Area Forecast Discussion
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FXUS61 KCTP 260917
Area Forecast Discussion
National Weather Service State College PA
517 AM EDT Wed Oct 26 2016
A dry and chilly day will be followed by a dose of cold rain
later tonight into Thursday with a wet snow accumulation possible
from the Northern Alleghenies into Poconos. Chilly air will hold
its ground through the end of the week with some moderation in
temperatures expected over the weekend and into next week.
.NEAR TERM /THROUGH TONIGHT/...
With the exception of the Harrisburg metro area, 08z observations
show freezing temperatures (26-32F) across the majority of Central
Pennsylvania. Expect temps to drop a few more degrees prior to
daybreak which should ensure an end to the growing season for the
counties in the Freeze Warning.
A sunny start should begin to fade behind increasing clouds from
west to east this evening as a low pressure system tracks from IA
over Lake Erie tonight into early Thursday morning. Strong warm
air advection and isentropic lift via strong SSW low level flow
will support expanding zone of pcpn overrunning a retreating cold
airmass ahead of the aforementioned area of low pressure. This
should bring a cold rain along with some wintry weather concerns
across the Northern Alleghenies into the Endless Mtns/Poconos.
Evaporative cooling/web bulb effects should allow for a period of
wet snow/sleet at the onset before a transition back to rain
occurs later Thursday morning as the boundary layer slowly warms
up. If temperatures can drop at or below freezing, would not rule
out some pockets of freezing rain as temperatures warm aloft, but
high resolution ensemble blend keeps surface temps AOA 32F. Marginal
low level temperatures near freezing and relatively warm surfaces
given time of year/first snow make for a difficult winter wx fcst
with snowfall somewhat dependent on elevation, rate and type of
surface (grass vs. pavement) with northern valleys likely having
little to no accumulation. The time of day (overnight-predawn) is
most favorable for early season accums and also most problematic
in terms of impact to the AM commute. Secondary roads over the
highest terrain could be slushy for a time with some slick spots.
Therefore, will highlight potential winter wx threats/risk in the
HWO for now and allow dayshift to assess possibility of an advy
given the early season aspect and timing impact. The 26/00z NCAR
ensemble and SPC SSEO both add confidence to winter ptypes with
the NCAR more in line with conceptual snow-mix-rain dominant
ptype progression while SSEO seems to favor more fzra/ice. The
dayshift should have the HRRR available for additional consult.
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Lingering wintry ptypes should transition to plain rain by later
Thursday morning in the northern tier. Elsewhere, a chilly rain
will expand south/east and continue through the afternoon before
tapering to showers from west to east into Thursday night.
Downstream blocking should allow northern stream energy to phase
with the lead trough and eventually result in a negatively tilted
trough over the Northeast U.S. by Friday morning. Models show the
formation of a triple point low deepening along the coast into the
Canadian Maritimes by Saturday morning.
A brisk northwest flow may afford some lake enhancement to shower
activity Thursday night and temps are cold enough to mention
rain/snow showers especially in the orographically favored areas
downwind of Lake Erie. Overall the trend should be toward drier
conditions into Friday. The wind gusts may end up being a little
stronger than forecast on the backside of the intensifying low.
High pressure briefly returns later Friday afternoon/evening
before shifting southeast Friday night into Saturday morning as
low pressure tracks across the Upper Great Lakes.
.LONG TERM /SATURDAY THROUGH TUESDAY/...
Models and ensembles generally agree in bringing a weakening cold
front through the area on Saturday and stalling it out near the
PA/MD border. POPs are in the chance range mainly over the NW 1/2
with little in the way of moisture/QPF. The EC/GFS seem to be
trending toward the idea of a wave of low pressure developing to
the west along the wavy boundary and possibly bringing a better
chance for appreciable rainfall /0.25-0.50 inch/ to southern PA on
Sunday. Beyond Sunday, high pressure dominates the pattern
favoring dry weather.
A noticeable rebound in temperatures appears likely on Saturday
with an ensemble blend yielding highs 10-15 degrees warmer than
Friday. Temperatures may fall back on Sunday before moderating
again into early next week as southerly flow develops ahead of low
pressure in the Upper Midwest.
.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
As the region has decoupled and temperatures have dropped,
generally VFR skies will dominate the pattern. However due to the
low temperatures and low dewpoint depressions there remains a
chance for patchy valley fog overnight through the early morning
hours. The mains area of concern is mainly along/in the river
valleys. The only TAF sites that are of concern is BFD and JST,
though JST still has weak northwesterly flow which should
alleviate any fog/mist from forming.
Wednesday will be VFR ahead of a storm system that will be moving
into the lower great lakes late in the day. A warm front will
lift north into the area Wednesday night and Thursday bringing
widespread reduced conditions.
Fri...AM shrasn/low cigs possible W Mtns.
Sat...SHRA w/IFR CIGs possible N PM. Otherwise no sig wx.
Sun...SHRA w/MVFR cigs N and W. Breezy NW wind.
Freeze Warning until 9 AM EDT this morning for PAZ026>028-035-