Scientific Forecaster Discussion
fxus61 kbtv 111133
Area forecast discussion
National Weather Service Burlington Vermont
633 am EST Wed Dec 11 2019
dry conditions linger through this morning, then expect snow showers
to increase across northern New York and northern Vermont this afternoon and
evening as a cold front crosses the region. Some of this activity
may be briefly heavy late in the day, especially in the St Lawrence
Valley and northern Adirondacks, where a couple inches of snow will
be possible. Drier but colder weather arrives on Thursday. The next
chance for widespread precipitation will occur Saturday, with
moderate rain generally expected across the north country.
Near term /through Thursday/...
as of 624 am EST Wednesday...no changes were needed with this
morning's update. Precipitation has ended across the area with
just high clouds streaming across southern and eastern sections.
Clearing can be seen across the St Lawrence Valley back into
Ontario, and this will shift eastward through the morning hours.
No precipitation expected until winds turn to the southwest and
lake enhanced snow develops. The forecast has this all covered,
so have just made slight tweaks to incorporate the latest obs.
Previous discussion...a fairly quiet start to the day today
with the cold front positioned offshore and well to our south. Much
drier air has spread across the north country behind the front,
shunting the steadier precipitation into southern and eastern New
England. Conditions will change this afternoon, however, as low
pressure currently over Lake Superior will scoot eastward, moving
north of our region later today and tonight. Winds will turn to the
southwest as the low approaches this afternoon, allowing enhanced
moisture to stream off of Lake Ontario and into northern New York.
Meanwhile, the low's attendant cold front will push eastward,
entering the St Lawrence Valley mid-late afternoon, and into the
Adirondacks early this evening. This front will serve to focus the
lake-enhanced moisture. Low-level lapse rates will steepen under
cold air advection; note that model guidance continues to show
SBCAPE values possibly exceeding 100 j/kg. Hence expect there will
be a sharp line of briefly heavy snow that will move through the St
Lawrence Valley into the northern Adirondacks late this afternoon
into early evening. Hi-res cams agree with this scenario, with the
band breaking up as it moves eastward through the Adirondacks and
into northern Vermont. One possible snag is the lack of substantial
moisture for those areas well away from Lake Ontario. Still, would
expect a quick inch or two of accumulation with this band along and
west of the western slopes of the Adirondacks, with lesser amounts
to the south and east. The aforementioned steepening lapse rates
will also serve to allow ample mixing of winds aloft down to the
surface. The latest NAM sounding for mss indicates mixing to 10 kft
Post-front, and with 30-40 kt at the top of the mixed layer, so
expect winds will be gusty with and after the front. The reinforcing
shot of cold and dry air behind the front will bring snow to an end
by midnight or shortly thereafter, lingering longest in the northern
Green Mountains. Temperatures will be cold under clearing skies,
though winds will remain gusty much of the night due to the
aforementioned cold air advection/mixing. Lows will range from the
single digits in the Adirondacks and Northeast Kingdom to the mid
teens in the wider valleys.
Thursday will be chilly but dry as high pressure slowly builds over
the northeast Continental U.S.. we'll start out with plenty of sunshine, but
high clouds will spread from west to east during the afternoon ahead
of weak low pressure pushing into the Great Lakes. Winds will turn
to the south/southwest late in the day as the ridge axis moves east,
but temperatures will remain well below normal for mid December.
Highs will range the upper teens in the Northeast Kingdom to the mid
and upper 20s in the more southern valley locations.
Short term /Thursday night through Friday/...
as of 331 am EST Wednesday...Thursday night into the pre-dawn hours,
quiet conditions are expected, though increasingly gusty will add
some disrupt the quiet. As high pressure shifts offshore, strong
pressure gradients will result in increasing southerly flow that
will channel through the Champlain Valley, in particular. A weak
upper shortwave will pass to our north with the main impacts being
cloud cover. With that and the increasing winds, overnight lows are
expected to remain in the teens to mid 20s.
Clouds should briefly clear as moisture continues to lift north and
east. We should also see gusty winds peak late morning/early
afternoon once warm air advection aloft reduces our mixing
potential. We should see highs creep above freezing across much of
the region, except at some of our colder hollows.
Towards the evening hours, some drizzle or flurries could begin to
work its way northward into south-central Vermont into the
Adirondacks as warm, moist advection continues to increase. Forecast
soundings are fairly complex with little cloud ice suggesting more
drizzle/flurry at onset. Deep layer moisture will continue to
increase. As the previous forecaster noted, some of our cold hollows
could hover around the freezing mark and remain near that point from
wet-bulbing. There is a very quick window where a brief snow and
freezing rain mix takes place as the warm nose strengthens and while
the wet-bulb effect maintains the colder air across those colder
Long term /Friday night through Tuesday/...
as of 331 am EST Wednesday...any mixed precip types should transition
to rain during the pre-dawn hours on Saturday. The higher summits
will be the exception and will be the last to transition to rain.
Forecast guidance remains consistent in strong warming with surface
temperatures warming into the 40s.
Negatively tilted trough will move in during the day on Saturday
with a coastal low likely tracking into interior New England. Not
much has changed in the overall details with near record pwats still
forecast along with the potential for downslope winds on the western
slopes from a 40-45kt east-southeasterly low-level jet to the north
of the low. With the mostly stable air, not anticipating too much
gustiness, but portions of the southern greens seems the most likely
to see gusty winds channeling through any gaps in the mountain
range. As for precipitation amounts, early indications are still for
about a 0.50"-1.00" with a locally up to 1.5", particularly east of
Active weather continues into Sunday as the mid-level low tracks
across the region. Plenty of synoptic scale forcing for ascent
remains as the cold front moves across during the morning hours.
Unfortunately, colder air will be slow to move in as the mid-level
low tracks north of US, keeping more west-southwesterly flow in
place. By the time cold air does arrive, the backside of the trough
begins to lift out of the region with moisture quickly thinning
Sunday night into Monday. Mainly mountain snow showers will be
possible Sunday evening/Monday. Otherwise, mostly dry conditions
Monday into early Tuesday. The next system will already be at our
doorstep on Tuesday.
Aviation /12z Wednesday through Sunday/...
through 12z Thursday.. dry conditions with prevailing VFR
expected through 18z today. Snow showers are expected to
develop over northern New York and western Vermont thereafter
and this could result in MVFR/local IFR visibilities. A cold
front will move across the region from west to east this
afternoon and evening. A band of moderate to briefly heavy snow
is expected with the front at kmss and kslk roughly 20z-01z
this afternoon/evening with MVFR/IFR conditions possible.
Elsewhere, the band will be more scattered in nature with MVFR
conditions for a brief period. All sites to see prevailing VFR
after 03z. North to northwest winds will persist through mid-
morning, mainly 5-10 kt. Eventually the winds will turn back to
the south and southwest after 15z remaining around 10 kt until
the front nears/moves through. Winds will then become gusty to
around 20 kt and turn to the west.
Thursday: VFR. No sig weather.
Thursday night: VFR. No sig weather.
Friday: mainly VFR, with local MVFR possible. No sig weather.
Friday night: MVFR/IFR conditions possible. Definite ra, chance
fzra, slight chance snow.
Saturday: mainly MVFR, with areas IFR possible. Definite rain.
Saturday night: IFR. Likely ra, likely snow.
Sunday: mainly MVFR, with local IFR possible. Likely sn, chance