Area Forecast Discussion
Issued by NWS Burlington, VT

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602 FXUS61 KBTV 130332 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 1032 PM EST Tue Dec 12 2017 .SYNOPSIS... Widespread light snow will become terrain focused overnight with additional snow accumulation likely...especially for the northern Adirondacks and central and northern Green Mountains. Much colder temperatures return on Wednesday with blustery west to northwest winds and occasional snow showers. Drier and cold weather continues for Wednesday Night into Thursday. && .NEAR TERM /UNTIL 6 PM WEDNESDAY EVENING/... As of 1023 PM EST Tuesday...We`ve cancelled the Winter Storm Warning across northern/southwestern St. Lawrence county...an area where snow has tapered off and won`t regenerate in any significant way owing to orographic forcing. The remainder of the forecast is on track. We anticipate combination of orographically blocked flow (Froude #s <0.5), increasingly deep saturated dendrite growth layer (-12 to -18C) and lake enhancement will allow a ramp up in radar reflectivity across the VT side of the Champlain Valley into the wrn slopes of the nrn Green Mtns within the next 1-2 hrs...and lasting thru 12-13Z Wednesday. This is expected to have an impact on the Wednesday morning commute, and travelers will want to leave extra time once again for commuting. Expecting storm total of 4 to 8 inches CPV/Northern Dacks...4 to 8 inches St. Lawrence Valley...6 to 10 inches southern/central VT/western slopes/nek...and 12 to 18 inches from Mt. Mansfield to Jay Peak by Wednesday evening. Please see our recently issued Public Information Statement and Local Storm Reports for the latest snowfall observations for this event. Whereas the western slopes of the Greens have lagged behind central/ern VT thus far, they will rapidly catch up with the developing NW flow during the overnight hrs into Wed morning. Previous Discussion...Surface analysis places elongated low pres from KSLK to KPOU this aftn with greatest 3hr pres falls toward Cape Cod with trailing cold front over central NY. Radar shows areas of light to moderate snow ahead of approaching boundary with some mix of rain in the cpv/western slopes with temps in the l/m 30s. Snowfall has been highly terrain/wind driven with generally 3 to 7 inches, except 8 to 10 inches central-southern Greens...while only an inch or two has fallen along the western slopes from Nashville to North Underhill. However, this will change tonight. The near-term forecast features our synoptic system transitioning to an mesoscale upslope event for the northern dacks into parts of the central/northern greens...including the western slopes. Water vapor shows potent shortwave trough across the eastern Great Lakes with embedded 5h vort located over western NY/PA helping to enhance ribbon of mid-level moisture and precip along boundary over the NE conus. As upper lvl trof approaches and surface boundary sweeps across our cwa btwn 21-03z...expect winds to shift from south to west/northwest with temps falling 3 to 6 degrees in an hour or two...along with another burst of mainly light to areas of moderate snowfall. At 23Z, we`ve already seen this NWLY wind shift occur across the Champlain Is. and Clinton County, where temperatures have fallen into the low-mid 20s. Further north in the Montreal Que. area, temperatures are in the upr teens, so colder air will be filtering in across the Champlain Valley with the wind shift. Continued snow and falling temps could result in areas of icy travel...especially in the Champlain Valley and point eastward. A quick inch or two is possible with boundary. After boundary...925mb to 850mb winds become west/northwest and the upslope machine will start. However...getting some mixed signals on amount of leftover deep layer moisture and associated qpf/snowfall. GFS/ECMWF continue to show very favorable backside commahead of enhanced 700mb moisture...along with embedded 5h vorts helping with synoptic scale lift in closing 5h/7h circulation. Meanwhile...RAP13/HRRR shows developing mid/upper lvl dry slot and precip quickly ending after fropa this evening with only minor upslope qpf/snowfall near Jay Peak. Given presentation on water vapor and mid/upper level system becoming negatively tilted...along with moderate to strong llvl caa a period of upslope snow is expected btwn 06z and 18z Weds. Also...local froude shows values around 0.50 thru 12z Weds...before flow becomes unblocked with values > 1.0...supporting snowfall downwind of the mountains. Thinking