Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 132347 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 647 PM EST Wed Dec 13 2017 .SYNOPSIS... * Update on present winds in the near-term ... 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 THURSDAY MORNING/... 7 pm update... Light snow associated with a clipper low fast approaching. From the SPC mesoanalysis, modest low-level frontogenetical push ahead of the storm yielding some decent output as discerned via local WSR-88D radars. Associated deep moist convergence on the backside with elements over-running ahead along the leading warm front, beneath strong ascent per main vortmax and parent H2 jet streak aiding with divergence aloft (H85-2 differential divergence). Decent warm air advection, can even see a nose of steep mid-level lapse rates sneaking into the OH River Valley. But downstream, the cold, dry airmass in place, low surface dew- points holding firm, keeping both the surface low and better moisture fetch well S offshore. Limitations on what can be lofted N of the low and as to the potential of that being lofted in reaching the ground. Nevertheless, do expect snow to move in beginning around 4a, the height around 7a (9z-12z). Reductions to visibility with light accumulations, the greatest of which around 1-2 inches confined to the S-coast and across the Islands, the Islands likely seeing the most. Potency of the system will need to be monitored closely as we go through the evening into the overnight. Have adjusted the forecast bringing PoPs up to highly likely. Echoing the prior forecaster, increasing clouds, snow settling in, at first appearing on radar to be overspreading the region, but likely initially not reaching the ground until a little later, closer to daybreak. As to winds ... holding onto the WIND ADVISORY. Still seeing peak wind gusts this hour that are actually at their strongest of the day in some spots, close if not exceeding 45 mph (40 kt) criteria. Vertical wind profilers from our radar and TDWR in Boston has H925 winds out of the west in excess of 50 mph (45-50 kts observed). Upstream winds are diminishing, so will continue to evaluate as to whether headlines need to be dropped or extended. Previous discussion... CAA continues across southern New England with SPC mesoanalysis indicating -19C air at 850 mb upstream over northern portions of NY and VT. Thus modified arctic air continues pour across the area on blustery WNW winds up to 45 mph, courtesy of 975 mb low over New Brunswick. This will result in very cold conditions this evening along with bitterly cold wind chills. Fairly robust mid level trough currently over the Great Lakes races eastward overnight with increasing clouds and light snow/flurries developing toward daybreak especially across CT, RI and south of the MA pike in western part of the state. Initially airmass is very dry so while radar may look impressive late tonight probably a few hours of virga before snow reaches the ground toward daybreak Thu. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... *** 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 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 /00Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Tonight... VFR. Blustery W winds, gusts 25-35 kts, subsiding towards morn. Increasing CIGs, lowering as -SN spreads SW-NE across terminals 9-12z, MVFR- IFR VSBY impacts expected as lighter W winds prevail. Thursday... BAF-ORH-BOS MVFR 3SM -SN beginning around 9-12z, through roughly 15z then tapering. S of the line, 2SM -SN again 9-12z, IFR yet MVFR VSBYs. Lowest conditions over S-coast and Islands with more appreciable accumulations around 1-2 inches subsequent of -SN/SN and VSBYs down to 1/2SM and IFR CIGs around 800 ft agl. Continued W winds, -SN/SN activity tapering 15-18z, becoming VFR and dry. Thursday night... VFR, dry, W winds becoming gusty again, especially along the S/SE coast. KBOS Terminal... Anticipating flurries to overspread the terminal around 9z then transitioning towards -SN with MVFR VSBYs. Brief period 12-15z and then improving thereafter. Light accumulations possible on area runways but mainly a dusting. A dry snow, easily blown around by jet turbines. KBDL Terminal... Flurries into the terminal airspace around 9z then becoming more light, potentially moderate, snow reducing VSBYs to 2SM / IFR. Light accumulation on runways possible up to an inch, but a light and fluffy snow easily blown around by jet turbines and expected W flow around 10 kts. Improvement after 15z, snow tapering as VSBYs improve. Outlook /Friday through Monday/... Friday: VFR. 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 Thursday Night/...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 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory until 10 PM EST this evening for MAZ007-019- 022>024. Wind Advisory until 8 PM EST this evening for MAZ002>006- 008>018-020-021-026. RI...Wind Advisory until 10 PM EST this evening for RIZ008. Wind Advisory until 8 PM EST this evening for RIZ001>007. MARINE...Gale Warning until 1 AM EST Thursday for ANZ230>235-237-251. Gale Warning until 9 PM EST this evening for ANZ236. Gale Warning until 3 AM EST Thursday for ANZ250-254>256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera/Sipprell SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell

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