Area Forecast Discussion
Issued by NWS Boston, MA

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317 FXUS61 KBOX 102058 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 358 PM EST Sat Dec 10 2016 .SYNOPSIS... Dry but unseasonably cold weather continues into Sunday afternoon. Low pressure and its attending warm front will bring accumulating snow to much of the region Sunday night into early Monday morning, which will likely change to rain across much of the area Monday morning, except mainly all snow for northwest Massachusetts. A brief period of snow is possible late Wednesday or Wednesday night in advance of a surge of arctic air invading southern New England Thursday and Friday. Another storm system may bring snow and mixed precip next Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 330 PM Update... Very cold and dry airmass over the region late this afternoon with dew pts in the single digits! This combined with mostly clear skies overnight along with high pres building into the area from the OH/TN valleys will result in diminishing winds and ideal radiational cooling conditions. Thus have derived tonight`s min temps from the coldest datasets...MAV and MET mos guidance. This yields low temps in the teens for most of the region and single digits in the valleys and typically colder spots of eastern interior MA including Marthas Vineyard. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/... Sunday... Very cold start to the day, although with neutral thermal advection eventually transitioning to weak warm advection will result in temps climbing a few degs higher than today with highs Sunday 30- 35...upper 20s high terrain. Morning sunshine fades behind increasing afternoon clouds. The onset of warm advection snows/flurries may enter western portions of CT/MA between 4 pm and 7 pm. Elsewhere snow holds off until Sunday evening. *** Accumulating snows Sunday night into early Mon morning *** Sunday night and Monday... Synopsis and Model Trends...00z models last night started the trend toward warmer solutions...holding onto a parent low longer over the Great Lakes Monday and delaying secondary low formation along with a more northeast position. All of this suggest warm air advancing quicker into southern New England faster than earlier model runs. Timing...good model agreement on light snow/flurries breaking out across western CT/MA 4 pm to 7 pm Sunday. Farther east dry air lingers and takes column longer to saturate. Thus snow begins 7 pm to 10 pm remainder of CT/MA and RI. A period of steady snow overspreads the region from about midnight to sunrise/daybreak Monday. Ptype/Changeover...the rain-snow line should begin lifting south to north across CT/RI and eastern MA between 4 am and 7 am Monday. Then the remainder of the region between 7 am and 10 am. However precip may remain all snow across northwest MA. Snow Accumulations and Impacts...most likely snowfall projections are 3-6" across northwest MA...2-4" for interior CT into central MA (including Worcester Hills) and Merrimack Valley...then tapering down to 1-2" for northeast MA (including Boston) into northern RI. Then a coating to an inch for the south coast of MA and RI. Snow will be changing to rain during the morning commute across RI and southeastern MA...probably into the Greater Boston area too. Thus snow covered roads in this area will transition to slushy conditions which will offer some improvement vs north and west of I495 in northeast MA. This is where the Monday morning commute will be most treacherous. Uncertainty...still 36-48 hrs out in model time so still some uncertainty. However model solutions have been converging the past two runs. Held off on issuing Winter Storm Watches for northwest MA for the following reasons...modest forcing for ascent/limited instability aloft/poor snow growth-ratios and very progressive system limiting snow duration. GEFS and SREF plumes both support sub- watch/warning criteria. In addition a superblend of all guidance sources only yields 20-30% risk of 6+ inches across northwest MA. Monday afternoon...dry slot overspreads the region rapidly from west to east...so expecting some partial clearing during the afternoon. However clouds and rain showers will linger longest over the south coast including Cape Cod and islands. With secondary low forming northeast of the region over the Gulf of ME...west-northwest winds develop after 18z and any leftover shallow cool air mixes out. This results in surface temps rising into the low and mid 40s across much of CT/RI and eastern MA...and near 50 over the islands. Cooler northwest MA with highs only mid to upper 30s. However these above temps above freezing will help snow covered roads across the interior transition to bare pavement. