Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 172349

Area Forecast Discussion
National Weather Service Burlington VT
649 PM EST Sun Dec 17 2017

Surface high pressure cresting over the North Country will
bring seasonably cold and quiet conditions through the first
half of tonight. A warm front approaching from southwestern New
York will bring increasing cloudiness and widespread light snow
for Monday morning into early Monday afternoon. Snow accumulations
between 1 and 3 inches are generally expected, with localized
totals around 4 inches for the southern Green Mountains. A
brief warm up is expected on Tuesday, with gusty south to
southwesterly winds. A frontal passage Tuesday evening will
bring a few rain or snow showers and a return to colder
temperatures. The next chance for significant precipitation will
arrive Friday night into Saturday. A low pressure system
passing to the west of the region is expected to bring mixed
wintry precipitation, transitioning to plain rain as warmer air
moves in on southerly winds.


As of 554 PM EST Sunday...Overall forecast in good shape with
just some tweaks to match current temperatures...which are wide
ranging at this time given relatively clear skies and light
winds. Eventually thicker cirrus will move in and temperatures
should stop falling and in some locations warm back up slightly.
Going forecast has this covered well.

Previous Discussion...
A period of light snow 12-18Z Monday will produce a general
1-2" snowfall across the region, with localized totals 2-4"
across the spine of the Green Mtns and across portions of
southern St. Lawrence and Franklin NY counties. While not a
significant event, may see a few travel slowdowns across the
region Monday morning into early Monday afternoon with
snow covered roads and brief periods of visibility less than
one mile. We`ve made this the focus of our social media posts
this afternoon and included mention in the Hazardous Weather

In the meantime, looking at a quiet evening and first half of
the night as 1030mb sfc anticyclone crests over the North
Country and srn Quebec before shifting ewd. This will result in
light winds and good radiational cooling after sunset. By
midnight, should see 2-meter temperatures generally zero to 10
above, and locally falling below zero across far nern VT. Will
see temperatures level off after 06Z Monday - or rise slightly -
with increasing high- mid level clouds from SW-NE in advance of
modest warm front across the srn tier of NY. Likewise, light
S-SE winds will develop after midnight.

Should see sufficient isentropic lift to allow light snow to
develop starting around 11Z across the nrn Adirondacks and
Rutland county, and lift nwd into the Champlain Valley by 12Z or
so. Combined with modest mid-level height falls as a shortwave
trough progresses ewd along the intl border, will see a widespread
light snowfall across the area thru 18Z or so. With warming
aloft, will see snow-to-liquid ratios drop into the 12:1 to 14:1
range. Best forcing for ascent shifts ewd during Monday aftn,
with some continued orographic snow in the Green Mtns and across
the Adirondacks. Temperatures will not be as cold, with highs
Monday generally 25-30F, except lower 20s with localized nely
winds in the St. Lawrence Valley, and across far nern VT where
low-level cold air will be slower to scour out.

Monday night will feature continued overcast conditions and
light S-SW winds, increasing a bit toward daybreak Tuesday (5-10
mph). Temperatures will hold relatively steady in the mid- upr
20s and slowly rise to near freezing by daybreak Tuesday. Not
much large- scale ascent, but continued light precipitation
across the nrn Adirondacks and nrn Greens will remain possible
with sw low-level flow and orographic ascent. Slowly warming
thermodynamic profiles may allow for intervals of sleet or even
plain rain in swrn St. Lawrence county.


As of 342 PM EST Sunday...A shortwave strengthening over the Great
Lakes Region will slightly amplify the upper-level flow Tuesday
evening and provide some stronger support aloft than Monday.
This will result in precipitation over northern New York/Vermont
becoming increasingly widespread into Tuesday evening when the
best dynamics move into the area. At the same time, warmer air
will continue to move into the forecast area Tuesday, working
it`s way east from Northern New York early Tuesday morning and
pushing into most of Vermont by early Tuesday afternoon. The
low-level warm air will support a transition from snow to rain
throughout the morning. Some mixed precipitation is possible in
areas of northern New York early Tuesday morning and in the
Northeast Kingdom of Vermont mid-day as cold air at the surface
takes a little longer to erode.

Tuesday night into early Wednesday morning, a cold front will push
into New England from southern Quebec, thus ending our brief early
week warmup. Colder air moving in will cool the entire column back
to below freezing, allowing precipitation to changeover back to all
snow throughout the morning Wednesday.  Flow will become
increasingly northwesterly midweek as the weakly amplified upper-
level longwave trough progresses downstream. Some lingering moisture
in the area Wednesday will support continued light snow showers, but
in the absence of any real forcing, these will mainly be
orographically focused.


As of 342 PM EST Sunday....High pressure centered over northern Ontario
will edge into New England from the west Wednesday night,
effectively ending any lingering precipitation. Thursday will be
the driest and coldest day of the work week with daytime highs
in the upper teens to low 20s.

The next impactful system on the horizon will start off as a weak
low forming in the lee of the Rocky Mountains Thursday afternoon. As
the system tracks northeastward towards the western Great Lakes
region Friday morning, it will intensify under favorable jet
dynamics aloft.  The low will then track into southern Ontario
Friday night, pushing an associated warm front across the forecast
area. The passage of the warm front will set off a brief but
strong warmup Saturday morning before the cold front marches
through Saturday afternoon/evening. The system will spread
precipitation over the area, however precipitation type will
depend greatly on the exact track of the low. Subsequent model
runs of the ECMWF and the GFS continue to shift the track of the
surface low north and west, which would put northern New York
and Vermont solidly in the warm sector of the cyclone by
Saturday morning. At this time, the current forecast reflects
this solution. Current thinking is that we`ll see some light
warm air advection snow initially Friday morning, followed by a
transition to rain Friday night through Saturday morning. After
the cold front pushes through Saturday night, precipitation will
taper off and change back over to some light snow. It`s worth
noting that this system is still a ways out in the forecast and
the impacts to the forecast area are highly dependent on the
exact track the low takes.

As the system departs the area, a colder arctic air mass will be
ushered in from Canada.  Looks like next week will likely feature
the coldest temperatures of the season thus far for Vermont/northern
New York.


Through 00Z Tuesday...VFR conditions will exist through about
09z with only mid and high level clouds over the area. Eventually
after 09z a warm front will approach from the southwest and
ceilings will lower into the MVFR category with light snow
developing as well. The snow will reduce visibilities into the
MVFR and IFR categories between 12z and 18z...then improve back
into the VFR category for the remainder of the period. Ceilings
however will remain in the MVFR category for the remainder of
the period. Winds will generally be variable and under 10 knots
through about 18z before become south for the remainder of the


Monday Night: MVFR/IFR conditions possible. Slight chance SHSN.
Tuesday: Mainly MVFR, with local VFR possible. Chance SHRA,
Chance SHSN.
Tuesday Night: Mainly VFR, with local MVFR possible. Chance SHSN,
Chance SHRA.
Wednesday: Mainly VFR, with areas MVFR possible. Slight chance
Wednesday Night: Mainly VFR, with areas MVFR possible. NO SIG WX.
Thursday: Mainly VFR, with areas MVFR possible. NO SIG WX.
Thursday Night: VFR. Slight chance SHSN.
Friday: Mainly VFR, with local MVFR possible. Chance SN.




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