Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170832 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 332 AM EST Fri Nov 17 2017 .SYNOPSIS...
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Intensifying low pressure over the Maritimes yields a dry but blustery and chilly day across New England. High pressure crest over the area tonight providing dry but cold weather. Strong low pressure then moves through the St Lawrence River Valley bring gusty showers and mild conditions late Saturday into Sunday morning. Its attending cold front sweeps across the region midday Sunday with drier but blustery and colder weather late Sunday into Monday. A brief return to mild weather arrives Tue before another cold front arrives Tue night into Wed.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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330 AM update... Closed mid level low and surface cyclone over Nova Scotia continues to intensify today given trough amplification and associated height falls. This results in modest CAA into southern New England with 850 temps lowering to -6C this morning, about -1 standard deviation colder than climo. This combined with tight pres gradient will result in a chilly morning across the region with NW winds gusting up to 35 mph. Temps will be in the 30s this morning but wind chills in the 20s. Cold advection peaks this morning then eases this afternoon as mid level trough moves offshore replaced by height rises across the region. Thus not as windy this afternoon. However temps will only rebound to 40-45, upper 30s across the high terrain, almost 10 degs cooler than normal. Scattered to broken cold air advection strato-cu this morning will give to mostly sunny conditions this afternoon as temps begin to warm aloft in response to height rises.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
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330 AM update... Tonight... Ridge of high pres builds across the area from west to east, resulting in a cold night especially outside the urban areas as mostly clear skies combine with light winds and a very dry airmass. For whatever reason MOS has been running too cold in these radiational cooling events this past month so we will follow a model blend here to offset the colder MOS guid. Nonetheless most locations fall into the 20s tonight, upper teens northwest MA and closer to 30 for Boston. Will have to watch mid and high clouds spilling into the area late tonight, which may have an impact on temps with mins possibly not quite as cold. Saturday... Chilly start to the day but low level WAA commences with increasing SSW winds during the afternoon and especially late in the day. Some morning sunshine thru high clouds possible, but will fade behind increasing clouds during the afternoon as mid level trough and 985 mb surface cyclone exits Great Lakes. All global guidance including ensembles keep dry weather across the entire region thru 18z Sat. Thereafter showers overspread the area especially western MA/CT. Will have to see how later model runs trend but it`s possible much if not the entire day remains dry across RI and eastern MA. Low level WAA should result in highs in the 50s across the coastal plain including the Boston to Providence corridor, with 40s farther inland where it will take longer to modify shallow cold air. SSW winds will increase as low level jet approaches the region. Model soundings suggest wind gusts up to 40 mph along the south coast by late in the day Saturday.
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&& .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
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*/ Highlights... - S/SE coast strong to damaging SW winds late Sat. into Sat. night - Widespread strong to damaging W/NW winds Sunday into Sunday night - Ebb and flow pattern of S warm air surges followed by N cold air */ Overview... Ebb and flow. So long as the doors remain open from the N Pacific, across the CONUS, and immediately downstream of New England, will the pattern remain progressive. So with the persistence of Pacific energy sheared S into the NW CONUS and over the N American Rockies, disturbances quickly race E such that forces acting yield greater curvature and subsequent negatively-tilting, dynamic and deepening activity, stronger storm development, over SE Canada rather than our next of the woods. This process aided by the reloading upstream of energy into the NW Pacific and resultant ridging over the W CONUS shearing downstream energy and the polar jet S, aiding in the afore- mentioned cyclogenesis into SE Canada with stronger thermal gradient tightening and energy transfer between equator and pole. So for the most part in our region we see quick moving disturbances with mild air lead ahead by a warm frontal boundary followed swiftly by cold frontal boundaries and temperatures below average. Ebb and flow, the transitions in-between distinguished by wet-weather and gusty winds to which any threat and impact of which will be highlighted in the discussion below. */ Discussion... Sunday into Monday... Winds shifting NW ushering colder air, remaining blustery. No doubt about dry adiabatic lapse rates through Monday up to H85 where the average of winds is around 45 to 50 mph (40 to 45 knots), again the high-res NAM slightly stronger. Widespread impacts however strongest winds may occur over N/E portions