Area Forecast Discussion
Issued by NWS Burlington, VT

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904
FXUS61 KBTV 120640
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
140 AM EST Tue Dec 12 2017

.SYNOPSIS...
A strong clipper low pressure system is expected to bring
widespread moderate to heavy snowfall across northern New York
and Vermont. Winter Storm Warnings and Advisories are in effect
for Tuesday and Wednesday. An arctic airmass will move in behind
the low pressure system bringing cold temperatures through
Friday before warming slightly over the weekend.

&&

.NEAR TERM /UNTIL 2 PM THIS AFTERNOON/...
As of 1017 PM EST Monday...Winter Storm Warnings and Winter
Weather Advisories remain in effect for the North Country as a
vigorous clipper low is expected to bring significant snow to
the region starting during the pre-dawn hrs Tuesday.

Quiet wx conditions in the meantime with high pressure in place
across srn Quebec and increasing mid-upper level clouds from
west to east. Temperatures have leveled off late this evening
with developing overcast. Chilly readings at 03Z range from the
low-mid teens across central/nwrn VT and the St. Lawrence
Valley, to the single digits across the nrn Adirondacks and far
nern VT. Some localized low-mid 20s exist across Rutland/Windsor
counties. Once southeasterly flow ramps up, as the clipper
starts to move into region, warm air advection will surge into
the region, and temperatures will slowly rise toward sunrise.

Warm advection snow will push into northern New York and into
Vermont during the pre-dawn hours. Mosaic composite reflectivity
shows an expanding area of 15-25dBZ echos across the Tug Hill
Plateau east of Lake Ontario and swwd across far wrn NY. PBL is
initially quite dry with cA airmass in place, but should see
very light snow developing across St. Lawrence County by 06Z.

The trends in the near term guidance continue to point to
significant snow accumulating across most of the North Country.
Omega in the snow growth zone amplifies with nearly 100%
saturation in the DGZ which all points to moderate to heavy snow
across the North Country.

As the midnight shift described, the storm will move through in
two waves with significant differences in the locations of snow
accumulations. With the first warm advection snow, expect
strong south to southeasterly winds to push into the region
which will cause downsloping effects in the western Greens and
Adirondacks. These areas will be the last to see accumulating
snow and likely to see only see 3-6 inches initially whereas the
eastern slopes of the Greens and across the Saint Lawrence
valley will see 6-10 inches. The closed surface low looks to
drop just south of the forecast area which will lead to the dry
slot perhaps not making it all the way into the North Country.
With the location of that low track and the warm air advection
all day, is possible in the southern Champlain Valley and
Connecticut river valley that the temperature profile will warm
up enough to see some rain mix in but for the time being I
haven`t included that in the forecast. The second phase is the
orographic snow described in the Short Term.

&&

.SHORT TERM /2 PM THIS AFTERNOON THROUGH WEDNESDAY NIGHT/...
As of 401 PM EST Monday...The second phase of the system will
come Tuesday night into Wednesday as the low pulls into Northern
Maine. Cold air advection returns and the flow on the backside
of the low turns northwest. Once this happens we`ll move into a
more favourable upslope pattern where the western slopes of the
Greens and Adirondacks start to pick up some bigger numbers.
Froude numbers initially will be blocked Tuesday night into
Wednesday and so the snow should back up into the Champlain
valley. Expect during this second phase that the upslope regions
will pick up another 3-6 inches of snow while the rest of the
North County sees an additional 1-2 inches. The flow aloft picks
up fairly quickly during the day on Wednesday and the flow
becomes unblocked so we`ll be looking at the potential for some
blowing snow across northern New York with accumulating snow
moving back into the Northeast Kingdom.

Its going to be rather chilly on Wednesday as cold air
advection rips into the North Country. Expect highs in the teens
across Northern New York and in the low 20s across Vermont.
With gusty winds its going to be quite the shock to the system
to feel wind chills back into the single digits above and below
zero.

