Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 150654
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
154 AM EST Fri Dec 15 2017

.SYNOPSIS...
A weak ridge of high pressure will slide across the region tonight
with cold, but mainly dry conditions.

Another clipper will move north of the region overnight Friday
and Saturday morning followed by high pressure and a brief
moderating trend in the temperature Sunday.

A series of cold fronts will move across the area late Sunday
into Wednesday. Windy and colder temperatures will follow.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Fair and cold conditions prevail early this morning as surface
high drifts over the state. Central Mountain valleys cooled to
the upper single digits late last evening, but have reboudned
slightly as clouds have thickened ahead of approaching
shortwave. Min temps will range from the upper single digits
across the northwest half of the forecast area, to the upper
teens in Lower Susq Valley.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/...
Thickening cloud cover ahead of approaching shortwave won`t
allow temps to recover much from the cold start Friday. Highs
only expected to range from the low 20s over the northern
mountains, to around freezing in the extreme southeast.

WAA ahead of shortwave could produce a few afternoon flurries
across the southeast half of the forecast area, while a
developing low off the Mid Atlantic coast could potentially
graze the extreme southeast corner of the forecast area during
the afternoon, so have included low POPs down there for a period
of light snow in the afternoon. Have kept the forecast dry for
much of Friday across the northwest mountains, where models
place a ribbon of low PWAT air. However, increasing moisture
works in late in the day ahead of approaching cold front,
bringing scattered snow showers toward evening.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
The period starts out with below normal 500 hPa heights and
below normal 850 hPa temperatures. The heights and temperatures
rise to above normal values over the northeastern United States
Sunday as a strong ridge retrogrades to our south. The really
warm air should stay to our southeast. The heights and
temperatures settle back toward normal Tuesday and Wednesday at
500 hPa and 850 hPa respectively.

Models indicate a developing band of low level fgen forcing
across northern Pa Saturday within left exit region of jet
streak. Model soundings also indicate a nearly saturated sfc-
700mb layer with upslope flow into the Alleghenies. Thus, have
mentioned the chance of spotty light snow across the entire
Allegheny Plateau Saturday, with higher POPs across the
northwest mountains, where best fgen forcing is indicated. WPC
qpf indicate a few inches of accum are likely Fri night-
Saturday across Warren/Mckean counties, with minimal amounts
likely further south along the Alleghenies.

Sunday into Monday a strong shortwave to our north will help
bring in warmer air. We may be cold air dammed near the surface
so the +1 to -2C air at 850 hPa may not translate to a warm
boundary layer. The warm advection should bring snow and snow to
rain in the south but QPF amounts are low and EFS based POPS are
not very high in central and southern areas.

As this northern stream wave zips by to our north it should drag
cold air back into the region Tuesday into Wednesday. The
northwest flow triggers snow in relatively high POPS in NW PA
due to LES. This cold surge should be relatively fleeting and
high pressure and a potential surge of warmer air in the
NAEFS/GEFS. This suggests relatively warmer on Thursday.

The Warm advection and approaching cold front will likely
produce increased potential for snow in NW Thursday into Friday.
Farther south any precipitation would likely be mixed or rain.

A strong frontal boundary will likely push another surge of
cold air into the region Friday into Saturday.

At this time and recent GEFS/CMCE and combined NAEFS runs keep
the strong ridge suppressed to our south. This keeps the warm
air from getting here. This keeps us relatively dry with all
the significant QPF events suppressed to our south.

There is considerable spread in the ensemble members. And we
have seen run-to-run variations in the solutions with the strong
southwest Atlantic ridge. The NAEFS deeps reforming vortex over
east-central Canada and maintains the sharp ridge over
northwestern North America. This seems to keep us in or on the
edge of cold air intrusions. Thus so far single model runs of
big warm ups appear to disappear in the ensembles.

&&

.AVIATION /07Z FRIDAY THROUGH TUESDAY/...
* Cloud/Ceiling info will be missing at KBFD until new part can
  be installed *

Mountain airfields of KBFD and KJST have dropped into MVFR
conditions, with occasional light snow being reported at KBFD.
VFR conditions at remaining central Pennsylvania airfields.

These conditions should prevail overnight. There could be some
brief periods of breaks in the ceilings in the west from time to
time overnight however it should be limited enough not to
mention in TEMPO conditions in the TAFs.

The chance of snow showers will be on the increase Friday, with
areas in the southeast being affected by WAA and potentially the
fringes of a coastal system, while the northern airfields will
see the potential of snow showers later in the day with the
approach of the next front.

Outlook...

Fri...Generally VFR...but with areas of MVFR in snow showers.

Sat...Scattered snow showers and local restrictions NW/Laurels.

Sun-Mon...Mainly VFR.

Tue...mainly VFR, but possible restrictions in the western and
northern mountains in snow showers.

&&

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

&&

$$
SYNOPSIS...La Corte
NEAR TERM...DeVoir/Fitzgerald
SHORT TERM...Fitzgerald/La Corte
LONG TERM...Grumm/Fitzgerald
AVIATION...Jung/Ceru


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