Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Burlington Vermont
927 PM EST Thursday Feb 11 2016

Synopsis...
a weak upper level trough across the St. Lawrence Valley will
track eastward with just a few flurries or snow showers through
the first half of tonight. As this feature moves east...skies will
becoming partly to mostly clear after midnight with surface high
pressure building in from the west. It will be cold with overnight
lows generally zero to 5 above in the Champlain and Connecticut
valleys...and generally zero to 10 below elsewhere. Another upper
level low and Arctic frontal zone will approach from the Great
Lakes and southeastern Ontario late in the day Friday into Friday
night. This feature will bring localized lake effect snows to
southern St. Lawrence County...and then the potential for snow
showers or squalls along the Arctic front as it moves through
during Friday night. This will usher in the coldest air mass of
the winter for the weekend...with bitterly cold temperatures. A
moderation in temperatures will begin Monday...with a storm system
bringing mixed wintry precipitation potentially for Tuesday.

&&

Near term /through Friday night/...
as of 927 PM EST Thursday...additional minor adjustments to
sky/T/TD/winds overnight based off current observational trends.
Lingering light shsn/flurries to generally end overnight leaving
clear/partly cloudy skies by morning as shortwave ridge passes
overhead. Temperatures are running colder than earlier forecast by
several degrees, but are quite variable depending on cloud cover
and wind. Thus took current 02z readings and lowered values by an
additional 3-5 degrees overnight per latest guidance trends. Also
opted to raise southerly winds and wind gusts considerably
tomorrow afternoon/evening ahead of the approaching Arctic boudary
with models offering a deeply mixed planetary boundary layer and a tightening p-grad.
Gusts into the 20-30 miles per hour range look reasonable, especially in the
broader valleys. Have a great night.

Prior discussion...
compact middle-level vorticity evident in infrared and visible imagery across the
St. Lawrence Valley will shift eastward along the international border and shift
east of Vermont by 06z. Looking at just a few light snow showers with
this feature through this evening...aided by steep low-level lapse
rates (in low-level cold air advection regime) and westerly 850mb flow of 15-20kts
providing some upslope into the Adirondacks and northern greens. Any
accumulation expected to be an inch or less...mainly confined to
areas of orographic ascent. Otherwise...it will be a cold night
with clearing skies as narrow surface ridge builds in briefly from the
west after 06z...cresting across Vermont around 12z Friday. Core of
coldest 850mb temperatures will be in place this evening...generally -24c
to -26c. Some radiational cooling and thin layer of snow cover
will aid in radiative cooling...especially late with clearing and
diminishing wind. Looking at overnight lows zero to 5 above for
the Champlain Valley and around Springfield Vermont...and generally
zero to -10f elsewhere.

Friday into Friday night: secondary Arctic boundary and associated
upper vorticity digs east-southeastward across the northern Great Lakes. This allows
surface-850mb flow to back ahead of the trough axis. Low-level
trajectories from Lake Ontario bring lake effect snow bands into
St. Lawrence County...likely starting around 14-15z Friday per
btv local WRF runs. Periods of lake effect snow will generally be
confined to St. Lawrence and Franklin counties during the daylight
hours Friday...with mainly dry conditions elsewhere with variably
cloudy conditions and weak S-SW flow. Lake effect snow will be
briefly augmented by low-level frontogenetic forcing and
convergence as Arctic boundary sweeps into northern New York between
02-05z...and across Vermont after midnight. Should see potential snow
squalls along the boundary as it moves across entire County Warning Area during the
overnight hours Friday night. Will See Lake effect snow end behind
Arctic boundary as mean flow veers rapidly from SW-NW.

Will be replacing the lake effect snow watch with a lake effect
Snow Advisory this afternoon for southern St. Lawrence County 14z Friday
through 06z Saturday to Cover Lake effect snow amounts generally
3-6". Will probably see a couple of communities in excess of 6"
snowfall around fine, New York...but anticipate that will be isolated given
transitional nature of the lake effect snow band as low-level flow
backs and then quickly veers with frontal passage. Otherwise...will see a
dusting to 2" Friday night with Arctic frontal passage with snow
showers and possible squall activity. Snow ratios generally 20:1
to 25:1 given very cold air mass...and not expecting any
significant impacts (also considering pre-dawn hours Saturday
morning with light traffic).

High temperatures on Friday generally generally teens to lower
20s. Strong cold air advection late Friday night will bring temperatures down to 5 to 10
below across the Adirondacks and St. Lawrence Valley by daybreak
Saturday...including wind chills 25 to 30 below (see forthcoming
short-term forecast section for more details on dangerously low
wind chills for the weekend). Lows across Vermont and the New York side
of the Champlain Valley Friday night mainly zero to +7f.