with caa and lowering snow growth zone...ratios will quickly jump from 12/14 to 1 this evening to > 25:1 on Weds...helping to support accumulating snowfall. Interesting the local 4km composite reflectivity shows several lake bands off lake champlain btwn 07z-12z when froude indicates flow is blocked...supporting snowfall along the western slopes/eastern cpv. Including just upslope snowfall thinking 2 to 4 inches northern dacks/cpv...with 3 to 6 inches western slopes...and 6 to 12 inches from Mt Mansfield to Jay Peak tonight into Weds. Little additional accumulation for the slv and southern parts of VT overnight into Weds. Sub 975mb low pres tracks into eastern Canada on Weds with cold and blustery conditions prevailing on Weds. Progged 850mb temps drop btwn -17c and -19c by 18z...with soundings showing good mixing of winds. This will create areas of blowing and drifting snow...mainly over the exposed trrn with temps near zero summits to mid/upper teens cpv/lower ct river valley. Wind chill values single digits cpv to well below zero in the mountains. As deep layer moisture slowly lifts north...anticipate the areal coverage/intensity of upslope snow showers to decrease...with mainly focus over the northern green mountains. && .SHORT TERM /6 PM WEDNESDAY EVENING THROUGH THURSDAY NIGHT/... As of 337 PM EST Tuesday...Deep cyclonic flow continues with chilly temps and mountains snow showers prevailing. If winds and clouds dissipate on either Weds or Thurs Night...temps will drop very quickly and be much lower than forecast...especially with fresh snow pack. Official forecast has lows near -5f slk to near 10f CT River Valley/CPV...with highs single digits to mid teens. Another cold night expected on Thurs with lows -10f to +5f. Mainly dry forecast prevails. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 242 PM EST Tuesday....Overall pattern for the long term from Friday through Tuesday will be a changeable one, but largely driven by broad upper troughing with brief breaks of ridging. Period starts off on Friday with a weak upper ridge over the forecast area in the morning, but an upper trough digging into the Great Lakes will turn the mid/upper level flow back to the southwest allowing a lake effect band to develop in the lee of Lake Ontario. Flow continues to back to the south allowing the band to move up the St. Lawrence River Valley before the upper trough swings in for Friday night through Saturday producing scattered snow showers across much of the forecast area. Brief break comes Saturday night into Sunday as an upper ridge builds overhead, but by Sunday evening deeper moisture associated with a potentially complex system moves into the region increasing chances for snow. Latest GFS and ECMWF continue to be in general disagreement with the pattern for early next week with the ECMWF depicting northern and southern stream energy attempting to phase, while the GFS offers more of a zonal flow keeping southern stream at bay. For now will offer a blended model approach and keep the chance for snow going in the forecast. Temps through the period will trend from below normal through the weekend, then back above normal for early next week. && .AVIATION /04Z WEDNESDAY THROUGH SUNDAY/... Through 00Z Wednesday...A brief lull in the snow will allow for mainly MVFR conditions with periods of IFR through about 04z, then IFR visibility with MVFR ceilings will prevail into Wednesday morning as the snow resumes. Winds will turn to the northwest at all terminals by 04z, generally at 5-15 kt. Winds will increase during the day Wednesday with gusts of 25-30 kt expected. Outlook... Wednesday Night: VFR. Slight chance SHSN. Thursday: VFR. NO SIG WX. Thursday Night: VFR. NO SIG WX. Friday: Mainly VFR, with local MVFR possible. Slight chance SHSN. Friday Night: Mainly VFR, with local MVFR possible. Scattered SHSN. Saturday: Mainly MVFR, with local IFR possible. Scattered SHSN. Saturday Night: Mainly VFR, with local IFR possible. Slight chance SHSN. Sunday: Mainly VFR, with areas MVFR possible. Slight chance SHSN. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Winter Weather Advisory until 7 PM EST Wednesday for VTZ001- 002-005-009-011. Winter Storm Warning until 7 PM EST Wednesday for VTZ003-004- 006>008-016>018. Winter Storm Warning until 7 AM EST Wednesday for VTZ010-012- 019. NY...Winter Weather Advisory until 7 PM EST Wednesday for NYZ027>031-034-035. && $$ SYNOPSIS...Taber NEAR TERM...Banacos/Taber SHORT TERM...Taber LONG TERM...Lahiff AVIATION...Hastings

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