Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 032031 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 PM EST Sat Dec 3 2016 .SYNOPSIS... * Near- and Short-term updates only... High pressure builds across the area allowing for clear conditions and light winds Sunday. A period of accumulating snows is forecast from early to mid Monday morning as a weather disturbance sweeps across the region. 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 late Tuesday night into Wednesday night, though timing and track remain uncertain. A polar front crosses the region late next week resulting in a trend toward colder than normal temperatures. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Into this evening... 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... * Temperatures running near to slightly below seasonal normals through midweek. * Low pressure looks slowly move across the Ohio Valley and mid Atlantic states, which may bring some mixed rain and/or snow into the region Tuesday night into Wednesday night, but confidence remains low. * A strong polar front approaches late next week, possibly bringing more unsettled conditions. Sunday night through Monday...Moderate confidence. A weak mid level short wave and associated surface reflection looks to bring a band of scattered rain and/or snow showers across the region during this timeframe. 00Z model suite in better agreement in holding on to band of light QPF amounts as it crosses. Low probability that there might be some light snow accumulations /0.5 inches or less/ mainly across the higher inland terrain late Sunday night into Monday morning. Precip should push offshore during Monday afternoon. Have kept low CHC POPs in the forecast. Monday night...Moderate confidence. Dry conditions and mostly clear skies prevail Monday night with surface high pressure centered over southern Quebec...and upper level ridging...being the main influence on our weather. Tuesday and Wednesday...Low confidence. Models showing a bit better continuity with next approaching system during this timeframe. High pres ridge pushes into Quebec, with southern extent bringing E-NE winds. Some model timing issues in place on how quickly the leading edge of the overrunning precip pushes into the region. Seems the latest GFS is bringing precip shield NE a bit faster than the GGEM and ECMWF. Looks like best chance for light rain and/or snow will occur Tuesday night and Wednesday as low pressure moves slowly up to mid Atlantic coast, associated with H5 short wave moving NE in the SW flow behind to exiting long wave ridge that crosses the region Tue night. Still uncertainty as to exact track of the mid Atlantic low pressure, as well as moisture field to the W of the region with the second low. Noting a rather strong cutoff H5 low S of Hudson Bay, which looks to be digging a deep long wave trough across the western Great Lakes and Mississippi valley. Also seeing arctic airmass diving SE out of western Canada and Alaska late Wed/Wed night. Light precip could linger through Wed night, depending upon track of the coastal low as well as developing system in the central U.S. Thursday and Friday...Low confidence. Models continue to signal development of H5 long wave trough with arctic airmass diving into the Plains states. Timing and track differences remain on the 00Z model runs, along with how the sensible weather evolves. At this point, looks like the coastal low swings across the region sometime Thu, with another front swinging out of the Ohio Valley late Thu night or Fri. Again, timing in question with how quickly the second front swings eastward. Have continued chance POPs for this forecast. && .AVIATION /20Z SATURDAY 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. Remainder of today... VFR. SCT-BKN low-end VFR cigs over N/W MA and CT high terrain as well as across the Outer Cape. Low risk MVFR. NW winds gusting as high as 35 kts with highest gusts over high terrain, coastline. 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 and Wednesday...Moderate confidence for overall trends, lower on exact timing. Should be mainly VFR Tuesday morning. May see MVFR CIGS/VSBYS move into central and western areas Tuesday afternoon in mainly -RA, but may see mixed precipitation across the interior. MVFR to local IFR CIGS/VSBYS possible Tue night into Wed with chance of -RA and/or -SN. && .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 Tuesday night...Winds and seas remain below small craft criteria for most of the waters. NE winds shift to E and increase, gusting to around 20 kt Tuesday night across the southern outer waters. Wednesday...E winds will gusts to around 20 kt, possibly up to 25 kt on the southern outer waters. Seas forecast to build to 5-6 ft there as well. && .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/EVT NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...EVT AVIATION...Sipprell/EVT MARINE...Sipprell/EVT

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