Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 172112 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 412 PM EST Fri Nov 17 2017 .SYNOPSIS... A brief area of high pressure over the region today will continue to provide dry weather to the North Country through most of Saturday before the next low pressure system brings another round of mainly rain Saturday night into Sunday morning. Rain showers transition to mountain snow showers Sunday afternoon, with dry conditions expected late Sunday night into Monday as high pressure returns. && .NEAR TERM /THROUGH SATURDAY NIGHT/... As of 316 PM EST Friday...It`s taken most of the day, but we`re finally seeing sunshine across the majority of the forecast area this afternoon as low pressure continues to exit northeast into the Gulf of St. Lawrence and a brief area of high pressure is building in from the west. Upper ridge is situated over the eastern Great Lakes which will allow some mid and high clouds to work into the region during the overnight hours, but skies should remain fairly clear through midnight before increasing, which allow temps to drop mainly into the teens. Different story across the high peaks which are sitting in the teens this afternoon, but will gradually warm through the night into the low/mid 30s by dawn as mid-level warm advection develops ahead of our next system. Next system begins to approach the region Saturday, but trends amongst the latest guidance continues to be towards a later arrival time of precipitation so feel most of the daylight hours of Saturday will actually be dry. Synoptic setup will be a developing area of low pressure over the Ohio Valley Saturday morning, tracking northeast into the eastern Great Lakes Saturday night to the vicinity of Ottawa by 12z Sunday morning. Increasing southerly flow ahead of the attending warm front will usher in seasonably warm temperatures Saturday afternoon in the 40s area-wide to near 50 for some locations. Precipitation arrives around the 6-8PM timeframe across western New York, and shifts east into Vermont through the overnight hours. By this time, warm air advection will be strong enough that temperatures while cooling slightly through the night, will still support rain as the dominant ptype. Area of some concern will be across the far Northeast Kingdom where some cold air damming looks likely keep temps at or below freezing for a period late Saturday night into early Sunday where some light ice accumulations are possible. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
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As of 409 PM EST Friday...To be honest, I was hoping the 12z guidance suite would be much closer together with their various solutions than they are. In a general sense, they all still support the idea that a deep (around 980mb) low pressure system will move northeastward out of the Great Lakes that will push a strong cold front through the region during the day and also be accompanied by strong gusty winds. However the exact track and timing differ enough (+/- 150 miles and 6 hours) that it results in a good deal of uncertainty with regard to specific details in the sensible weather (when rain changes to snow, how strong wind gusts get etc). The GFS & Canadian show the surface low moving right up the St Lawrence Valley. The 12z NAM has moved westward compared to prior runs and now shows this track as well. This is a climatologically favored low track. However, the 12z ECMWF shifted a bit east, and is now bringing the low across the Adirondacks on it`s way to Maine. This is not a typical storm track. Perhaps it`s the start of a model trend pointing toward a solution that has a low taking a more easterly track. That said, most of the ensemble model runs still have the westerly St Lawrence track. Long story short, for this forecast cycle, we are maintaining a forecast scenario where the low center moves up the St Lawrence Valley early Sunday morning, with a strong cold front moving through by mid-day. With that, here is what is expected at this point: Sunday morning should be plain rain across the region as temperatures aloft and at the surface will be above freezing. However a cold front will sweep across northern NY by late morning and then into eastern VT by early afternoon. 