Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 140600 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 100 AM EST Thu Dec 14 2017 .SYNOPSIS... A fast moving low pressure will track S of New England late tonight and early Thursday bringing a period of accumulating snow to the south coast and especially the Islands. Cold persists into the weekend with a chance of snow showers Friday night into Saturday. Blustery and cold for Sunday but the beginning of a milder trend Monday ahead of which there`s the possibility of a mixed wintry precipitation event. Quiet and dry with an ebb and flow pattern during the week up till late Thursday into Friday during which time there`s the possibility of another mixed wintry precipitation event. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1 AM update... Leading edge of light snow reaching the ground is across SE NY. Dry air in low levels with dewpoints in the single numbers will initially erode the snow so onset timing of 3 to 6 am still looks reasonable. Previous discussion... Current forecast on track for the most part. Only change was to keep a dry forecast tonight for much of eastern MA north of TAN and east of ORH. Low level dry air will hang tough in this region and will likely prevent any snowfall through 6 am. Timing of onset of snowfall for SW New Eng and south coast region is 3-6 AM with best chance for a coating to up to an inch by 6 AM from HFD to southern RI and the Islands. Will allow wind advisory to expire at 10 pm.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... *** Accumulating Snow Likely Immediate South Coast and Islands Thursday Morning *** 4 PM update... Thursday... Mid level trough moves across the region from west to east during the morning. Good synoptic scale lift associated with this system as seen on Q-vector maps. This results in surface low jumping from near Cleveland at 06z to redeveloping south of Long Island 12z Thu and tracking east along the 40N parallel during the day. Good model agreement that modest omega (15 -ubar/s) occurs in the DGZ (snow growth layer of -10C to -20C) 12z-15z Thu along the immediate south coast and Islands. Thus should see at least modest size flakes (dendrites) which will help reduce vsby and yield an efficient snow system with 15-1 snow ratios. In addition, models suggest some instability in this layer too in the form of -epv. So the combination of modest synoptic scale lift, good Fgen in snow growth layer may result in this system over performing. However limiting factors for more snow is very dry airmass on the front end of this system. Will likely lose a few hours of qpf to virga as blyr slowly saturates. Furthermore, system is very progress which leaves just a 1-3 hr window for moderate snowfall. Thus have leaned toward a 1-3 inch snowfall for immediate south coast and islands with impact to the morning commute from Westerly to Newport to New Bedford to Cape Cod. Given cold antecedent conditions snow will immediately stick to all surfaces. Will have to see how 00z models trend as these systems sometime trend northward. This would result in accumulating snow possibly as for north as Hartford to Providence to Plymouth. Nonetheless there will be a very sharp cutoff on the northern edge of the snow shield. Given trough is very progressive expecting sunshine to develop during the afternoon. Still colder than normal with highs only in the upper 20s and lower 30s. Not nearly as windy as today but still a chilly day. Thursday night... Becoming blustery during the evening behind departing low. However winds drop off late as weak high pres builds in. This will allow for another cold night including the south coast given a fresh snow cover. Lows in the teens most areas with single digits well inland. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Light snow showers possible Friday night into Saturday - Cold and blustery Saturday night into Sunday - Possible mixed precipitation Sunday night through Monday night - Perhaps quiet, cold until Thursday night into Friday with another mixed precipitation event */ Overview... Interpreting through late December. Considerable forecast warming in the stratosphere, polar low / night-time jet displaced to the other side of the N hemisphere subsequent of noteworthy anomalous ridging across Northern N America. Beneath which warm air continues to surge into the Arctic from the NW Pacific (with perhaps MJO contributions) and the NW Atlantic displacing colder air S where it becomes bowled up across NE Asia / NE N America. Ensemble means continue to signal an active weather pattern with an amplified H5 ridge-trof pattern over the CONUS, anomalous H85 T warming over the N Pacific with cold anomalies continually reloaded over Central/E N America, however a trend towards higher heights / surface mean sea level pressure with stratospheric cross-polar flow shifting into NW Europe. Making sense of it all, NW Pacific disturbances translating E across the CONUS seemingly look to flatten the H5 pattern, lock colder air N for a time, the NE CONUS falling into the transitional zone of air- masses before colder air reloads and surges S again. An ebb and flow setup however not barred from N shots of colder air, the lack of S- stream dominance. Yet a trend of higher heights as signaled from the ensemble means, cold shots may not be as deep, storm development may not be so pronounced long-term. Perhaps La Nina signals are starting to emerge across N America with a lesser pronounced H5 trof pattern over Central/E N America downstream of stout ridging / warmth over the NW Pacific. Tough to get down on the details and specifics, taking a broad view approach with the discussion above. Will hit on any threats/impacts in the details below. With perhaps a flatter pattern evolving and the lack of downstream traffic over the N Atlantic, the move into a zone of transition between the airmasses as noted above, as forecast guidance suggests, could end up in a regime of quick-moving systems with mixed precipitation type outcomes that later deepen downstream and yield that reloading