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  Monday November 20, 2017

 

NWS Area Forecast Discussion


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FXUS61 KBTV 180837
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
337 AM EST Sat Nov 18 2017

.SYNOPSIS...
Strong low pressure will pass through the region tonight into
Sunday with widespread rainfall and milder temperatures. Rain
will taper to snow showers on Sunday afternoon as much colder
air surges back into the area along with gusty winds. The
general weather pattern quiets down from Monday onward into much
of next week with mainly dry and seasonably cold weather
expected for the Thanksgiving holiday.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 337 AM EST Saturday...Quite the variety of weather is
expected over the next 36 hours as deepening low pressure tracks
from the Ohio Valley into the St. Lawrence Valley by Sunday
morning. From a broader perspective the overall theme of the
forecast remains the same with thickening clouds expected today
before widespread precipitation arrives by early evening into
the overnight hours. Strengthening southerly flow and warm
thermal advective processes will push boundary layer
temperatures above freezing in most locales by mid to late
afternoon such that mainly a steady light rain is expected. The
only exception would be a brief period of mixed precipitation at
onset at mountain summit level and possibly across portions of
far northeastern VT. Given the strength of the synoptic
background southerly flow have generally downplayed this threat
however. Highs today to top out in the upper 30s to mid 40s for
most spots with readings holding steady in VT overnight, and
falling through the 30s toward morning across northern NY as
cold front sweeps through.

By Sunday the cold front surges quickly across the remainder of the
forecast area, clearing well east by afternoon as the ~980 mb
surface low races northeast toward the Gulf of St. Lawrence by early
evening. Robust 3-hourly pressure rises behind the frontal boundary
should ensure a windy afternoon as steadier rains transition to snow
showers and flow trends northwesterly. Accumulations should be
minor, though amounts from a dusting to 2 inches will be possible
across the St. Lawrence Valley and higher terrain of the
Adirondacks/Greens by early evening. Using an upward adjustment
factor from 160-190 percent yields gusts into the 30 to 45 mph range
with highest gusts in favored westerly downslope areas of the
eastern Adirondacks and eastern Greens/CT River Valley. Temperatures
will take on a typical cool-season non-diurnal trend as highs from
the upper 30s to mid 40s will be reached in the morning before
values slowly fall into the upper 20s to mid 30s by late
afternoon.

&&

.SHORT TERM /SUNDAY NIGHT/...
As of 409 PM EST Friday...To be honest, I was hoping the 12z
guidance suite would be much closer together with their various
solutions than they are. In a general sense, they all still
support the idea that a deep (around 980mb) low pressure system
will move northeastward out of the Great Lakes that will push a
strong cold front through the region during the day and also be
accompanied by strong gusty winds. However the exact track and
timing differ enough (+/- 150 miles and 6 hours) that it results
in a good deal of uncertainty with regard to specific details
in the sensible weather (when rain changes to snow, how strong
wind gusts get etc). The GFS & Canadian show the surface low
moving right up the St Lawrence Valley. The 12z NAM has moved
westward compared to prior runs and now shows this track as
well. This is a climatologically favored low track. However, the
12z ECMWF shifted a bit east, and is now bringing the low
across the Adirondacks on it`s way to Maine. This is not a
typical storm track. Perhaps it`s the start of a model trend
pointing toward a solution that has a low taking a more easterly
track. That said, most of the ensemble model runs still have
the westerly St Lawrence track.

Long story short, for this forecast cycle, we are maintaining a
forecast scenario where the low center moves up the St Lawrence
Valley early Sunday morning, with a strong cold front moving
through by mid-day.

With that, here is what is expected at this point:

Sunday morning should be plain rain across the region as
temperatures aloft and at the surface will be above freezing.
However a cold front will sweep across northern NY by late
morning and then into eastern VT by early afternoon. 850mb
temperatures will drop from about +4C to -5C in a few hours.
This will turn the rain showers to snow showers first at higher
elevations, and eventually lower down. It`s a strong front, so
we may have a narrow band of heavier showers with the front, but
at this point the hi-res models aren`t showing this feature.
This means high temperatures will be first thing in the day.
Exactly how warm it gets depends on the track of the low.
Further west means a better chance of warmer air getting farther
north. Right now have some low-mid 40s highs in the morning,
dropping back into the 30s for the afternoon. Could be some
places reaching 50F. Or if the low stays further east, then 40F
will be about it.

As the precipitation changes over to snow, the deepest moisture
is moving out, so not anticipating any significant
accumulations. Perhaps 1" or so at elevations primarily above
1000ft, and only some flakes that melt when they hit the ground
in the valleys.

