Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 040831 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 AM EST Sun Dec 4 2016 .SYNOPSIS... High pressure builds across the area today resulting in winds diminishing along with dry but cold conditions. A period of light snow Monday morning yields a minor accumulation before tapering off to a mix of light rain and snow in the afternoon. High pressure returns Monday night with mainly dry conditions into Tuesday. Low pressure off the mid Atlantic coast may bring some mixed light rain and/or snow into southern New England Tuesday night into Wednesday night. A polar front crosses the region Thursday with rain showers possibly changing to snow showers. Very windy and cold conditions Friday and next Saturday with wind chill indices in the single digits and teens at night. && .NEAR TERM /THROUGH TODAY/... 315 AM update... Dry but cold NW flow aloft continues across the region today. Patchy strato-cu clouds this morning over the region will dissipate this afternoon as heights rise slowly across the region and mid level flow becomes less cyclonic. Thus more sunshine this afternoon than this morning. However sunshine today will be ineffective given the low Dec sun angle and cold airmass in place with h85 temps down to about -9C this morning and -5C at 925 mb. Model soundings indicate blyr mixing will be limited to about 875 mb due to subsidence inversion. This will yield highs only in the upper 30s to lower 40s...about 5 degs colder than normal. However less wind than yesterday will make it more tolerable. Ocean effect snow showers early this morning affecting the outer Cape and possibly into Nantucket will come to an end later this morning as subsidence inversion lowers and squashes updraft potential as high pres crest over the region. && .SHORT TERM /TONIGHT THROUGH 6 PM MONDAY/... Snow Potential... Good model agreement on snow overspreading CT and western-central MA between 09z-12z...then into RI and eastern MA 12z-15z. Here are a few limiting factors that will keep snowfall potential low... 1) short wave trough is very progressive, thus short duration of snowfall 2) deep layer moisture is brief as well 3) POPS from guidance are only high chance/low likely. This could be a potential red flag. Prefer to see guid offering category pops if expecting accumulating snow. 4) Forcing for ascent is fairly weak and brief in the snow growth region /-10c to -18c/ with dry air entering this layer rapidly from 15z- 18z. Given this not expecting large snow ratios. 5) short wave trough is deamplifying along with weakening wind fields with time as it approaches and moves across southern New England. This will limit forcing for ascent. 6) warm air at 925 mb surging northward will likely change snow to rain toward midday across CT/RI and south of the MA turnpike. This will also limit snowfall potential. While blyr wet bulb temps are below freezing...precip may be too light to take advantage of evaporative cooling or from melting precip. Given all of the above factors will play this portion of the forecast conservative for now. Potential snowfall looks to be from a coating to an inch for most locations. A few locations in the higher terrain of CT/MA will have a low risk for up to 2 inches. Overall not a high impact event. Best chance of roads briefly becoming snow covered will be across CT/western-central MA including the Greater Hartford-Springfield areas before snow shield weakens as it moves eastward. Given these factors above will hold off on any headlines or statements for the moment. Temperatures... Temps will fall rapidly this evening with sunset given cold/dry airmass in place combined with clear skies and light winds. Thus blended some of the colder MOS guidance to account for this. Then clouds arrive around or shortly after midnight resulting in temps leveling off or perhaps rising a few degs toward daybreak. Temps begin to rise late Mon morning and afternoon as blyr winds become light SE. Winds... Winds will be light given weak pres gradient as feeble low pres tracks south of New England. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Very active weather pattern * Mixed precipitation possible late Tuesday night and Wednesday * Very strong polar front blasts through on Thursday * Windy and very cold into next weekend with strong gales over the waters Monday night... Turning quiet and once again cold. High pressure building back into the region parent with rising heights aloft. Winds should become light out of the N as conditions become mostly clear. Especially for those areas with any snow cover, should see radiational cooling proceed resulting in lows well down into the 20s, perhaps teens for the high terrain. Leaning with the coldest MOS guidance. Tuesday... High confidence. High pressure over eastern Quebec and northern New England will provide a mostly sunny day. However there will be cool northeast to east winds and increasing high and mid cloudiness from the southwest during the afternoon. Highs only upper 30s to lower 40s. Tuesday night through Wednesday night...Moderate confidence. The upper level closed low from the southwest states will have opened up and what is left aloft will be moving across the Mid- Atlantic states then passing the south of New England. Meanwhile a strong upper low will be moving across western Ontario. The net effect will be to cause a splitting of the flow over us. Low pressure at the surface off the VA/NC coast will cause cloudiness and some light precipitation to spread into mainly western and southern portions of southern New England Tuesday night. The precipitation could be a mix of snow and rain in northern CT and central/northeast MA and mainly light snow in northwest MA...with rain elsewhere. On Wednesday...GEFS and ECMWF ensembles all show the main low moving straight east and passing well to our south. However...with light moist easterly flow over the area we should remain cloudy and there could still be some light rain southeast and a light rain/snow mix northwest. Not expecting any more than an inch of snow in the northwesternmost