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  Wednesday December 13, 2017

 

NWS Area Forecast Discussion


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812
FXUS61 KBTV 130614
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
114 AM EST Wed Dec 13 2017

.SYNOPSIS...
Widespread light snow will become terrain focused overnight
with additional snow accumulation likely...especially for the
northern Adirondacks and central and northern Green Mountains.
Much colder temperatures return on Wednesday with blustery west
to northwest winds and occasional snow showers. Drier and cold
weather continues for Wednesday Night into Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 1023 PM EST Tuesday...We`ve cancelled the Winter Storm
Warning across northern/southwestern St. Lawrence county...an
area where snow has tapered off and won`t regenerate in any
significant way owing to orographic forcing. The remainder of
the forecast is on track. We anticipate combination of
orographically blocked flow (Froude #s <0.5), increasingly deep
saturated dendrite growth layer (-12 to -18C) and lake
enhancement will allow a ramp up in radar reflectivity across
the VT side of the Champlain Valley into the wrn slopes of the
nrn Green Mtns within the next 1-2 hrs...and lasting thru 12-13Z
Wednesday. This is expected to have an impact on the Wednesday
morning commute, and travelers will want to leave extra time
once again for commuting.

Expecting storm total of 4 to 8 inches CPV/Northern Dacks...4
to 8 inches St. Lawrence Valley...6 to 10 inches
southern/central VT/western slopes/nek...and 12 to 18 inches
from Mt. Mansfield to Jay Peak by Wednesday evening. Please see
our recently issued Public Information Statement and Local Storm
Reports for the latest snowfall observations for this event.
Whereas the western slopes of the Greens have lagged behind
central/ern VT thus far, they will rapidly catch up with the
developing NW flow during the overnight hrs into Wed morning.

Previous Discussion...Surface analysis places elongated low
pres from KSLK to KPOU this aftn with greatest 3hr pres falls
toward Cape Cod with trailing cold front over central NY. Radar
shows areas of light to moderate snow ahead of approaching
boundary with some mix of rain in the cpv/western slopes with
temps in the l/m 30s. Snowfall has been highly terrain/wind
driven with generally 3 to 7 inches, except 8 to 10 inches
central-southern Greens...while only an inch or two has fallen
along the western slopes from Nashville to North Underhill.
However, this will change tonight.

The near-term forecast features our synoptic system
transitioning to an mesoscale upslope event for the northern
dacks into parts of the central/northern greens...including the
western slopes. Water vapor shows potent shortwave trough across
the eastern Great Lakes with embedded 5h vort located over
western NY/PA helping to enhance ribbon of mid-level moisture
and precip along boundary over the NE conus. As upper lvl trof
approaches and surface boundary sweeps across our cwa btwn
21-03z...expect winds to shift from south to west/northwest with
temps falling 3 to 6 degrees in an hour or two...along with
another burst of mainly light to areas of moderate snowfall. At
23Z, we`ve already seen this NWLY wind shift occur across the
Champlain Is. and Clinton County, where temperatures have fallen
into the low-mid 20s. Further north in the Montreal Que. area,
temperatures are in the upr teens, so colder air will be
filtering in across the Champlain Valley with the wind shift.

Continued snow and falling temps could result in areas of icy
travel...especially in the Champlain Valley and point eastward.
A quick inch or two is possible with boundary. After
boundary...925mb to 850mb winds become west/northwest and the
upslope machine will start. However...getting some mixed signals
on amount of leftover deep layer moisture and associated
qpf/snowfall. GFS/ECMWF continue to show very favorable backside
commahead of enhanced 700mb moisture...along with embedded 5h
vorts helping with synoptic scale lift in closing 5h/7h
circulation. Meanwhile...RAP13/HRRR shows developing mid/upper
lvl dry slot and precip quickly ending after fropa this evening
with only minor upslope qpf/snowfall near Jay Peak. Given
presentation on water vapor and mid/upper level system becoming
negatively tilted...along with moderate to strong llvl caa a
period of upslope snow is expected btwn 06z and 18z Weds.
Also...local froude shows values around 0.50 thru 12z
Weds...before flow becomes unblocked with values >
1.0...supporting snowfall downwind of the mountains. Thinking
with caa and lowering snow growth zone...ratios will quickly
jump from 12/14 to 1 this evening to > 25:1 on Weds...helping to
support accumulating snowfall. Interesting the local 4km
composite reflectivity shows several lake bands off lake
champlain btwn 07z-12z when froude indicates flow is
blocked...supporting snowfall along the western slopes/eastern
cpv.

