Area Forecast Discussion
Issued by NWS Boston, MA

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623 FXUS61 KBOX 162026 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 326 PM EST Thu Nov 16 2017 .SYNOPSIS...
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Wet weather moves offshore this evening. Blustery winds follow this departing storm system tonight and Friday. Strong to damaging wind threats ahead of and rearward of a sweeping storm system Saturday into Sunday, 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.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Latest runs of many of the near term forecast models, such as the HRRR, appear to have a very good handle on the precipitation timing this afternoon. Expecting the rain across the eastern half of southern New England to move offshore by early evening, as a triple point low passes us by. Contemplating canceling the Wind Advisory early. Winds diminish briefly this evening, until the colder air arrives later. Peak gusts to this point in the afternoon have been only about 35 mph. Attention then turns to the gusty west to northwest winds and potential for additional showers to get into portions of western MA overnight. At the moment, thinking most of the showers will be farther north, closer to the better dynamics aloft. Strong upslope with the west to northwest winds would place the higher terrain of the east slopes of the Berkshires at risk, with a few showers possibly surviving past the spine of those mountains. The corresponding downslope flow would also work to keep things dry farther east. Below normal temperatures continue tonight. Wind chills mainly in the 20s.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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Low pressure continues to intensify over the Maritimes Friday, maintaining a gusty northwest wind. Max temperatures remain below normal, with wind chills mainly in the 30s. High pressure centered near the Carolina coast should extend in our direction Friday night. This will permit winds to become less gusty, while also keeping dry weather for most of southern New England. Thinking the peak winds and gusts will remain below Wind Advisory thresholds Friday and Friday night.
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&& .LONG TERM /SATURDAY 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... Saturday into Saturday night... Monitoring for strong, possibly damaging SW winds. Strongest winds 1p Saturday to 7a Sunday, the crux of which are over SE New England. Model consensus H925 SW winds 45 to 60 mph (40 to 55 knots) with the high-res NAM suggesting over 65 mph (60 knots). Also best low-level lapse rates up to H9 averaging around 7 C/km within strongest winds around the I-95 corridor and SE, at least 6 C/km through the event. Looking at a +2 standard deviation SW event per SREF. A SW event with winds off the maritimes requires faster winds aloft to net WIND ADVISORY criteria. Based on local climatology, typically like to see winds at H925 around 60 mph (55 knots) to support WIND ADVISORIES. Per latest forecast grids we meet WIND ADVISORY criteria along the S/SE coast of RI and MA including S Bristol and S Plymouth Counties in MA beginning Saturday evening and continuing Sunday morning. We stay just below HIGH WIND WARNING criteria. For that in SW cases H925 winds over 65 mph (60 knots) to which a consensus of model forecasts falls just below. No headlines this early as we are currently dealing with a storm presently and don`t want to create confusion. Will continue to highlight the strong to damaging wind threat in the HAZARDOUS WEATHER OUTLOOK. Otherwise, warm front sliding NE late Saturday ahead of which will see isentropic upslope lead to showery, nuisance weather, the better chance of which will be closer to the low center NW. Bulk of rain comes towards Sunday morning with the cold front given deep lift and ascent of higher thetaE. However, quick moving, deeper sub-tropical air well S along with higher precipitable waters, and erosion rear- ward by a sharply impinging dry-slot out of the SW. Looking at not much in the way of outcomes a couple to few tenths, no threat of thunder. Warming all the way up prior to the cold frontal passage. Lows Saturday morning but highs may come just prior to midnight, upper 40s to low 50s. 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 /21Z THURSDAY 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 Friday Night/... Through 00Z...Moderate confidence. Mix of conditions across the region this afternoon, ranging from IFR to VFR. Think we have reached the lowest conditions, with all trends pointing to improving from west to east. Tonight...High confidence. Any lingering MVFR in showers early, quickly improves to VFR along with a wind shift from SW to NW with gusts of 25 to 30 kt. A few gusts up to 35 knots possible. Friday...High confidence. VFR, dry weather and NW winds 15-25 kt with a few gusts up to 30 kt possible. Friday night...High confidence. VFR. Gusty NW winds diminish. KBOS Terminal...Timing in the TAF the main concern. High confidence in winds backing through north to the west, but when exactly. Timing still points to being about the time of the evening push. High confidence in trends. KBDL Terminal...High confidence in trends. Only moderate confidence in timing through 17/00Z. Outlook /Saturday through Tuesday/...Moderate to high Saturday: VFR. Windy with gusts to 35 kt. Chance SHRA. Saturday Night: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. SHRA. 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. Short Term /through Friday Night/...High confidence in trends. Moderate confidence in the details. Intensifying low pressure over the Gulf of Maine tonight will move into the Maritimes Friday. This will keep gusty NW winds across the waters through Friday. Gusts could approach gale force, particularly across the waters east of MA. Will continue the Gale Warnings. Although some of the nearshore sounds and bays might come up a little short of meeting the 34 kt criteria, it should be very close. Winds diminish Friday night, but Small Craft Advisories will likely be needed. Outlook /Saturday through Tuesday/...Moderate confidence. Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain showers. 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 Warning until 4 PM EST Friday for ANZ231>235-237-254>256. Small Craft Advisory until midnight EST Friday night for ANZ230. Gale Warning until 7 PM EST this evening for ANZ236. Gale Warning from 7 PM this evening to 4 PM EST Friday for ANZ250-251.
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&& $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell MARINE...Belk/Sipprell

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