of MA considering the proximity to the deepening low into SE Canada and subsequent pressure falls, the greatest of which and subsequent isallobaric contribution occurring Sunday morning. Could see additional emphasis on the winds as energy continues to rotate round the deepening storm system lifting further into E Canada, such emphasis extending into Monday. For WIND ADVISORY headlines with W/NW flow you`d like to see H925-85 winds 35 to 45 mph (30 to 40 knots) which we easily meet across the entire region. Especially Sunday into Sunday evening, likely we`ll need widespread WIND ADVISORY headlines just based on the consensus of synoptics and considering that some trees still have plenty of leaves. Early for headlines, will highlight this in the HAZARDOUS WEATHER OUTLOOK. The lake effect snow machine going, could see some rain and/or snow showers into N/W portions of MA and CT depending on 2m temperatures. Likely the greater threat on Sunday given the cold fetch over the warm lakes and greater cyclonic flow aloft. Light accumulations for N/W MA and CT are not out of the question. Winds and any additional threats diminish Monday night. Highs Sunday likely right after midnight, possibly into early morning, before temperatures plummet with cold air advection. Around the upper 30s for highs Monday with scattered to broken cloud decks, sunshine peaking through, while overnight lows in the 20s, the colder of which progged into Tuesday morning given clear conditions and light winds, could be talking down into the teens for some. However coldest wind chills Sunday night into Monday morning, also down into the teens. Tuesday... Quiet beneath high pressure. Return S flow. Warming back up nicely given SW flow at all levels. However, influence via ageostrophic E flow in response to developing low pressure over the N Great Lakes, leaning that warmest temptress will occur over the SE interior, highs into the 50s, while slightly cooler with an onshore flow over the E/SE coastline. Tuesday night into Wednesday... Potential weather. Two pieces of energy, a N and S stream that given the progressive flow look to phase downstream well away from the NE CONUS per the latest 16.12z guidance and noting ensemble means. Could be looking simply at a dry cold frontal passage. Will put some slight chance PoPs for the period. Wednesday night through the Thanksgiving weekend... Leaning with a dry pattern. Scattered to broken cloud decks initially with NW flow and possible snowy influences off the lakes. Potential for reinforcing cold air over the region as high pressure settles in. A temperature rebound as the high shifts E ahead of the next upstream disturbance. The ebb and flow pattern continues in the mainly zonal flow.
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&& .AVIATION /09Z 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/... 06z update... Today...high confidence. Other than isolated MVFR cigs across the higher terrain of western MA this morning, VFR conditions will prevail along with dry conditions. Main issue will be gusty NW winds today with speeds up to 30 kt except up to 35 kt across Cape Cod and the Islands. Tonight...high confidence. VFR, dry weather along with diminishing winds. Saturday...high confidence. VFR, light winds and dry weather thru the morning. MVFR and rain begin to increase from west to east, with current timing 21z- 00z. Also SSW 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. KBOS Terminal... High confidence in TAF. Main issue today will be gusty NW winds up to 30 kt. KBDL Terminal... High confidence in TAF. Main issue today will be gusty NW winds up to 30 kt. Outlook /Saturday Night through Tuesday/...Moderate to high Sunday: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. Chance SHRA. Sunday Night: VFR. Strong winds with gusts to 40 kt. Monday: VFR. Windy with gusts to 35 kt. Monday Night: VFR. Windy with gusts to 30 kt. Tuesday: VFR. Breezy.
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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. 330 AM update... Short Term /through Saturday/...high forecast confidence. Today... NW gales across the eastern MA waters, sub gales elsewhere with strongest winds this morning. Dry weather and good vsby prevail. Tonight... Light winds as high pressure crest over the New England waters. Saturday... light winds to start the day but increasing SSW winds during the afternoon and possibly reaching Gales by sunset across the RI and southern MA waters. Showers may also arrive late in the day. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Low risk for storm force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain showers. Sunday: Low risk for storm force winds with gusts up to 50 kt. Rough seas up to 15 ft. Chance of rain showers. Sunday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 12 ft. Slight chance of rain showers. Monday: Moderate risk for gale force winds with gusts up to 35 kt. Areas of rough seas. 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. Areas of rough seas.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ232>237-255-256. Small Craft Advisory until 7 PM EST this evening for ANZ230- 232>237-255-256. Gale Warning until 11 AM EST this morning for ANZ231-250-251- 254.
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&& $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell

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