Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 110832

Area Forecast Discussion
National Weather Service Burlington VT
332 AM EST Mon Dec 11 2017

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.


As of 330 AM EST Monday...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.


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.


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.


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.


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.


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.


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