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  Wednesday December 13, 2017

 

NWS Area Forecast Discussion


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000
FXUS61 KBTV 110552
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1252 AM EST Mon Dec 11 2017

.SYNOPSIS...
Lake effect snow from Lake Ontario will move across parts of
the region tonight with minor snow accumulations
possible...mainly over portions of northern New York. Relatively
dry weather is expected on Monday before a low pressure system
moves across the area Tuesday into Wednesday and brings
widespread light to moderate snowfall to the North Country.

&&

.NEAR TERM /UNTIL 2 PM THIS AFTERNOON/...
As of 1020 PM EST Sunday...Forecast remains on track this
evening with plenty of mesoscale factors influencing weather
conditions late this evening and thru the overnight hrs. At 03Z,
well-defined arctic bndry extending NE-SW had pushed through
Ottawa, Ont., with nwly wind shift. Temperatures drop off
rapidly with cA air mass across nrn Ontario...which will be our
air mass for Monday. South of the front, low-level flow has
backed just enough to bring lake Ontario snowband nwd into srn
St. Lawrence and Franklin NY counties. May see localized
snowfall accumulations of 2-4" across far srn St. Lawrence
county, before frontal passage rapidly shifts band south of our
forecast area after 08Z or so. With 850mb flow briefly 25-30kts
this evening, may see occasional snow showers or flurries extending
ewd across the Champlain Valley into the nrn Green Mtns. A
light accumulation (coating to 1") is possible in localized
areas with ern extent of the lake streamer overnight, mainly
across the higher terrain.

The cold front itself will begin to shift across the St.
Lawrence Valley around 06-07Z, and thru the Adirondacks and nrn
Champlain Valley around 09-10Z. May see some brief enhancement
of snowfall as lake effect band is entrained into frontal
circulation/convergence. However, overall effect will be to
shift the Lake Ontario snowband south of our CWA during the pre-
dawn and toward sunrise on Monday.

Colder and drier air will advect in behind the front as well,
bringing the snow showers to an end by mid morning Monday. All
told, expect southern St Lawrence and Franklin Counties in NY to
see the most accumulation, generally 2 to 4 inches. Elsewhere,
snowfall will be 2 inches or less, a combination of lake effect
snow showers and snow showers along the frontal bndry as it
shifts to the south and east. Snow ratios will be relatively
high with good dendrite snow growth, generally 18:1 to 22:1.
Temperatures will be relatively steady much of the night, but
drop quickly late with passage of the arctic front. Should see
readings in the mid-teens across the St. Lawrence Valley and
Adirondacks toward daybreak, but generally low-mid 20s across
VT.

For Monday and Monday night...once the aforementioned front moves
through...again by mid morning or so... expect drier weather to
briefly move in as high pressure quickly skirts across the area.
Highs will be in the 20s, with the coldest readings coming in the
northern mountains. As we head into Monday night, the focus turns to
a clipper system approaching from the west. Precipitation associated
with warm air advection out ahead of this system will spread from
west to east overnight, but with dry air in place at lower levels,
it will take a bit before the snow is able to accumulate. Lows will
be in the teens pretty much areawide.

&&

.SHORT TERM /2 PM THIS AFTERNOON THROUGH WEDNESDAY NIGHT/...
As of 345 PM EST Sunday...A winter storm watch has been issued
for snow accumulations of 4-7 inches as a well defined northern
stream clipper low tracks just south of the North Country.

By Monday night a clipper low will be tracking through the
Great Lakes and lifting a warm front into the North Country.
Warm advection snow will overspread the North Country producing
widespread plowable snowfall. The best moisture convergence and
lift will be during the morning hours on Tuesday as we see 1-2
inches across southern Vermont before sunrise and then another
with another 2-4 inches during the mid morning hours Tuesday.
Storm Total accumulations are anticipated to be in the 4-7 inch
range so just on the border between advisory vs warning. Being
early in the season I opted for a watch to highlight the
potential for adverse travel on Tuesday morning.

The trends in the 12z guidance were to increase the amount of
liquid qpf with the system so even accounting for the normal
high bias in warm advection we are in the range of seeing
between 0.25-0.4 inches of liquid with snow ratios in the
14-18:1. Will need to watch to see if trends continue to
increase the QPF because thats generally not favourable to see
high qpf events for the North Country with clipper type systems.

Expect with the warm air pushing in on Tuesday temps will be in
the upper 20s to low 30s before the cold front from the parent
upper level trough swings through. Expect overnight temps to
generally be in the 20s until just before daybreak when lows
drop into the teens.

Then as the upper level trough swings through Tuesday night
into Wednesday the pattern will shift to more orographic showers
in the western slopes of both the Greens and the Adirondacks.
The moisture will be limited but even so another 1-3 inches will
be possible in the higher terrain.

Cold air moves into the region Wednesday night with both the
GFS and EC showing -18C to -20C at 850mb. With a fresh snow pack
I`ve got lows in the single digits on Wednesday night. I did
trend up a couple of degrees from guidance as even with the snow
pack, expect mostly cloudy skies which will dampen how fast we
cool.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 345 PM EST Sunday...The large scale pattern continues
with a mean upper level trough over over eastern Canada. We
should see one brief quiet day on Thursday but by Friday another
weaker clipper system will bring a chance for measurable snow.
With the colder airmass still in place anticipate temperatures
to remain below normal with teens for highs on Thursday and a
gradual warming trend towards upper 20s to low 30s by Saturday.

&&

.AVIATION /06Z 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 Storm Watch from late tonight through late Tuesday
     night for VTZ001>012-016>019.
NY...Winter Storm Watch from late tonight through late Tuesday
     night for NYZ026>031-034-035-087.

&&

$$
SYNOPSIS...Hastings
NEAR TERM...Banacos/Hastings
SHORT TERM...Deal
LONG TERM...Deal
AVIATION...Nash



 
 
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