Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 110552 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 1252 AM EST Mon Dec 11 2017 .SYNOPSIS... Lake effect snow from Lake Ontario will move across parts of the region tonight with minor snow accumulations possible...mainly over portions of northern New York. Relatively dry weather is expected on Monday before a low pressure system moves across the area Tuesday into Wednesday and brings widespread light to moderate snowfall to the North Country. && .NEAR TERM /UNTIL 2 PM THIS AFTERNOON/... As of 1020 PM EST Sunday...Forecast remains on track this evening with plenty of mesoscale factors influencing weather conditions late this evening and thru the overnight hrs. At 03Z, well-defined arctic bndry extending NE-SW had pushed through Ottawa, Ont., with nwly wind shift. Temperatures drop off rapidly with cA air mass across nrn Ontario...which will be our air mass for Monday. South of the front, low-level flow has backed just enough to bring lake Ontario snowband nwd into srn St. Lawrence and Franklin NY counties. May see localized snowfall accumulations of 2-4" across far srn St. Lawrence county, before frontal passage rapidly shifts band south of our forecast area after 08Z or so. With 850mb flow briefly 25-30kts this evening, may see occasional snow showers or flurries extending ewd across the Champlain Valley into the nrn Green Mtns. A light accumulation (coating to 1") is possible in localized areas with ern extent of the lake streamer overnight, mainly across the higher terrain. The cold front itself will begin to shift across the St. Lawrence Valley around 06-07Z, and thru the Adirondacks and nrn Champlain Valley around 09-10Z. May see some brief enhancement of snowfall as lake effect band is entrained into frontal circulation/convergence. However, overall effect will be to shift the Lake Ontario snowband south of our CWA during the pre- dawn and toward sunrise on Monday. Colder and drier air will advect in behind the front as well, bringing the snow showers to an end by mid morning Monday. All told, expect southern St Lawrence and Franklin Counties in NY to see the most accumulation, generally 2 to 4 inches. Elsewhere, snowfall will be 2 inches or less, a combination of lake effect snow showers and snow showers along the frontal bndry as it shifts to the south and east. Snow ratios will be relatively high with good dendrite snow growth, generally 18:1 to 22:1. Temperatures will be relatively steady much of the night, but drop quickly late with passage of the arctic front. Should see readings in the mid-teens across the St. Lawrence Valley and Adirondacks toward daybreak, but generally low-mid 20s across VT. For Monday and Monday night...once the aforementioned front moves through...again by mid morning or so...expect drier weather to briefly move in as high pressure quickly skirts across the area. Highs will be in the 20s, with the coldest readings coming in the northern mountains. As we head into Monday night, the focus turns to a clipper system approaching from the west. Precipitation associated with warm air advection out ahead of this system will spread from west to east overnight, but with dry air in place at lower levels, it will take a bit before the snow is able to accumulate. Lows will be in the teens pretty much areawide. && .SHORT TERM /2 PM THIS AFTERNOON THROUGH WEDNESDAY NIGHT/... As of 345 PM EST Sunday...A winter storm watch has been issued for snow accumulations of 4-7 inches as a well defined northern stream clipper low tracks just south of the North Country. By Monday night a clipper low will be tracking through the Great Lakes and lifting a warm front into the North Country. Warm advection snow will overspread the North Country producing widespread plowable snowfall. The best moisture convergence and lift will be during the morning hours on Tuesday as we see 1-2 inches across southern Vermont before sunrise and then another with another 2-4 inches during the mid morning hours Tuesday. Storm Total accumulations are anticipated to be in the 4-7 inch range so just on the border between advisory vs warning. Being early in the season I opted for a watch to highlight the potential for adverse travel on Tuesday morning. The trends in the 12z guidance were to increase the amount of liquid qpf with the system so even accounting for the normal high bias in warm advection we are in the range of seeing between 0.25-0.4 inches of liquid with snow ratios in the 14-18:1. Will need to watch to see if trends continue to increase the QPF because thats generally not favourable to see high qpf events for the North Country with clipper type systems. Expect with the warm air pushing in on Tuesday temps will be in the upper 20s to low 30s before the cold front from the parent upper level trough swings through. Expect overnight temps to generally be in the 20s until just before daybreak when lows drop into the teens. Then as the upper level trough swings through Tuesday night into Wednesday the pattern will shift to more orographic showers in the western slopes of both the Greens and the Adirondacks. The moisture will be limited but even so another 1-3 inches will be possible in the higher terrain. Cold air moves into the region Wednesday night with both the GFS and EC showing -18C to -20C at 850mb. With a fresh snow pack I`ve got lows in the single digits on Wednesday night. I did trend up a couple of degrees from guidance as even with the snow pack, expect mostly cloudy skies which will dampen how fast we cool. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 345 PM EST Sunday...The large scale pattern continues with a mean upper level trough over over eastern Canada. We should see one brief quiet day on Thursday but by Friday another weaker clipper system will bring a chance for measurable snow. With the colder airmass still in place anticipate temperatures to remain below normal with teens for highs on Thursday and a gradual warming trend towards upper 20s to low 30s by Saturday. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... Through 06Z Tuesday...the combination of a streamer of snow showers extending from Lake Ontario across the Adirondacks and into northwest Vermont and a cold front just entering the St Lawrence Valley is resulting in a mix of MVFR and local IFR conditions at this time. The front will slowly push southeast, and cause the lake effect band to pivot more to the south and dissipate by mid-morning. For the most part, the lake effect band will affect SLK and from time to time light snow will make it to BTV as well. Tried to handle the snow at those locations through tempo groups. Light snow showers may occur at most other TAF sites through 12z or so, but confidence is low, so just some VCSH. Expecting VFR conditions to become widespread after 15z, and stay that way into Monday evening. Toward the end of the TAF period, expecting light snow to develop across western sections. Most of the impacts of a lowering to MVFR/IFR will occur after 06z, so will tackle more of that for the 12z TAFs. Outlook... Monday Night: Mainly VFR, with local MVFR possible. Chance SN. Tuesday: Mainly MVFR, with local IFR possible. Definite SN. Tuesday Night: Mainly MVFR, with areas IFR possible. Chance SHSN. Wednesday: Mainly MVFR, with areas VFR possible. Chance SHSN. Wednesday Night: Mainly VFR, with areas MVFR possible. Slight chance SHSN. Thursday: Mainly VFR, with local MVFR possible. NO SIG WX. Thursday Night: Mainly VFR, with local MVFR possible. Slight chance SHSN. Friday: Mainly VFR, with local MVFR possible. Chance SHSN. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Winter Storm Watch from late tonight through late Tuesday night for VTZ001>012-016>019. NY...Winter Storm Watch from late tonight through late Tuesday night for NYZ026>031-034-035-087. && $$ SYNOPSIS...Hastings NEAR TERM...Banacos/Hastings SHORT TERM...Deal LONG TERM...Deal AVIATION...Nash

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