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  Wednesday December 13, 2017

 

NWS Area Forecast Discussion


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000
FXUS61 KBTV 122039
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
339 PM EST Tue Dec 12 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 /THROUGH WEDNESDAY/...
As of 337 PM EST Tuesday...All current warnings and advisory continue
with no significant changes to storm total snowfall. Expecting
storm total of 3 to 7 inches CPV/Northern Dacks...4 to 8 inches
SLV...6 to 10 inches southern/central VT/western
slopes/nek...and 12 to 18 inches from Mt Mansfield to Jay Peak
by Weds evening.

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 short wave trof 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. This combined with falling temps could result in areas of
slippery travel...especially cpv 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 /WEDNESDAY NIGHT 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 /21Z TUESDAY THROUGH SUNDAY/...
Through 18Z Wednesday...Overall conditions through the remainder
of the daylight hours will continue to be IFR or below in
moderate snow. Brief lull in precipitation later this evening
will raise conditions to MVFR through about midnight, before
IFR snow returns through much of the remainder of the forecast
period. Wind mainly 5-10kts from the southeast this afternoon shift
northwest overnight with gusts 20-30kts after 12Z.

Outlook...

Wednesday Night: VFR. Slight chance SHSN.
Thursday: VFR. NO SIG WX.
Thursday Night: VFR. NO SIG WX.
Friday: VFR. NO SIG WX.
Friday Night: Mainly VFR, with areas MVFR possible. Slight chance
SHSN.
Saturday: Mainly VFR, with areas MVFR possible. Slight chance
SHSN.
Saturday Night: Mainly MVFR, with areas VFR possible. Slight
chance SHSN.
Sunday: Mainly MVFR, with local IFR 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...Taber
NEAR TERM...Taber
SHORT TERM...Taber
LONG TERM...Lahiff
AVIATION...Lahiff


 
 
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