Area Forecast Discussion
Issued by NWS Boston, MA

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437 FXUS61 KBOX 040254 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 954 PM EST Sat Dec 3 2016 .SYNOPSIS... High pressure builds across the area allowing for clear conditions and light winds Sunday. A period of light accumulating snow is possible Monday morning into early afternoon before ending as light rain. 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, though timing and track remain uncertain. 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 /UNTIL 6 AM SUNDAY MORNING/... 10 PM update... Chilly evening across the region with dry but cold northwest flow. 850 temps currently down to about -9C across the area (per latest SPC mesoanalysis) with -12C 850 mb air upstream along the Canadian/northern New England border. Thus cold night ahead. Winds on the verge of decoupling in the valleys and typical sheltered locations. Cold air streaming across the relatively warm ocean waters is yielding ocean effect rain/snow bands east of Cape Ann and Cape Cod. As low level winds back to the NNW overnight and into early Sunday morning these bands may clip the outer cape per Hi Res guidance. Previous forecast captures these details nicely so no major changes with this update. Previous discussion... Gradual pressure rises as high pressure builds in from the W as low pressure exits E. Slight dampening of the winds but remaining blustery out of the NW with gusts up to 30 mph. Scattered to broken cloud decks continue as mid-level energy continues to rotate through the overall cyclonic flow. Plenty of moisture and steep lapse rates below H8-7. Tonight... A quiet, cold night of weather. Scattered to broken cloud decks are forecast to continue across the N/W high terrain of S New England and over the Outer Cape given N/NW flow relaxing through the night as high pressure builds in from the W. Given the breezy conditions somewhat cautious of the coldest guidance. Should be clear over S/E interior S New England and should winds become light, especially within the sheltered valleys, then it should drop off. So with a combination of cold air advection and/or radiational cooling, most locations will see lows in the 20s. Add in the winds, going to feel like the teens for several locations by Sunday morning. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/... Sunday... Pleasant but cool day on tap. Surface high pressure building into the region as heights rise aloft, winds should dampen beneath the anti-cyclonic flow. Clouds dissipating giving all of S New England mostly clear conditions. Recent dousing rains along with a h925 airmass aloft around -4 to -6C, hesitant to go with the warmest of guidance but believe we will come close. Thinking highs around the upper 30s with warmest locations around the low 40s across the lower CT Valley and interior S/E portions of S New England. Light NW wind. Sunday Night into Monday... * Snow and impacts anticipated for the Monday morning commute. * Takeaways: reduced visibilities at times, slippery conditions. * Light accumulations forecast, coating up to 1 inch. * Highest snow accumulations possible above an inch over the east slopes of the Berkshires. Mid to upper level feature flattening while becoming sheared by the strong W flow aloft. Overall weakening as synoptics stretch, there is still a measure of lift of a continental tropical airmass over- running, especially along the 285-295K theta surfaces (H9-7), across a diffuse warm frontal boundary beneath a measure of mid to upper level forcing, enough to generate some low to mid level fronto- genesis, squeezing the available moisture out of the atmosphere as a mix or rain or snow. Went closely with the EC with this forecast. One thing for certain is the environment ahead of inclement weather. Prior to clouds on the increase W to E, conditions will be clear as winds will be light allowing 2m temperatures to drop into the 20s. Much of the area except along the immediate coast will be below freezing before onset of precipitation towards early Monday morning. Also will be fairly dry dewpoints as well. But as to the availability of moisture aloft, especially within the snow growth region (-12 to -18C), there is some uncertainty. Dry air will be eroding the continental tropical airmass especially within the stronger W flow. Anticipating top-down drying with mechanical mixing given the faster flow aloft. Aside, will lean towards the presence of ice (thinking its availability down to -8 to -10C). Will keep any and all wintry precipitation as snow, no freezing rain. So altogether, looking at a window of impacts around early to mid morning Monday, roughly 4 am to 10 pm. Moistening low levels with time, ran a wetbulb procedure closer to temperature with this event given how light it is, but keep it below freezing longer along with the presence of snow. A consensus of forecast solutions along with ensemble probabilistics, at least a coating for much of interior S New England with highest amounts of a tenth or two inch over W New England with up to half an inch over the N/W high terrain. May be too bullish on amounts as latest forecast has on average a half an inch for much of S New England. But aside, snow is expected and the main takeaway is that travel will become hazardous given the timing of the event during the Monday morning commute and should roads become snow covered. At a minimum snow will result in reductions in visibility. Towards midday conditions should quickly improve with winds becoming breezy out of the W and conditions clearing out. Temperatures remain stable during the morning given cloud cover and W winds. More than likely by midday into afternoon with clearing we`ll see temperatures bounce to highs around the upper 30s, low 40s. 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. && .LONG TERM /TUESDAY 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 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 /03Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. 10 pm update... No major changes from 00z TAFs. Earlier discussion below. ============================================================= Tonight... VFR. NW winds diminishing but initially blustery. Gusts up around 25 kts dampening. Cigs dissipating. Sunday... VFR. Mostly clear with light NW winds. Sunday Night into Monday... Mostly -SN with coastal -RA for the Monday morning push. Will see conditions deteriorate around 9-15z to IFR with -SN as VSBY will most likely be impacted. Cigs mainly MVFR, IFR for the high terrain. W winds around 5 to 10 kts. Improving VFR beginning around 18z Monday. Snow accumulations possible, at least a coating, for all interior terminals. Monday Night... VFR. SKC. Light NW winds. KBOS TAF...Gusty NW winds continuing around 25 to 30 kts. Diminishing through the overnight period. KBDL TAF...Gusty NW winds continuing around 25 to 30 kts. Diminishing through the overnight period. Outlook /Tuesday through Thursday/... 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 Night/... Through Sunday... High pressure building into the waters will allow winds and seas to relax. Initially, NW gusts up to 30 kts expected, dropping out overnight into Sunday morning. Wave action dampens through Sunday. This will allow small craft advisories to conclude. Ocean effect rain showers likely to be an issue throughout, especially over the outer waters. Sunday night into Monday night... Weak disturbance will sweep W to E across the waters during the early to mid Monday morning period. Winds will overall be light and out of the W. Expect showers, perhaps some slight reductions in visibility. Winds increase late going into Tuesday morning out of the NW, could see some gusts up to 20 kts. Waves remaining below 5 feet throughout. Outlook /Tuesday 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 Sunday for ANZ230>237-251- 256. Small Craft Advisory until 4 PM EST Sunday for ANZ250-254-255. && $$ SYNOPSIS...Sipprell/GAF NEAR TERM...Nocera/Sipprell/GAF SHORT TERM...Sipprell LONG TERM...GAF AVIATION...Sipprell/GAF MARINE...Sipprell/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.