Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180617 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 117 AM EST Sat Nov 18 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 THIS MORNING/...
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1 AM update... Cold ridge of high pressure cresting over the region tonight providing ideal radiational cooling with light winds, mostly clear skies and a dry airmass. Most locations at 1 AM already down into the 20s. Some cirrus spilling in from the west but cloud cover appears thin enough not to impact temps overnight. Previous forecast remains on track so no major changes with this update.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... 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 NIGHT 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 /06Z 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 Sunday/... 06z... Today...high confidence. VFR and dry weather prevails. The exception will be across western MA/CT where MVFR conditions in light rain will increase between 21z-00z. Light winds this morning increase sharply this afternoon from the SSW with gusts approaching 30 kts by sunset (22z). Elsewhere SSW winds increasing to 10-15 kt. Tonight...High confidence. VFR-MVFR to start in areas of light rain but trending toward IFR-MVFR after midnight. Strong SSW 20-25G45kt winds along the south coast including Cape Cod and Islands. Elsewhere winds not as strong and this results in LLWS across much of RI and eastern MA including BOS. Sunday...moderate confidence. Some uncertainty on probability of -TSRA and areal coverage along and ahead of strong cold front. MVFR-IFR with widespread showers, some with locally heavy downpours. Could be a fine line of low top showers with embedded thunder roughly 15z-18z. Gusty winds may accompany this convection. Strong SSW winds 20-25G45kt continue along the south coast including Cape Cod and Islands. Away from the south coast expect LLWS to continue thru the morning across RI and eastern MA. Then a strong cold front sweeps across the area late morning into the early afternoon, with improving conditions behind the front and a wind shift to the west. KBOS Terminal...High confidence in TAF thru 00z then some timing issues on arrival of lower cigs and rain. Then period of rough weather tonight into midday Sunday with LLWS along with heavy showers. Isolated thunder possible 15z-18z Sunday, then strong cold front sweeps across the area with improving conditions after 18z Sunday. KBDL Terminal...High confidence in TAF thru 21z then some timing issues on arrival of lower cigs and rain. Unsettled weather tonight into Sunday morning with heavy showers and possible isolated Thunder. Outlook /Sunday Night through Wednesday/... 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...
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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 Sunday/...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 Night 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Gale Warning from 5 PM this afternoon 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 this evening to 7 PM EST Sunday for ANZ236. Gale Warning from 3 PM this afternoon to 6 AM EST Monday for ANZ235-237-255-256. Gale Warning from 7 PM this evening to 6 AM EST Monday for ANZ250-254.
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&& $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Nocera/Sipprell SHORT TERM... LONG TERM...Sipprell AVIATION...Belk/Nocera/Sipprell MARINE...Belk/Sipprell

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