Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 180121
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
821 PM EST Fri Nov 17 2017

.SYNOPSIS...
An intensifying storm system will move through the eastern Great
Lakes Saturday. followed by a strong cold front for Saturday
night. Cold gusty winds and some lake effect snow will then
affect the area Sunday into Monday. High pressure will then
bring improving conditions through the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Clear skies will give way to increasing high clouds overnight
with the first of the rain forecast by the latest HRRR to be
entering my western stripe of counties right around sunrise.
Temperatures will continue to fall this evening, with readings
already freezing or below over the northern tier. While the
clouds will eventually put a brake on the fall as they thicken
up, some of the normal chilly nooks could be vulnerable to a
short period of spotty light freezing rain as the precip
overspreads the CWA. Right now the confidence is too low to
issue an advisory, but the potential will continue to be
monitored as the rain approaches.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The storm system currently emerging from the front range of the
Rockies will deepen on its track into the eastern Gr Lakes
Saturday and Saturday evening.

The warm front south of the state will be very slow to advance
into the central ridge-valley region Saturday with the models
showing a strong cold air damming scenario setting up. With the
right entrance region of an upper jet streak sliding by just to
the north of the area, the thermally indirect vertical
circulation will support a strong NELY component to the low
level ageostrophic flow out of the north-northeast. With this
strong northerly component to the low level flow and precip
falling into an initially dry airmass contributing to further
stabilization through evaporational cooling, it points to a
chilly rain for the forecast area during the day, with the
steadiest stripe of rain likely over about the northern 2/3 of
the CWA.

Another concern for Saturday will be for an elevated
thunderstorm. SPC has us in its general outlook, but it`s hard
to find even a hint of elevated instability in the face of the
intense cold air damming. While I won`t rule it out in the
strong warm advection pattern that is expected to develop, I
think the probability is low enough to not mention at this
point.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
One-two punch of gusty West to Northwest wind, then a quick
transition to the first significant bout of Lake Effect Snow
this season across the NW mtns of PA in the wake of a strong
cold frontal passage Saturday night.

The synoptic set-up of a deepening low moving down the St
Lawrence River valley and a potent short wave trough aloft
swinging around the base of the longer trough is quite favorable
for strong winds.

The center of an area of very sharp pressure rises (10 mb/3
hours) moves ENE right over the region during the mid to late
morning hours Sunday. We could easily see Wind Advisory Criteria
with this type of fall/rise couplet passing directly overhead,
and possibly even a High Wind Watch/Warning scenario with west
to northwest gusts in the 50-60 mph range. It`s still far
enough out to look for consistency in this feature and decide
whether to go the High Wind Watch route at ~4th period.

The second punch is the deep/strong cold air advection with -10C
air at 850 mb crossing Lake Erie Water temps in the mid-upper
40s, which results in an extreme Lake-850 mb Delta T of 18C
Sunday afternoon and evening across the perennial snowbelt of NW
PA.

Winter Weather Advisory criteria will likely be reached from
this upcoming bout of LES, considering anticipated 4-8 inch
forecast snow amounts over an extended 24-36 hour period.
Elsewhere, much lighter snow amounts of a coating to 2 inches
will occur from sinuous bands of snow showers and briefly heavy
squalls that should make it well into the Central Ridge and
Valley Region and portions of the Susq Valley based on fairly
strong 35 to 45 kt nw winds in the deep and cold boundary
layer.

High pressure sliding by to our south will influence the
weather to start Thanksgiving week, bringing a couple of cool-
dry days. Temperatures will begin to moderate Tuesday with a
gusty SW flow expected to develop as potent sub-1000 mb low
pressure passes by across southern Ontario and Quebec. Tuesday
will be the lone day in the Mon-Fri period when 850 mb temps
will poke a few to several deg C above zero.

Though the jet stream will remain north of the region, a quick
moving trough will brush the region on Wed followed by a weak
shortwave Wed night, which will bring chance for periods of snow
showers (with light accums of a coating to 2 inches) back into
the NW zones, along with a return of slightly cooler air.
Flurries or brief snow showers will occur just SE of the
Allegheny Front.

12Z GFS and EC have trended into better agreement on a deepening
upper low across southeastern Canada or the NE U.S. over the
Thanksgiving weekend. Surface and 850 mb temps could be a few
deg colder than our upcoming cold outbreak later this weekend.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Widespread VFR expected overnight before conditions trend
steadily downhill into Saturday. Rain expected to overspread the
airspace 10-15z with a few pockets of freezing rain possible on
the leading edge over the north-central airspace. Confidence is
low on fzra risk and it would only last for a short duration.
Periods of rain will continue through the day with variable
cigs/vis ranging from low VFR to LIFR. Introduced LLWS at
western sites through midday with another period of LLWS
possible over the eastern 1/2 of the airspace ahead of a strong
cold front Saturday night. Southerly winds will increase
through the day with gusts 20-25kts by evening across the
western airspace.

Outlook...

Sun...Windy with sfc gusts 30-40kts from 270-310 degrees. IFR
snow showers NW trending MVFR central to VFR east.

Mon...MVFR cigs/sct -shsn psbl early NW 1/4.

Tue...Breezy. Mainly VFR.

Wed...Cold fropa. Snow showers possible NW.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...La Corte/Gartner
NEAR TERM...La Corte/Gartner
SHORT TERM...Dangelo/Gartner
LONG TERM...Lambert
AVIATION...Steinbugl



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