shot of colder air, yet that colder air not as deep, perhaps simply swiping the NE CONUS more to the N. */ Discussion... Friday through Saturday... Cold, blustery at times, chance of snow showers Friday night into Saturday. A nod to ensemble means for 24-hour rainfall probabilities in excess of 0.01 inches as N-stream clipper energy translates to a dominant S-stream disturbance across the offshore baroclinic front. Yet leaning very light outcomes. Lack of ascent within moist snow growth regions parent with the positively-tilted H5 trof undergoing neutral transition along the surface transition boundary. It isn`t till better mid-level curvature and ejecting vortmax above the S- stream can get things going, in this instance further downstream. Leaning snow accumulations along the N/W slopes of high terrain given orographic support, and along the SE coast as energy / ascent begins to come together before ejecting out to sea. Highest snow accumulations possibly over an inch for S/SE coast, especially over the Islands and Outer Cape. Visibility impacts, perhaps some slick road conditions. Behind the disturbance, winds kick up, colder air drives back in, wind chills drop back down into the single digits Sunday morning, lows in the teens. Sunday into Monday... Potential mixed precipitation event. Ascent overall light, seemingly quasi-parallel flow along the lifting warm front. Above a cold air damming signature with an indication of northerly ageostrophic flow, the surface high to the N however a weaker low over the Great Lakes, could see a period of freezing rain / drizzle centered around late Sunday night into early Monday morning, especially in the interior sheltered valleys where cold will be more difficult to dislodge. A nod again to ensemble mean for 24-hour rainfall probabilities in excess of 0.01 inches which is roughly 70 percent or greater. Again light outcomes but enough to potentially make for hazardous travel and could yield WINTER WEATHER ADVISORIES for a trace of ice. Some question as to how quickly the warm front will lift N and how long threats / impacts may linger. Could hold through Monday evening. A low confidence forecast given poor handling via operational guidance on individual upstream waves. Tuesday onward... Leaning a flatter, progressive flow regime with transitioning air- masses. With any additional waves will be looking for potential mixed precipitation events with the ebb and flow of 2m temperatures. Weaker energy ejecting E and deepening, expecting behind each system the return of colder conditions ushered by breezy N/W winds. Lack of confidence in the forecast beyond 96 hours (which does include the Sunday into Monday timeframe). In leaning with ensemble means, after the early week disturbance, our next event may not occur until late Thursday into Friday, an initial cold blustery pattern transitioning warmer, more seasonable in-between. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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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/...High confidence. Through 12z... Areas of MVFR developing near the south coast 09-12z as light snow develops. Localized IFR vsbys. Today...High confidence. Period of snow with MVFR/IFR conditions moves along the south coast and Cape/Islands 12-16z. Northern edge of MVFR BDL-PVD with areas of IFR to the south, especially Cape/Islands. Mainly VFR cigs along and north of BAF-ORH-BOS. Improving conditions this afternoon with gradual clearing from NW to SE. WNW gusts 20-25 kt develop in the afternoon, strongest higher terrain. Tonight...High confidence. VFR. Clear skies and light winds. Friday...High confidence. VFR with mid level clouds moving in during the afternoon. Areas of MVFR cigs developing late afternoon Cape/Islands. KBOS Terminal...High confidence in TAF. A few flurries possible this morning but any vsby restrictions should remain to the south. KBDL Terminal...Moderate confidence in TAF. Brief MVFR cigs/vsbys in light snow possible for a few hours this morning, but lower conditions should remain to the south. Outlook /Friday Night through Monday/... Friday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHSN. Saturday: VFR. Windy with gusts to 30 kt. Saturday Night: VFR. Windy with areas gusts to 30 kt. Sunday: Mainly VFR, with local MVFR possible. Slight chance SN. Sunday Night: Mainly VFR, with areas MVFR possible. Chance SN, chance RA, slight chance FZRA. Monday: Mainly VFR, with local MVFR possible. Breezy. Chance RA, chance SN, slight chance FZRA.
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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/...High confidence. Tonight... WNW gales this evening then diminishing toward midnight as 975 low over New Brunswick lifts farther north into Canada. Thursday... Weak low moves east across 40N south of New England with a period of morning snow lowering vsbys especially near south coast of MA/RI. Low races out to sea with improving conditions during the afternoon and winds becoming NW. Thursday night... Modest NW winds in the evening becoming light late as weak high pres builds in from the west. Dry weather and good vsby. Outlook /Friday Night through Monday/... Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of snow showers. Saturday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain showers, slight chance of snow showers. Saturday Night: Low risk for gale force winds with gusts up to 35 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night through Monday: Winds less than 25 kt. Chance of rain.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ231- 232-235-237-251. Small Craft Advisory until 4 AM EST early this morning for ANZ230-233-234. Small Craft Advisory until 3 AM EST early this morning for ANZ236. Small Craft Advisory until 7 AM EST Friday for ANZ250-254>256.
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&& $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...KJC SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...Nocera/Sipprell

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