It`s the winds that will become the most noticeable aspect
of this storm. After the front passes, strong cold air advection
will result in deep boundary layer mixing. At the same time a
tight pressure gradient will produce winds of 40-50 knots a few
thousand feet above the surface. A rapid pressure rise on the
order of 10mb/3 hours also suggests ageostrophic enhancement to
the winds. Net result, there is the potential of wind gusts on
the order of 35-45 mph for much of Sunday afternoon and evening.
Some downslope enhancement on the eastern sides of the `Dacks
and Greens as the westerly winds flow across, so perhaps
localized higher gusts in those areas. Later forecasts will fine
tune the need for any wind related headlines (probably advisory
level).

Sunday night, the snow showers will come to an end, but the wind
will continue to blow. Expecting 20-30 mph gusts nearly
everywhere as cold air advection keeps the boundary layer mixed.
Lows will bottom out in the teens to 20s, so wind chills will be
primarily in the single digits.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 409 PM EST Friday...Stuck pretty close to the guidance
blend for the period. In general, the 12z guidance was pretty
similar with the large scale features. So overall a reasonably
quiet weather week as large scale trough settles in over eastern
Canada. Several weak systems will zip through in the fast flow.
Temperatures will fluctuate some day to day, but overall
average at to just below normal. A few notes on each of the
days:

Monday: Still a gusty/brisk day thanks to a reasonably tight
gradient still existing between the departed low pressure and a
ridge of high pressure that will start to build in. Thinking
we`ll still see winds of 25-35 mph across much of the region
during the day. By evening, winds slack off and start to
transition to a southerly flow. This may cause some weakening
lake effect snow showers streaming off Lake Ontario to shift
position and move into the St Lawrence Valley. Not expecting any
significant accumulation, and have only 20-30% PoPs for that.

Tuesday: Looking pretty nice. Southwest flow will bring in
warmer temperatures. Weak ridging should result in fairly clear
skies. Highs well into the 40s for most of the region.

Wednesday: A weak cold front will move through. Perhaps a few
rain transitioning to snow showers (especially across the
northern NY & VT mountains), but it looks pretty weak. Not
thinking it will have any impact on those traveling for the
holiday.

Turkey Day: Chilly but mostly dry. Typical late November
weather. Couldn`t rule out a flurry here or there as westerly
flow will advect some moisture off the Great Lakes causing
fairly cloudy skies. Given trajectory of the overall flow,
any organized lake effect snow bands will remain south of
the forecast area.

Friday: Models indicate a weak shortwave moving across the
region. Could be a few light snow showers. Don`t see anything in
the models at this point indicating anything major precipitation
wise, so continued good traveling for those enjoying the
holiday. Temperatures again only in the 30s for highs, cooler at
higher elevations.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
Through 06Z Sunday...VFR through 00Z Sunday with thickening
mid/high cloud cover as light winds trend south/southeasterly at
5 to 10 kts (northeasterly at KMSS). After 00Z Sunday
widespread rain affects all terminals with ceilings lowering to
MVFR in the 015-025 AGL range in general. Some occnl VFR may
continue at KBTV/KPBG/KRUT.

Outlook...

Sunday: Mainly MVFR, with local IFR possible. Windy with gusts to
35 kt. Likely RA, Chance SHRA, Chance SHSN.
Sunday Night: VFR. Windy with gusts to 30 kt. Slight chance SHSN.
Monday: Mainly VFR, with local MVFR possible. Windy with gusts to
30 kt. NO SIG WX.
Monday Night: Mainly VFR, with areas MVFR possible. Slight chance
SHSN.
Tuesday: VFR. Slight chance SHRA.
Tuesday Night: Mainly VFR, with local MVFR possible. Chance SHRA,
Chance SHSN.
Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA,
Chance SHSN.

&&

.MARINE...
As of 337 AM EST Saturday...A Lake Wind Advisory is in effect
for today as southerly winds increase into the 15 to 25 knot
range by this afternoon and significant wave heights build into
the 2 to 4 foot range with a moderate chop. These conditions may
prove hazardous to those operating small craft, especially on
the broad lake and in bays/inlets with southerly exposures.
Gusty south winds will gradually abate tonight, though increase
sharply from the northwest by Sunday as a strong cold front
sweeps across the area.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...None.
NY...None.

&&

$$
SYNOPSIS...JMG
NEAR TERM...JMG
SHORT TERM...Nash
LONG TERM...Nash
AVIATION...JMG
MARINE...JMG


 
 
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