Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 171637

Area Forecast Discussion
National Weather Service State College PA
1137 AM EST Sun Dec 17 2017

A weakening wave of low pressure will move in from the midwest
late today, generating some spotty rain showers over south
central Pennsylvania. Slightly colder air moves in for Monday,
but temperatures will warm a bit for Tuesday. A back door cold
front will bring colder air in from the north on Wednesday.
Generally dry weather with a warming trend is then expected for
the last part of the week.


Clouds cover the entire area this morning. Most areas just have
high clouds, but thicker low clouds linger in the
north/northeastern mtns. No precip out of it, though. Still
looks like a brief brush of rain showers for the Laurels between
5 and 9 PM. But, QPF only 5 hundreths on avg from most readily-
available CAMs and NAM/GFS 12z runs. This stuff fizzles into
barely DZ this evening as it lifts slightly into the central
mtns. Will continue low POPs tonight with measurable precip
unlikely at most places.

GOES 16 shows broken to overcast skies covering NE third to
half of central PA this morning thanks to shallow warm advection
above the boundary layer, while SW 2/3 is clear to pc. High
clouds are starting to increase from the Ohio Valley ahead of
an upper trof over the central and southern plains which will
track towards the region this afternoon and evening.

Clouds in the NE third should begin to break up this morning,
but just in time for a new batch of warm advection to ride in
from the west. Models have backed off on POPs and precip amounts
from this wave, but we still expect some very 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 aforementioned upper trof over the Ohio Valley will shear
and weaken as it rides over the flat upper ridging along the
eastern seaboard tonight. Still, there will be sufficient
forcing and moisture to keep low chance of rain/snow showers
across central and southern areas primarily. Much of the
precipitation will be hard pressed to measure. Clouds will be
widespread keeping mins up tonight...with most areas remaining
above freezing.

Mild southwest boundary layer flow will persist on Monday, as
another shearing trof drops towards the region from the central
and eastern GLAKS. Slgt to chc pops are again favored on Monday
in this scenario, with best chc along and west of the
Alleghenies which will have slightly enhanced Pops due to
upslope zonal flow. Highs will range from the upper 30s north to
the mid to upper 40s south.


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.


VFR all locations at 16Z very light wind in the west and calm
east. A very brief SHRA is expected at JST this evening
between 22Z and 01Z. Otherwise, some patchy DZ is all we should
see overnight as deeper moisture diminishes quickly after 00Z.
There is a small chance for some FZDZ overnight, but chc too
slim to mention with upcoming pkg.

Light winds continue with MVFR vsbys forming at UNV and VLIFR
continuing at BFD. Restrictions at BFD will continue for the
next few hours and latest CAMS due show some improvements
between now and 15Z and have put in a TEMPO group accordingly. 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.


Mon...Mainly VFR. Sct SHRA/SN W.

Tue...Mainly VFR, Iso SHSN NW.

Wed-Thu...No sig wx.

Fri...SHRA poss W.




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