Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kbtv 130332 
afdbtv

Area forecast discussion
National Weather Service Burlington Vermont
1032 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 /until 6 PM Wednesday 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 Vermont 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 Vermont/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 Vermont thus far, they will rapidly catch up with
the developing northwest 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 New York. 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 New York/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 northwesterly 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/European model (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... 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 Vermont
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 Wednesday 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 European model (ecmwf) continue to be in general
disagreement with the pattern for early next week with the European model (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 /04z Wednesday through Sunday/...
through 00z Wednesday...a brief lull in the snow will allow for
mainly MVFR conditions with periods of IFR through about 04z,
then IFR visibility with MVFR ceilings will prevail into
Wednesday morning as the snow resumes. Winds will turn to the
northwest at all terminals by 04z, generally at 5-15 kt. Winds
will increase during the day Wednesday with gusts of 25-30 kt
expected.

Outlook...

Wednesday night: VFR. Slight chance shsn.
Thursday: VFR. No sig weather.
Thursday night: VFR. No sig weather.
Friday: mainly VFR, with local MVFR possible. Slight chance shsn.
Friday night: mainly VFR, with local MVFR possible. Scattered
shsn.
Saturday: mainly MVFR, with local IFR possible. Scattered shsn.
Saturday night: mainly VFR, with local IFR possible. Slight
chance shsn.
Sunday: mainly VFR, with areas MVFR possible. Slight chance shsn.

&&

Btv watches/warnings/advisories...
Vermont...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.
New York...Winter Weather Advisory until 7 PM EST Wednesday for
nyz027>031-034-035.

&&

$$

National Weather Service Glossary of Abbreviations