Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 140814 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 314 AM EST Thu Dec 14 2017 .SYNOPSIS... A fast moving low pressure will track south of New England this morning bringing a light snowfall 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 PM THIS EVENING/... *** Light snow accumulations likely immediate south coast and Cape/Islands this morning *** Fairly robust mid level shortwave and attending sfc low tracks south of New Eng this morning. Large scale synoptic forcing assocd with this shortwave and left exit of the upper jet will combine with an area of low level frontogenesis to bring modest omega to the south coast and Islands. Cross sections indicate this forcing for ascent punches through favorable DGZ 12-16z with some instability above this layer which should bring a brief period of light to moderate snow this morning to the south coast and Islands. The limiting factor is dry air in low levels which will erode initial snowfall. Dewpoints currently in the single numbers across SNE, but dewpoints are rising along the immediate south coast and Islands and light snow is breaking out along the CT coast. Used a blend of global guidance and HREF ensemble guidance for QPF with 15-20:1 snow ratios to derive snowfall. Not much change from previous forecast with potential for 1-2 inches along the south coast and 2-3" over the islands. May see a coating to less than an inch up to BDL-PVD-TAN corridor, with just some flurries to the north where low level dry air is more pronounced with sharp drop off in omega. Snow will exit the Cape/Islands by midday with gradual clearing and increasing sunshine moving in from NW to SE during the afternoon. Another unseasonably cold day with highs mostly 25-30 degrees with some lower 30s near the south coast. Becoming blustery in the afternoon with WNW gusts 20-30 mph developing making it feel much colder. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Clear skies and diminishing wind as high pres moves to the mid Atlc coast with ridging extending into New Eng. This will result in good radiational cooling which will be enhanced by fresh snow cover near the coast. Lows dropping into the single numbers, except teens near the coast. Friday... Next mid level trough and shortwave approaches from the west as coastal low pres develops well to the south. Sunshine will give way to increasing clouds in the afternoon as the column moistens. Modest large scale forcing develops late in the day ahead of the mid level trough with best chance for any late day snow showers across SE New Eng assocd with a weak inverted trough and increased low level moisture. Still rather cold with 925 mb temps -8 to -10C which will yield another day with highs mostly in the 20s, but winds will be lighter and less of a factor. && .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 /08Z THURSDAY THROUGH MONDAY/... 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. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. Today... Diminishing winds through the morning, but WNW winds increase this afternoon with gusts to 25-30 kt developing. Area of snow this morning will reduce vsbys, mainly over south coastal waters. SCA all waters. Tonight... Lingering SCA wind gusts in the evening then diminishing winds overnight as high pres builds in from the west. Friday... NW winds shift to the SW in the afternoon. Speeds below 20 kt with seas below SCA. Outlook /Friday Night through Monday/... 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. && .BOX WATCHES/WARNINGS/ADVISORIES... 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. && $$ SYNOPSIS...KJC NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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