AM EST Monday for ANZ250- 254. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Nocera/Sipprell SHORT TERM... LONG TERM...Sipprell AVIATION...Belk/Nocera/Sipprell MARINE...Belk/Sipprell

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