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Some light snow possible late Wed and Wed night * Blast of arctic air Thu/Fri * More snow and mixed precip possible next Sat Overview... Large scale pattern features an anomalous ridge across Alaska and blocking ridge over northern reaches of Europe. This results in anomalously low heights across Canada as polar vortex rotates south and east from north central Canada. Main feature in the extended will be the blast of arctic air at the end of the week as the vortex moves to eastern Canada. Ahead of the arctic intrusion we will have to watch for a potential coastal wave which may spread some light snow to coastal New Eng late Wed and Wed night. With lack of downstream blocking over the favorable location in the north Atlantic, pattern is transitory so arctic air will be relatively short lived, and there is a potential for another winter weather event next Sat as the arctic air departs and SW flow aloft becomes re-established. Details... Monday night through Tuesday night... Quiet period with mainly dry and seasonable conditions. High pres moves to the mid atlc coast with dry westerly flow. Some mid/high clouds will move through Tue. Seasonable temps expected. Wednesday into Wednesday night... This is expected to be the transition period ahead of the arctic intrusion during the end of the week. Mid level shortwave diving SE from the Gt Lakes will sharpen the trof somewhat leading to potential offshore wave developing. Consensus of the guidance favors this to remain far enough offshore, but northern extent of the moisture may at least clip coastal areas with some light snow sometime late Wed and Wed night. There are a modest number of GEFS/EPS members that have some snow with low probs for 0.10" QPF. So we will continue to advertise chc pops, mainly for Wed night ahead of the arctic air. Thursday and Friday... There is high confidence for arctic intrusion with coldest air of the season. Ensemble mean 850 mb temps down to -20C late Thu into Fri with deterministic ECMWF even several degrees colder. Expect highs in the teens and 20s both days with Fri the coldest day, and potential for near zero mins in the interior high terrain Fri morning, and single numbers for much of the interior. Bitter cold wind chills well below zero possible late Thu night and early Fri. Looks mainly dry during this period, with low prob for some ocean effect snow showers over the outer Cape if winds veer more than forecast. Low level wind forecast is W/NW which would keep best chance of ocean effect snow offshore but this is still several days away. Saturday... Low confidence forecast at this time range but pattern setting up for some overrunning snow as very cold air in place at 850 mb before arctic air departs. With high pres positioned to the east, may also see an eventual changeover to rain/mixed precip. Still lots of uncertainty with the details so stay tuned. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1 pm update... Thru 00z...high forecast confidence VFR...dry weather and a modest NW wind of 10 to 20 kt. After 00z...high forecast confidence VFR...dry and diminishing winds. Sunday...high forecast confidence VFR...dry weather and light winds. Exception will be across western MA/CT where cigs may lower to MVFR in light snow 21z-22z. Sunday night...moderate confidence. MVFR in snow trending toward IFR toward morning along with areas of LIFR. Snow changing to rain 09z-12z from south to north across RI and eastern MA. Monday...moderate confidence. IFR/LIFR in snow interior Monday morning...rain across RI and eastern MA but improving to MVFR and VFR Monday afternoon. Probably only improving to MVFR across the higher terrain. KBOS TAF...High confidence in TAF. Snow begins 00z-03z Mon with steadiest snow 06z-09z Mon before changing to rain 09z-12z Mon. KBDL TAF...High confidence in TAF. Snow begins 22z-01z Mon with steadiest snow 03z-09z then changing to rain 09z-12z but possibly a brief period of freezing rain 09z-12z. Outlook /Monday Night through Thursday/... Monday night through Tuesday night...Moderate confidence. VFR. Wednesday into Wednesday night...Moderate confidence. A period of MVFR possible sometime late Wed or Wed night with some light snow, especially near the coast. Thursday...Moderate confidence. Mainly VFR cigs. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Tonight...leftover NW winds 20-25 kt and 5+ seas will warrant SCA for the outer waters. Downward trend later this evening and overnight. Dry weather and good vsby. Sunday...light wind with high pres overhead. Dry weather and good vsby continue. Sunday night...south to southeast winds increasing to 20 to 30 kt toward Monday morning. Vsby lowers to 1 mile or less at times in snow. Monday...southeast winds 20 to 30 kt in the morning shift to WSW in the afternoon. Vsby 1 mile or less in morning snow changing to rain from south to north. Improving and drying in the afternoon but slowest improvement on vsbys will be southern waters as rain lingers. Outlook /Monday Night through Thursday/... Monday night...High confidence. West gusts to 25 kt likely with SCA conditions. Tuesday into Wednesday...Moderate confidence. Diminishing westerly winds below SCA by Tue and remaining below SCA into Wed. Seas subsiding. Thursday...Moderate confidence. Increasing W/NW winds with gusts to 30 kt likely and moderate prob of gales and G35-40kt over outer waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EST Sunday for ANZ250-254>256. && $$ SYNOPSIS...KJC/Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...KJC AVIATION...KJC/Nocera MARINE...KJC/Nocera

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