areas. Highs upper 30s northwest to mid 40s southeast. The light precipitation may continue into Wednesday night with the best chances of light snow still in the slopes of the Berkshires especially in northwest MA. Some light accumulations still possible there Wednesday night. Thursday and Thursday night...Moderate confidence. Still some big timing differences in models regarding the progression of a very sharp upper level trough and arctic cold frontal passage. The GFS and its ensembles are roughly 12-24 hours quicker than the ECMWF and its members. We have taken a blend. Southerly winds ahead of the front will cause temperatures to rise to the upper 30s northwest and upper 40s southeast Thursday. Showers are likely ahead of the front. They will mainly be rain showers, but if the front enters northwest MA before evening, the rain showers could change over the snow showers there late in the day. The arctic front should pass through Thursday night, accompanied by a chance of snow showers across most of the region, except rain in southern RI, Cape Cod, and the Islands. Lows 25 to 30 except mid to upper 30s Cape Cod and Islands. Friday and Saturday... Moderate confidence. Ensemble means show low pressure bombing out over southeastern Canada...anywhere from Labrador to Nova Scotia. A very strong pressure gradient over New England will cause northwest winds to gust to 25-35 mph inland and to 35-45 knots over the coastal waters, with perhaps some 50 knot gusts over the waters. Depending on timing, the max winds could be Friday afternoon or Friday night. Highs Friday will struggle to rise through the 30s except lower 40s Cape Cod and Islands. Models are indicating that lake effect snow showers from the eastern Great Lakes may actually make it over the Berkshires and into portions of western and central Massachusetts. Have gone with 20-30 percent PoPs for those areas. With clearing skies Friday forecasting temperatures to drop to below model consensus guidance levels...into the upper teens in the Worcester hills and slopes of the Berkshires with 20 to 25 elsewhere and upper 20s to near 30 Cape and Islands. Winds will still be 20-30 mph and this causes some single digit wind chill index values in northwestern Massachusetts Friday night. High pressure builds into the Mid-Atlantic states on Saturday, but despite sunshine it will still be blustery and cold across southern New England. Highs will only reach the upper 20s to mid 30s. Wind chill indices Saturday night could dip to near zero in northwest Massachusetts. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1 am update... Short Term /through Monday/...High confidence. Thru 12z... VFR with any cigs AOA 4 kft. The exception will be over the outer Cape and possibly into Nantucket where snow showers at times will yield MVFR conditions. Light NW winds except gusty NNW winds over Cape Cod and the islands with gusts up to 30 kt. After 12z... VFR and diminishing winds. Lingering MVFR in snow showers over Cape Cod and Nantucket early will improve to VFR by afternoon. Tonight...VFR and light winds. MVFR in snow overspreading CT and western-central MA between 09z-12z...holding off til 12z-15z across RI and eastern MA. Monday...a period of light snow in the morning then then tapering off during the afternoon. Light snow will mix with or change to rain along the south coast and possibly north to PVD-BOS. Snow accumulations will be light...mainly a coating to an inch. Low risk for a few spots across CT receiving up to 2 inches. MVFR conditions will trend to VFR toward sunset across CT and western- central MA...MVFR likely persisting across RI and eastern MA. KBOS TAF...high confidence for VFR/dry runways and NW winds not nearly as gusty as Sat. MVFR in light snow overspreads Logan 12z-15z Monday. Snow accumulations from a coating to an inch possible. KBDL TAF...high confidence for VFR/dry runways and NW winds not nearly as gusty as Sat. MVFR in light snow overspreads BDL 09z- 12z Monday. Snow accumulations of a coating to 2 inches possible. Outlook /Monday Night through Thursday/... Monday Night... VFR. SKC. Light NW winds. Tuesday...High confidence. VFR. Tuesday night through Wednesday night... Low to moderate confidence. MVFR to local IFR ceilings in light rain RI and southeast MA... a mix of rain and snow northern CT, central and northeast MA, and light snow northwest MA. Thursday... Low to moderate confidence. Mainly MVFR ceilings in areas of light rain showers ahead of an approaching arctic cold front. Depending on timing, rain showers could change to snow showers with local IFR conditions in western MA late in the afternoon. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Today... Gusty NW winds diminish as the day progresses. Ocean effect snow showers east of Cape Cod will dissipate by midday. Tonight... light winds as high pres crest over the waters. Dry weather and good vsby. Monday... light southeast winds as weak low pres tracks near the southern New England coast. Outlook /Monday Night through Thursday/...Moderate confidence. Tuesday through Wednesday night...Winds and seas remain below small craft criteria for most of the waters. NE to E winds may gust to 15 to 20 knots at times. Seas may build to near 5 ft over the southern outer waters by Wednesday night. Thursday...Small craft advisories will likely be needed. Ahead of an approaching Arctic front, winds will shift to the southeast to south and may gust up to 25 to 30 kt by late Thursday afternoon. Seas will be building to 5 to 7 ft over the outer waters. Attention mariners... Although it is just beyond this current should be aware that northwest to west strong gales with some potential for storm force gusts are expected to develop Friday into early next weekend. Stay tuned to the forecast. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 AM EST this morning for ANZ230>237-251-256. Small Craft Advisory until 4 PM EST this afternoon for ANZ250- 254-255. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...GAF AVIATION...Nocera/GAF MARINE...Nocera/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.