Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 180837 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 337 AM EST Sat Nov 18 2017 .SYNOPSIS...
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Strong low pressure will pass through the region tonight into Sunday with widespread rainfall and milder temperatures. Rain will taper to snow showers on Sunday afternoon as much colder air surges back into the area along with gusty winds. The general weather pattern quiets down from Monday onward into much of next week with mainly dry and seasonably cold weather expected for the Thanksgiving holiday.
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&& .NEAR TERM /THROUGH SUNDAY/...
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As of 337 AM EST Saturday...Quite the variety of weather is expected over the next 36 hours as deepening low pressure tracks from the Ohio Valley into the St. Lawrence Valley by Sunday morning. From a broader perspective the overall theme of the forecast remains the same with thickening clouds expected today before widespread precipitation arrives by early evening into the overnight hours. Strengthening southerly flow and warm thermal advective processes will push boundary layer temperatures above freezing in most locales by mid to late afternoon such that mainly a steady light rain is expected. The only exception would be a brief period of mixed precipitation at onset at mountain summit level and possibly across portions of far northeastern VT. Given the strength of the synoptic background southerly flow have generally downplayed this threat however. Highs today to top out in the upper 30s to mid 40s for most spots with readings holding steady in VT overnight, and falling through the 30s toward morning across northern NY as cold front sweeps through. By Sunday the cold front surges quickly across the remainder of the forecast area, clearing well east by afternoon as the ~980 mb surface low races northeast toward the Gulf of St. Lawrence by early evening. Robust 3-hourly pressure rises behind the frontal boundary should ensure a windy afternoon as steadier rains transition to snow showers and flow trends northwesterly. Accumulations should be minor, though amounts from a dusting to 2 inches will be possible across the St. Lawrence Valley and higher terrain of the Adirondacks/Greens by early evening. Using an upward adjustment factor from 160-190 percent yields gusts into the 30 to 45 mph range with highest gusts in favored westerly downslope areas of the eastern Adirondacks and eastern Greens/CT River Valley. Temperatures will take on a typical cool-season non-diurnal trend as highs from the upper 30s to mid 40s will be reached in the morning before values slowly fall into the upper 20s to mid 30s by late afternoon.
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&& .SHORT TERM /SUNDAY NIGHT/... 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. && .LONG TERM /MONDAY THROUGH FRIDAY/... 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 traveling 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 traveling for those enjoying the holiday. Temperatures again only in the 30s for highs, cooler at higher elevations. && .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... Through 06Z Sunday...VFR through 00Z Sunday with thickening mid/high cloud cover as light winds trend south/southeasterly at 5 to 10 kts (northeasterly at KMSS). After 00Z Sunday widespread rain affects all terminals with ceilings lowering to MVFR in the 015-025 AGL range in general. Some occnl VFR may continue at KBTV/KPBG/KRUT. Outlook... Sunday: Mainly MVFR, with local IFR possible. Windy with gusts to 35 kt. Likely RA, Chance SHRA, Chance SHSN. Sunday Night: VFR. Windy with gusts to 30 kt. Slight chance SHSN. Monday: Mainly VFR, with local MVFR possible. Windy with gusts to 30 kt. NO SIG WX. Monday Night: Mainly VFR, with areas MVFR possible. Slight chance SHSN. Tuesday: VFR. Slight chance SHRA. Tuesday Night: Mainly VFR, with local MVFR possible. Chance SHRA, Chance SHSN. Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA, Chance SHSN. && .MARINE...
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As of 337 AM EST Saturday...A Lake Wind Advisory is in effect for today as southerly winds increase into the 15 to 25 knot range by this afternoon and significant wave heights build into the 2 to 4 foot range with a moderate chop. These conditions may prove hazardous to those operating small craft, especially on the broad lake and in bays/inlets with southerly exposures. Gusty south winds will gradually abate tonight, though increase sharply from the northwest by Sunday as a strong cold front sweeps across the area.
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&& .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...JMG NEAR TERM...JMG SHORT TERM...Nash LONG TERM...Nash AVIATION...JMG MARINE...JMG

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