Some lingering mountain snow showers will continue in the
evening hours on Wednesday however the flow will be turning more
west to west northwesterly so with cold air and flow across
warm lakes we`ll be switching gears to a lake effect snow again.
Most of the accumulating lake effect snow will fall south of
Saint Lawrence county. Across the North Country, with a fresh
snow pack and cold air still pushing into the region expect a
chilly evening with lows in the single digits above and below
zero.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 330 AM EST Monday...Large scale picture shows mid/upper
prevailing across the eastern Conus...while energy and moisture
from the Pacific NW ejects into the fast flow aloft every couple
of days. Latest guidance supports the northern tier staying on
the cool side of these systems with several mainly light snow
events possible. Below are my daily thoughts for days 4 thru 7.

Thursday...deep mid/upper level trof across eastern canada will
result in favorable upslope flow...but 1000 to 700mb rh is
progged to slowly dissipate by 18z Thurs. Based on llvl caa and
upslope flow...expecting remaining moisture to be squeezed out
across the northern dacks/greens. Have mentioned chc pops with
only minor accumulation expected. Temps aft a chilly start will
only warm into the single digits/lower teens mountains to
mid/upper teens warmer valleys with brisk northwest winds. Weak
surface high pres builds into our cwa on Thurs Night into
Friday...but next system approaches our western fa by Friday.
Very difficult to determine cloud cover/winds and impacts of
fresh snow pack will have on temps Thurs Night/Friday Morning.
Have trended toward cooler side of guidance envelope with -10f
slk/nek to single digits to around 10f cpv. Some lake effect
clouds could impact temps near BTV.

Friday/Saturday...Both gfs/ecmwf show next weak clipper like
system and associated boundary crossing our cwa. This system
will be weaker and have less impact than current clipper. Still
anticipating a period of light snow activity...given good 5h
energy...some enhanced mid level moisture...and great lake
moisture interaction with approaching boundary. Several inches
likely in the mountains with a dusting to an inch possible in
the valleys. Have continued to mention high chc pops in the
grids. Progged 850mb temps btwn
-14c and -16c...support highs mainly single digits summits to
 teens mountain towns to l/m 20s valleys.

Sunday...weak mid/upper level ridge and associated 1025mb high
pres builds into the ne conus. Extremely difficult to time
systems in fast flow aloft...along with amount of clearing and
impacts on temps. Have lowered pops slightly for Sunday...but
timing of next system could arrive quicker based on fast flow
aloft. Clouds and winds will impact temps...but have kept temps
close to superblend with lows mainly in the teens and highs in
the 20s.

Monday...Guidance supports another clipper like system moving
thru the Ohio Valley into the NE CONUS. Have noted lots of
uncertainty in ensemble data and between 00z/12z guidance in
track of surface low...with some to our northwest...while others
show a system to our south. Have continued on the cooler side
of guidance...thinking surface pres will keep system suppressed
to our south...with another light snow event possible. 12z ECMWF
shows favorable mid level moisture...good deep layer lift...and
moderate llvl waa signature. Plenty of time and
uncertainty...so have mention chc pops for now...with temps
holding in the 20s. Overall pattern will support a net increase
in snow pack across the mountains with cool temps keeping
snowpack fresh.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Through 06Z Wednesday...Deteriorating conditions as moderate
snow moves into the region associated with clipper system
crossing the area today. Widespread IFR to LIFR develops at
all sites by early Tuesday Morning with significant impacts
anticipated through 04z, before widespread snow develops from
southwest to northeast across our TAF sites between 04z-08z.
Visibility will quickly drop to IFR with locally VLIFR in bursts
of heavier snow expected at all sites between 09z-15z Tuesday.
Ceilings will vary from IFR at MSS to MVFR at
RUT/MPV/BTV/SLK/PBG...with periods of IFR possible. Localized
southeast winds gusts to 25 to 30 knots at RUT on Tuesday
morning with breezy northeast winds at MSS. Conditions will
slowly improve at RUT after 18z. Some low level wind shear
likely at MSS/MPV with change of speed and direction...causing
increased shear and turbulence on Tuesday Morning.

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 until 7 PM EST Wednesday for VTZ001-
     002-005-009-011.
     Winter Storm Warning until 7 PM EST Wednesday for VTZ003-004-
     006>008-016>018.
     Winter Storm Warning until 7 AM EST Wednesday for VTZ010-012-
     019.
NY...Winter Weather Advisory until 7 PM EST Wednesday for
     NYZ027>031-034-035.
     Winter Storm Warning until 7 PM EST Wednesday for NYZ026-087.

&&

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



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