Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 170440
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1140 PM EST Sat Dec 16 2017

.SYNOPSIS...
High pressure is moving in for tonight and last into Sunday. A
weakening wave of low pressure will move in from the midwest
late Sunday. Scattered snow showers will be in store for the
west during first part of the week. Fast west to east flow aloft
will keep the weather unsettled.

&&

.NEAR TERM /UNTIL 7 AM SUNDAY MORNING/...
Clouds will likely hold on all night in the NErn half to 1/3rd
of the the area with warm advection and lift just above the
blyr. However, moisture is decreasing and the light snow has
dwindle to flurries which should end around midnight.
Temperatures under the clouds are running a bit warmer than
expected along with west-southwest winds over the clearer areas
of the southwest. bumped everyone up 4 to 7 degrees. This makes
min temps tricky. Locations to the east of the Susq will likely
fall the hardest with snow on the ground.

&&

.SHORT TERM /7 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Clouds in the NE half/third should begin to break up, but just
in time for a new batch of warm advection to ride in from the
west later in the day. So, there will likely be an increase in
clouds over the SW, and a waffle from clouds to no clouds and
back again in the central/NE. We are expecting some light
precip to break out in the SW/SC counties by nightfall. The
temps will get into the 40s in the south with sun. without much
sun in the NE, it will likely stay below 35F.

The low pressure over the mid-MS valley will fall apart as it
goes into the OH valley and a loss of moisture will kill much of
the precip as the feature fills over us and moves eastward Sun
night. The clouds will likely hang low and thick, keeping temps
up. Many places should stay above freezing.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Fast westerly zonal flow will turn a bit more SW by Friday as a
trough digs over the upper Plains.

A western trough kicks a strong shortwave out across the Plains
Sunday into Monday. The associated low center will move
northeast across the Great Lakes staying to our north. This
system will drag in some warmer air in southwest flow ahead of
it. However, 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 light. Will have to watch for
potential for some ice also Sunday night.

Then through much of the week, we move to fast moving pattern
of weak shortwaves and chances for precipitation (mainly over
the northern mountains) through the week. As a northern stream
wave zips by to our north by Monday night, it will bring 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 relatively warmer air follows Thursday.

The warm advection and approaching cold front will likely
produce increased potential for snow/ice in the west Thursday
night into early Friday. As precip works in on Friday,
precipitation looks to transition through mixed to light rain.
Temps could fall below guidance over the east Thu night if skies
remain clear as exiting surface high pressure will leave a dry
airmass in place.

A strong frontal boundary will likely push another surge of
cold air into the region Friday into Saturday as the eastern
long wave trough tries to re-establish itself. GFS and ECMWF
diverge by this time, with the EC moving surface low slower and
thus forecasting higher pops than GFS.

&&

.AVIATION /04Z SUNDAY THROUGH THURSDAY/...
Lingering snow shower activity has ended across the region as of
04z according to radar and observations. Satellite continues to
show a slow but steady erosion from the south of the cloud deck
which covers the northern third of Central Pennsylvania.

Winds have decoupled at most locations, with generally 3 knots
or less being reported at most locations. KJST is the outlier,
with winds still westerly sustained at 10 knots.

Forecasting challenge for the overnight is determining when and
where any fog and lower stratus may form. Guidance suggests that
northern airfields will redevelop at least MVFR ceiling
conditions overnight...with some areas of IFR/LIFR.

Based on current conditions being a bit better than projected
by models, have somewhat backed off ceiling and visibility
restrictions in the TAFs. Last few nights have seen aggressive
fog formation in areas where skies have cleared.

Sunday, any areas of low clouds and fog may be stubborn to
lift given light wind conditions. Warm advection on developing
southerly wind will bring increasing mid and high clouds from
the Southwest. Expect a brief period of mainly VFR conditions
across the region, before precipitation spreads in later in the
day into the overnight.

Outlook...

Sun...Areas of morning MVFR/IFR conditions, most likely across
the north, otherwise VFR. Lowering conditions moving in from the
southwest late in the day.

Mon...Mainly VFR. Spotty SHSN poss W.

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

Wed-Thu...No sig wx.

&&

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

&&

$$
SYNOPSIS...Dangelo
NEAR TERM...Watson/Dangelo
SHORT TERM...Dangelo
LONG TERM...RXR
AVIATION...Jung


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