Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
1248 am EST Friday Feb 12 2016
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...then surface high pressure builds from
the west. 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 /until 1 PM this afternoon/...
as of 1226 am EST Friday...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
boundarywith 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 /1 PM this afternoon 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 /06z Friday through Tuesday/...
through 06z Saturday...trend toward VFR overnight as upper level
ridge moves over the area and northwest flow relaxes generally after 06z.
Btv and mpv currently still have MVFR ceilings. 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 20 kts...then generally less than 10 kts then shifting
to the S and SW around 10kts through Friday morning. S winds may
be 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. 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.
Outlook 06z Saturday through Tuesday...generally VFR after 12z
Saturday with flurries possible. Northwest wind gusts to 25kt possible
Saturday afternoon and evening. Monday night into Tuesday precipitation will be
beginning and spreading south to north across our area. Depending
on ptype...will see some MVFR/IFR possible.
Vermont...wind chill watch from late tonight through Sunday evening for
New York...wind chill watch from late tonight through Sunday evening for
Lake effect Snow Advisory from 9 am this morning to 1 am EST
Saturday for nyz029-087.