Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 231121
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
621 AM EST Thu Nov 23 2017

.SYNOPSIS...
Generally fair weather is expected across the North Country for
Thanksgiving through Friday with a period of lake effect snow
showers across the western Adirondacks tonight. Temperatures
will be seasonably cool today, but moderate to above normal
levels on Friday. The next chance for widespread precipitation
will be on Saturday, mainly in the form of rain, with another
cold front crossing the area.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 621 AM EST Thursday...Quick update to earlier forecast,
mainly to adjust temps and sky cover slightly. Also, based on
surface obs, added in some flurries to central and northeastern
Vermont through mid-morning.

Previous Discussion...Mainly quiet weather is expected for most
of the region over the next 36 hours with the only real concern
being some light lake effect snow working into the western
Adirondacks for a period tonight.

Thanksgiving Day will start with mainly dry conditions and
varying amounts of cloud cover as low level moisture remains
trapped below a subsidence inversion. Latest IR imagery does
indicate that the drier air aloft is beginning to win out, so I
do expect low clouds to scour out through the morning into
partly sunny skies by this afternoon. Also, based on a few
surface obs and radar imagery, can`t rule out some flurries
where low clouds are more persistent. Highs will be on the
chilly side of normal and range through the 30s.

Heading into this evening, a weak surface front and shortwave
trough passing north of the border will shift the low/mid level
flow to the southwest allowing a lake effect snow band to
develop off Lake Ontario. Consensus among the hi-res CAM models
is that the band will briefly shift over portions of southern
St. Lawrence and Franklin counties in New York from about 00-06Z
before moving south and dissipating as the flow weakens and turns
more westerly. Don`t expect any big travel impacts, but we
could see a dusting to perhaps an inch in spots, especially in
the NY Route 3 corridor around Star Lake and Wanakena.

In the wake of the upper trough passage tonight, high pressure
builds back into the region for Friday with a quiet yet mild day
on tap as southwesterly flow begins to increase again with
another low passing to the northwest of the region. No
precipitation is expected over the forecast area, far removed
from any deep moisture, but temps should warm nicely into the
40s area-wide under partly to mostly sunny skies, especially
across central and southern areas.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
As of 322 AM EST Thursday...
Friday Night: for the most part, it looks dry for the period.
South/southwest winds will be on the increase as the pressure
gradient tightens in advance of a front. We should see winds in the
925-850mb layer (roughly 2000-4500 ft) increase into the 45-55kt
range as well. Given a stable near surface boundary layer, most of
this wind will stay above the surface, however it still will get
breezy especially in the Champlain and St Lawrence Valleys as the
flow is channeled up the valleys. Expect gusts on the order of 25-30
mph. It will probably blow even stronger over Lake Champlain as the
relatively warmer waters will promote better mixing of the near
surface boundary layer. The winds also make for a tricky temperature
forecast. Think in general lows from the Champlain Valley westward
will occur Friday evening, with a steady to slow rise the rest of
the night. Eastern Vermont will be more protected from the winds,
but even late at night will likely see temperatures begin to rise.
Most guidance suggests that precipitation with the front will be
just getting into the St Lawrence Valley very late, so have painted
in some 20-40% PoPs in that part of the world after 10z Saturday.
Temperatures aloft will be too warm (+3 to +5C at 850mb)for any
snow, so looking for only light rain.

Saturday: Band of light rain showers treks rapidly across the
region, and should be east of the forecast area by early afternoon.
Not really a well defined front, so surface convergence isn`t
strong, nor are the upper level dynamics. Thus the band of rain
showers will be weakening as it goes across. QPF amounts are
expected to be minimal, generally less than 1/10" of rain. Blended
in BTV4km, NAM3km and NAM output to drive the hourly forecast for
the band of precipitation. As mentioned above, temperatures aloft
will be too warm for snow, even at the highest summits.  The colder
temperatures aloft will start to filter in by mid/late afternoon, so
although the main batch of showers will have ended, enough low level
moisture along with westerly winds will drive a few residual very
light rain/snow showers across the higher terrain.

Saturday Night: a fast moving sharp shortwave will approach the
region overnight. With the dynamics and increasing moisture
(partially from flow coming across the Great Lakes), expecting snow
showers to re-develop across the region, especially the higher
terrain of the Adirondacks and northern/central Green Mountains with
orographic enhancement. Light accumulations are expected.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 322 AM EST Thursday...
Sunday: All guidance indicate the fast moving shortwave will zip
across the region during the day. Thus expect scattered snow
showers, especially during the first half of the day. Looks like
flow turns northwest and strengthens, so this should enhance
orographic enhancement, especially for the northern Greens and the
`Dacks. Could see additional light accumulations.  The trough should
drive enough snow shower activity that even lower elevations may see
a trace to 1/4" of snow. The snow shower activity will quickly fade
during the afternoon as the trough moves away. Although not looking
at a lot of snow, there could still be some slick travel conditions.

Monday/Tuesday: The changeable pattern continues. Flow will begin
turning southwest again as a trough develops well to the west and
high pressure moves off the east coast. Thus a moderating trend will
start. Monday will be a bit warmer than Sunday, and Tuesday is
certainly looking mild. 925mb temperatures will be around +5C, so we
may see some valley locations make a run at 50F.

Wednesday: More uncertainty in the models this far out.  Both GFS
and ECMWF suggest a cold front will sweep in during the day. However
timing and overall strength of the front is very uncertain. Just
went with the guidance blend at this point to indicate roughly 30-
40% chance of precipitation. With uncertainty in the boundary layer
and temperature profile above the surface, I just painted in
rain/snow showers for the potential weather.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Through 12Z Friday...Low clouds continue to slowly erode over
the region but are still holding tough across portions of
northern Vermont. As we progress through the morning, expect low
clouds to completely scatter out while mid and high level
clouds increase through midnight before generally lowering to
MVFR and locally IFR at KSLK where lake effect snow showers
shift into portions of the Adirondacks after 02Z. Northwest
winds mainly 5-10kts early this morning turn south/southwest by
mid-day.

Outlook...

Friday: VFR. NO SIG WX.
Friday Night: VFR. Chance SHRA.
Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA,
Chance SHSN.
Saturday Night: Mainly MVFR, with local IFR possible. Chance
SHRA, Chance SHSN.
Sunday: Mainly MVFR, with local IFR possible. Chance SHSN.
Sunday Night: Mainly MVFR, with local VFR possible. Slight chance
SHSN.
Monday: Mainly VFR, with areas MVFR possible. NO SIG WX.

&&

.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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