Including just upslope snowfall thinking 2 to 4 inches northern
dacks/cpv...with 3 to 6 inches western slopes... and 6 to 12
inches from Mt Mansfield to Jay Peak tonight into Weds. Little
additional accumulation for the slv and southern parts of VT
overnight into Weds.

Sub 975mb low pres tracks into eastern Canada on Weds with cold
and blustery conditions prevailing on Weds. Progged 850mb temps
drop btwn -17c and -19c by 18z...with soundings showing good
mixing of winds. This will create areas of blowing and drifting
snow...mainly over the exposed trrn with temps near zero summits
to mid/upper teens cpv/lower ct river valley. Wind chill values
single digits cpv to well below zero in the mountains. As deep
layer moisture slowly lifts north...anticipate the areal
coverage/intensity of upslope snow showers to decrease...with
mainly focus over the northern green mountains.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/...
As of 337 PM EST Tuesday...Deep cyclonic flow continues with
chilly temps and mountains snow showers prevailing. If winds and
clouds dissipate on either Weds or Thurs Night...temps will
drop very quickly and be much lower than forecast...especially
with fresh snow pack. Official forecast has lows near -5f slk to
near 10f CT River Valley/CPV...with highs single digits to mid
teens. Another cold night expected on Thurs with lows -10f to
+5f. Mainly dry forecast prevails.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 242 PM EST Tuesday....Overall pattern for the long term
from Friday through Tuesday will be a changeable one, but
largely driven by broad upper troughing with brief breaks of
ridging. Period starts off on Friday with a weak upper ridge
over the forecast area in the morning, but an upper trough
digging into the Great Lakes will turn the mid/upper level flow
back to the southwest allowing a lake effect band to develop in
the lee of Lake Ontario. Flow continues to back to the south
allowing the band to move up the St. Lawrence River Valley
before the upper trough swings in for Friday night through
Saturday producing scattered snow showers across much of the
forecast area.

Brief break comes Saturday night into Sunday as an upper ridge
builds overhead, but by Sunday evening deeper moisture
associated with a potentially complex system moves into the
region increasing chances for snow. Latest GFS and ECMWF
continue to be in general disagreement with the pattern for
early next week with the ECMWF depicting northern and southern
stream energy attempting to phase, while the GFS offers more of
a zonal flow keeping southern stream at bay. For now will offer
a blended model approach and keep the chance for snow going in
the forecast.

Temps through the period will trend from below normal through
the weekend, then back above normal for early next week.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Through 06Z Wednesday...Snow will resume at BTV and MPV with IFR
conditions at times. Other terminals are mostly done with the
snow this time around. Ceilings have improved to VFR at most
places with MVFR ceilings remaining at MSS and SLK. Winds will
turn to the northwest at all terminals by 08z, generally at
5-15 kt. Winds will increase during the day Wednesday with gusts
of 25-30 kt expected. Ceilings will continue to improve through
the period.

Outlook...

Thursday: VFR. NO SIG WX.
Thursday Night: VFR. NO SIG WX.
Friday: Mainly VFR, with local MVFR possible. Slight chance SHSN.
Friday Night: Mainly VFR, with areas MVFR possible. Scattered
SHSN.
Saturday: Mainly MVFR, with local VFR possible. Scattered SHSN.
Saturday Night: Mainly MVFR, with areas VFR possible. Slight
chance SHSN.
Sunday: Mainly MVFR, with areas VFR possible. Slight chance SHSN.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...Winter Weather Advisory until 7 PM EST this evening for VTZ001-
     002-005-009-011.
     Winter Storm Warning until 7 PM EST this evening for VTZ003-
     004-006>008-016>018.
     Winter Storm Warning until 7 AM EST this morning for VTZ010-
     012-019.
NY...Winter Weather Advisory until 7 PM EST this evening for
     NYZ027>031-034-035.

&&

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



 
 
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