Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 111050 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 550 AM EST Mon Dec 11 2017 .SYNOPSIS... Weak high pressure will bridge across the area today with any morning light snow showers ending and seasonably cold temperatures expected. Low pressure will track across the region on Tuesday and intensify to our northeast on Wednesday bringing a widespread moderate to locally heavy snowfall to our region. Behind this system, a cold airmass settles into the region into Friday before temperatures moderate by next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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As of 550 AM EST Monday...Minor update to increase light snow/snow shower activity across northern counties through mid- morning to account for current activity. Minor accumulations of a dusting to an inch still look reasonable through 8 am, though localized amounts to 2 inches will be possible here and there. Have a great day. Prior discussion... The forecast remains largely on track for today as a moderating polar front drops through the area this morning. Scattered light snows/snow showers will continue across mainly central and northern counties in association with the boundary this morning, though accumulations should be light and generally less than an inch. Mainly dry weather is expected across the south. High temperatures will average about 4 to 8 degrees colder than yesterday across the north under light west to northwesterly flow - mainly upper teens to mid 20s, though a few low 30s will be possible far south where the front will arrive later in the morning.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/... As of 330 AM EST Monday...The most active period in the 7-day forecast then takes shape later tonight into the Tuesday/Wednesday time frame as a well-advertised clipper low takes a favorable track across our area with a moderate to locally heavy snowfall expected for our area. The low will then deepen rapidly into Wednesday night as it moves to our north and east. As discussed last night, the storm will have two phases, the first occurring Tuesday morning and afternoon as favorable warm advective processes interact with deeper moisture and isentropic lift to produce a widespread moderate snowfall as the low approaches. Southeasterly flow in the lower to mid-levels should allow precipitation to take on a typical orographic character with the heaviest totals along the eastern slopes of the southern Greens where low-end Winter Storm Warnings have been issued for a general 4- 8 inch snowfall. As a side note I`ve leaned on the cooler side of guidance in regard to mid- level warming in these areas and have leaned away from the milder NAM output anticipating wet-bulb processes should be enough to cool the column sufficiently to keep p- type all frozen. Elsewhere, we`re mainly looking at advisory-level totals ranging from 3-7 inches with the highest amounts along the eastern slopes of the northern Greens/Dacks and in the St. Lawrence Valley where where northeasterly channeling may enhance QPF slightly. Lows tonight should generally range through the teens with corresponding highs on Tuesday in the upper 20s to lower 30s. By Tuesday evening the low will track to our north and east dragging the northern nose of a distinct dry-slot into central and southern areas. During this period steadier snows should taper off in intensity and lessen in overall coverage, especially across the south where some patchy freezing drizzle may be possible. At this point it appears there will be a saturated layer above the dry-slot such that light seeder-feeder processes should keep any light pcpn still mainly in the form of light snows/flurries. As we progress into Wednesday phase two of the system affects our area as deeper moisture wraps back into the region on increasingly gusty northwest flow. During this period snows should reblossom in earnest across the northern mountains with some backbuilding into eastern portions of the northern Champlain Valley as flow trends blocked over time. While additional accumulations should be light in most spots, favorable upslope areas of the western slopes and northern Greens stand to pick up several more inches where longer- term Winter Storm Warnings will be in effect, both for the front-end warm advective snows, and the back side upslope phase. Here two-day totals will likely range in the 6-10 inch range with localized heavier totals. With the pressure gradient tightening over time areas of blowing snow will also be possible as northwesterly winds gust into the 15 to 30 mph range. This will put quite the chill in the air and with temperatures holding nearly steady from the mid teens to lower 20s apparent T values will range through the positive and negative single digits. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 330 AM EST Monday...00z guidance didn`t really have too much in the way of changes for the period. GFS and ECMWF both similar with colder temperatures hanging on into Saturday as eastern trough hangs in. Both models do show more of a westerly flow pattern for Sunday with moderating temperatures as a Pacific airmass spreads in, pushing the arctic air out. But there are differences between the two regarding how much warming there may be. A fairly active pattern, albeit each disturbance will be fairly weak, so at this point it doesn`t look like any significant precipitation. Just several chances for light snow showers, along with a good deal of clouds. A few notes on specific time periods follow: Wednesday Night: Fairly steep pressure gradient in the wake of the low pressure. With cold air advection, we should see gusty northwest winds linger all night. The airmass moving in will be the coldest we`ve seen so far this young winter season, with lows expected to be in the single digits, when adding in winds gusting 20-25 mph that gives wind chills of -10F to nearly -20F. Should be a few higher elevation snow showers across northern Vermont prior to midnight. Thursday to Friday: A cold couple of days. With 925mb temperatures generally running -10C to -15C during the period, daytime highs are going to struggle to reach 20F at lower elevations with teens at higher elevations. May see some breaks in the clouds Thursday night, and with fresh snow cover and lighter winds, there is a chance for pockets of good radiational cooling. At this point, stuck with guidance blend and have lows ranging from -5F to +5F, though could easily have our normally cold spots in the `Dacks and Northeast Kingdom go colder than -10F if we can get some clear skies. Guidance does suggest a weak trough coming in Friday, so have some low PoPs for a few light snow showers. Friday night will be another cold one, but not as cold as Thursday night given clouds and a few flurries. Saturday/Sunday: As mentioned above, guidance points to a transition to a more westerly flow pattern with moderating temperatures. Still chilly Saturday with highs in the high teens to low 20s, but by Sunday 925mb temperatures will be somewhere in the -5C to 0C range indicating highs near to above freezing in lower elevations. With the warm air advection aloft, still looking at plenty of clouds with a spotty flurry. && .AVIATION /12Z 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 Weather Advisory from 1 AM Tuesday to 1 AM EST Wednesday for VTZ001-002-004-005-007>009-011. Winter Storm Warning from 1 AM Tuesday to 1 AM EST Wednesday for VTZ010-012-019. Winter Storm Warning from 1 AM Tuesday to 7 PM EST Wednesday for VTZ003-006-016>018. NY...Winter Weather Advisory from 1 AM Tuesday to 1 AM EST Wednesday for NYZ026>031-034-035-087. && $$ SYNOPSIS...JMG NEAR TERM...JMG SHORT TERM...JMG LONG TERM...Nash AVIATION...Nash

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