&&

Short term /Saturday through Sunday/...
as of 347 PM EST Thursday...wind chill watch in effect early
Saturday morning through Sunday.

Cold Arctic air will continue to filter into the north country in
the wake of strong cold front Saturday. Temperatures will fall
throughout the day...with maxes occurring in the morning in the
negative single digits across most of northern New York...and the
positive single digits across the Champlain Valley eastward.
Mountain tops will remain below zero. Pressure gradient increases
as departing surface low moves northeastward and ridging builds over the
mid-west. This will result in breezy conditions at the surface
with gusts 15-25kts possible. Cold temperatures combined with
winds will result in wind chills of 20 to 40 degrees below zero
throughout the weekend as 850mb temperatures fall into -30c to
-25c early Sunday. This cold frigid air is expected to persist
until Sunday night when the ridge of high pressure moves overhead
and winds subside while temperatures begin to increase aloft. Air
temperatures Saturday night will be below zero...ranging from
negative teens to negative 20s. Sunday will see maximum temperatures
in the single digits above and below zero.

Precipitation-wise...some residual moisture in the northwesterly
winds with Froude numbers showing slow moving/blocked upslope flow
will keep snow showers across the Northeast Kingdom and western
slopes of The Greens and northern Adirondacks for Saturday with a
drying trend thereafter. With Lake Champlain water temperatures
still in the middle 30s, cannot rule out some steam clouds developing
in colder air and possible Tea Kettle type snow showers in the
Champlain Valley Saturday and Sunday.

&&

Long term /Sunday night through Thursday/...
as of 347 PM EST Thursday...surface high pressure will slowly
shift eastward Sunday night into Monday with slight ridging aloft.
On the western side of the ridge axis, southerly flow will help
moderate temperatures Monday...with maxes reaching the 20s.

The next significant system will evolve from digging 500mb trough
that approaches from the west Monday night into Tuesday. While
models still differ on track of associated surface that develops,
expect low to begin bringing precipitation in from the south on
Tuesday. Both GFS and European model (ecmwf) currently bring significant warming to
the north country and result in above freezing temperatures on
Tuesday. Temperatures may be warm enough for precipitation to transition
to rain late Tuesday. But as the low lifts north and east, cold
air returns to the region on the backside of the system with snow
showers expected. Multiple vortices in 500mb trough may keep
chance for precipitation into Wednesday as the main surface low
continues tracking northward into coastal Canada.

&&

Aviation /03z Friday through Tuesday/...
through 00z Saturday...trend toward VFR overnight as upper level
ridge moves over the area and northwest flow relaxes generally after 06z.

At btv...northwest flow across the 37 degree waters of Lake Champlain
may bring flurries or even a snow shower for a while tonight. For
now just mentioning as vcty snow showers but its entirely possiblethat
ceilings go MVFR and visibility IFR for a brief period before 04z. Not
expecting much accumulation on runways.

Otherwise MVFR ceilings (2-3 kft) generally limited to slk otherwise
the trend to VFR late tonight into Friday morning.

Will begin to see MVFR ceilings and locally IFR visibility at slk/mss
with -shsn possible after 17z associated with increasing southwesterly flow
from Lake Ontario and fringe impact from lake effect snow bands.

Northwest winds briefly gusty 15 to 20kts through early evening...then
generally less than 10kts tonight and Friday morning then shifting
to the S and SW around 10kts through Friday morning.

S winds maybe a problem at kbtv ground operations Friday
afternoon into the night with drifting or blowing snow by 00z Sat and
may be gusting over 30kts until the Arctic front passes early Sat
a.M.

Outlook 00z Saturday through Monday...local vlifr in snow squalls
briefly possible Friday night 02-05z across northern New York and 05-09z
across Vermont as Arctic front moves through...with visibility briefly less than
1/2 mile and a wind shift to northwest with gusts 20 to 30 kts. Modest
impact to Airport ground operations with dusting to 2" snowfall
possible. Generally VFR after 12z Saturday with flurries possible. Northwest
wind gusts to 25kt possible Saturday afternoon and evening.

&&

Btv watches/warnings/advisories...
Vermont...wind chill watch from late Friday night through Sunday evening
for vtz001>012-016>019.
New York...wind chill watch from late Friday night through Sunday evening
for nyz026>031-034-035-087.
Lake effect Snow Advisory from 9 am Friday to 1 am EST
Saturday for nyz029-087.

&&

$$
Synopsis...banacos
near term...banacos/jmg
short term...kgm
long term...kgm
aviation...banacos/Sisson

National Weather Service Glossary of Abbreviations