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  Wednesday December 13, 2017

 

NWS Area Forecast Discussion


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306
FXUS61 KBTV 121942
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
242 PM EST Tue Dec 12 2017

.SYNOPSIS...
Deepening low pressure will cross the region today into
Wednesday with a widespread moderate to locally heavy snowfall
expected. Behind this system chilly temperatures are expected
through the end of the week with periodic chances of flurries
and light snow, with the best chances occurring later in the
weekend.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 1246 PM EST Tuesday...Update this afternoon featured to
decrease snowfall amounts along the western slopes near
Nashville and North Underhill...as latest web cams show limited
accumulation. Also...have integrated latest snowfall amounts
into 6 hr snowfall grids to capture what has already fallen
across the cwa...which ranges from a 0.5 to over 6 inches in
spots. See latest PNSBTV. As expected...southeast winds btwn 30
and 50 knots at 925mb to 850mb has influenced qpf/snowfall
today. Highest snowfall amounts have been across the southeast
upslope regions of the central/southern Green Mtns...especially
near Ludlow to Waterbury with much less on the Western Slopes.
This afternoon better dynamics associated with waa lift is
shifting north of our cwa...while mid/upper level dry slot
develops across the dacks. Expecting precip to become more
showery...with best forcing/moisture over central/eastern VT.
Additional snowfall thru this aftn will range between a dusting
to 2 or 3 inches. Areas of light snow will continue across the
SLV until surface boundary shifts winds and drier air develops
later this aftn. All advisories and warnings continue for now.
Second part of the storm develops tonight...as winds becoming
west/northwest and upslope precip develops. Still plenty of
questions on depth of backside moisture and amount of additional
snowfall. Will continue to examine 12z data...for potential
minor adjustments in snowfall. Also...would not be surprised if
a few rain drops mix with snowfall over the southern/central
CPV...as sfc temps warm to low/mid 30s. With temps near freezing
and lighter precip rates treated roadways should be wet for
most of this aftn...especially in the cpv.

Prior discussion...
The forecast remains on track over the next 36-hours as clipper
energy tracks from the eastern Ohio Valley through central New
England today before exiting into the Maritimes by tonight. All
current advisories and warnings remain intact with this package.
As discussed quite thoroughly over the last few nights the
storm will come in two stages - a warm advection stage today,
and an orographic upslope stage later tonight into Wednesday.
The overall idea advertises steady light to moderate snow
overspreading the entire forecast area through the morning hours
before tapering off somewhat later today into the first part of
this evening as surface energy and associated isentropic lift
exit east. By later tonight into Wednesday the flow transitions
to northwesterly allowing a general reblossoming of activity
across northern counties, especially in favored upslope areas.
Snowfall will show considerable variability over any given
6-hour time frame into tomorrow with the proximity of the
surface low track allowing a gradual backing of the low level
flow over time. That said storm average totals remain largely
unchanged and should generally range from 3-7 inches in advisory
areas, 5 to 8 inches or so in southeastern VT and the St.
Lawrence Valley, and from 6 to 10 inches in the northern Greens.
Totals that do fall out of these ranges should be on the order
of just an inch or two in the lower elevations and well within a
typical standard deviation profile for complex events such as
this. Northwesterly winds will also trend quite gusty on
Wednesday as p-gradient tightens so this will create areas of
blowing snow. High temperatures today should top out in the
upper 20s to lower 30s before bottoming out mainly in the teens
overnight. Readings won`t really recovery too much on Wednesday
under strong cold thermal advection and the aforementioned gusty
winds - mainly steady in the mid teens to lower 20s.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 303 AM EST Tuesday...Light snow and snow showers wind
down across the northern mountains Wednesday night before ending
by Thursday as high pressure slowly builds into the region. It
will remain quite chilly under modestly gusty west/northwest
flow with lows Wednesday night in the single digits a highs on
Thursday once again in the teens to locally near 20F across the
southern valleys. Wind chills Wednesday night look to generally
range through the negative single digits to negative teens so
slightly warmer than advisory level thresholds - at least that`s
the way it looks at this point.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
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 /20Z 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...JMG
NEAR TERM...JMG/Taber
SHORT TERM...JMG
LONG TERM...Lahiff
AVIATION...Lahiff



 
 
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