850mb temperatures will drop from about +4C to -5C in a few hours. This will turn the rain showers to snow showers first at higher elevations, and eventually lower down. It`s a strong front, so we may have a narrow band of heavier showers with the front, but at this point the hi-res models aren`t showing this feature. This means high temperatures will be first thing in the day. Exactly how warm it gets depends on the track of the low. Further west means a better chance of warmer air getting farther north. Right now have some low-mid 40s highs in the morning, dropping back into the 30s for the afternoon. Could be some places reaching 50F. Or if the low stays further east, then 40F will be about it. As the precipitation changes over to snow, the deepest moisture is moving out, so not anticipating any significant accumulations. Perhaps 1" or so at elevations primarily above 1000ft, and only some flakes that melt when they hit the ground in the valleys. It`s the winds that will become the most noticeable aspect of this storm. After the front passes, strong cold air advection will result in deep boundary layer mixing. At the same time a tight pressure gradient will produce winds of 40-50 knots a few thousand feet above the surface. A rapid pressure rise on the order of 10mb/3 hours also suggests ageostrophic enhancement to the winds. Net result, there is the potential of wind gusts on the order of 35-45 mph for much of Sunday afternoon and evening. Some downslope enhancement on the eastern sides of the `Dacks and Greens as the westerly winds flow across, so perhaps localized higher gusts in those areas. Later forecasts will fine tune the need for any wind related headlines (probably advisory level). Sunday night, the snow showers will come to an end, but the wind will continue to blow. Expecting 20-30 mph gusts nearly everywhere as cold air advection keeps the boundary layer mixed. Lows will bottom out in the teens to 20s, so wind chills will be primarily in the single digits.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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As of 409 PM EST Friday...Stuck pretty close to the guidance blend for the period. In general, the 12z guidance was pretty similar with the large scale features. So overall a reasonably quiet weather week as large scale trough settles in over eastern Canada. Several weak systems will zip through in the fast flow. Temperatures will fluctuate some day to day, but overall average at to just below normal. A few notes on each of the days: Monday: Still a gusty/brisk day thanks to a reasonably tight gradient still existing between the departed low pressure and a ridge of high pressure that will start to build in. Thinking we`ll still see winds of 25-35 mph across much of the region during the day. By evening, winds slack off and start to transition to a southerly flow. This may cause some weakening lake effect snow showers streaming off Lake Ontario to shift position and move into the St Lawrence Valley. Not expecting any significant accumulation, and have only 20-30% PoPs for that. Tuesday: Looking pretty nice. Southwest flow will bring in warmer temperatures. Weak ridging should result in fairly clear skies. Highs well into the 40s for most of the region. Wednesday: A weak cold front will move through. Perhaps a few rain transitioning to snow showers (especially across the northern NY & VT mountains), but it looks pretty weak. Not thinking it will have any impact on those travelling for the holiday. Turkey Day: Chilly but mostly dry. Typical late November weather. Couldn`t rule out a flurry here or there as westerly flow will advect some moisture off the Great Lakes causing fairly cloudy skies. Given trajectory of the overall flow, any organized lake effect snow bands will remain south of the forecast area. Friday: Models indicate a weak shortwave moving across the region. Could be a few light snow showers. Don`t see anything in the models at this point indicating anything major precipitation wise, so continued good travelling for those enjoying the holiday. Temperatures again only in the 30s for highs, cooler at higher elevations.
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&& .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/... Through 18Z Saturday...Mix of MVFR and VFR currently will trend to VFR/SKC at all sites by 00Z and remain VFR through the remainder of the period with mid/high clouds increasing from west to east after 00Z. Northwest winds gusting 20-25kts through the afternoon will go light/calm after sunset before turning southerly and increasing to 5-15kts after 12Z Saturday. Outlook... Saturday Night: Mainly MVFR, with areas IFR possible. Definite RA, Likely SN. Sunday: MVFR/IFR conditions possible. Windy with gusts to 35 kt. Likely RA, Chance SHRA, Chance SHSN. Sunday Night: Mainly VFR, with local MVFR possible. Chance SHSN. Monday: VFR. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday Night: VFR. Chance SHRA, Chance SHSN. Wednesday: Mainly VFR, with local MVFR possible. Slight chance SHRA, Slight chance SHSN. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Lahiff NEAR TERM...Lahiff SHORT TERM...Nash LONG TERM...Nash AVIATION...Lahiff

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