Area Forecast Discussion
Issued by NWS State College, PA

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121
FXUS61 KCTP 151912
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
212 PM EST Fri Dec 15 2017

.SYNOPSIS...
A weak upper level disturbance will swing down across the region
today as a surface low forms over the Outer Banks and tracks
out to sea. High pressure and a brief moderating trend in the
temperature are expected by Sunday. A series of cold fronts will
move across the area late Sunday into Wednesday. Windy and
colder temperatures will follow.

&&

.NEAR TERM /THROUGH TONIGHT/...
A fairly potent upper level shortwave/Alberta clipper is
diving SE out of the Gr Lakes. Forcing in the favorable left
front quad of the upper jet streak is helping support a brief
period low level convergence/frontogenetic forcing and fairly
widespread - but light snow. CAMs suggest the best chance for a
coating to an inch or so of snow will be over my SERN zones
where advisory criteria is small and travel issues common with
even light snowfalls. At this point it doesn`t seem to warrant a
headline, but we did issue a Special Weather Statement to alert
for the potential of locally slippery travel.

Models use the shortwave to develop a weak surface wave over
coastal NC and speeding it well offshore later today and
overnight bringing a quick end to the SERN snows.

Moisture will also increase over the northwest throughout the
day as cold front approaches. That will bring increasing snow
shower chances the deeper into the day we get.

As the winds shift more westerly behind the upper trough this
evening and overnight, cold flow over the lakes will become
more favorable for lake effect snow showers. The flow will favor
a more westerly than NWLY direction suggesting the best snows
will stay closer to the lake, mainly affecting NRN Warren and
McKean counties.

Lows in the teens and 20s will average about 3-6 deg colder than
normal.

&&

.SHORT TERM /SATURDAY/...
Lake effect snow showers will continue Saturday morning, tending
to diminish during the afternoon as large scale subsidence
begins to develop under rising heights aloft.

It will still be colder than normal with highs ranging from the
upper 20s northwest to around 40 over the south.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
This coming week will see the long-wave upper level trough that
has been dominating out weather shift east as a deep trough digs
over the Western U.S. Lake enhanced snow showers should be
ending by Saturday night with the surface high pressure ridge
moving off the coast and the winds becoming move west to
southwesterly.

The 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 and west.
This system will bring in 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.

We look to move into a 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 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 relatively warmer air follows 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 as the eastern
long wave trough tries to re-establish itself.


&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
* Cloud/Ceiling info will be missing at KBFD until new part can
  be installed *

Areas of light snow and pockets of MVFR/IFR over central and
eastern terminals will continue into the late afternoon with
improving conditions between about 21-00Z.

Over the NW the flow will take on more of an over-lake
trajectory which will bring occasional snow showers into the
normal snow belt areas and Laurel Highlands. Expect mainly VFR
conditions with locally IFR/LIFR under heavier bands of snow
showers.

Outlook...

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

Sun-Mon...Mainly VFR.

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

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 PM this evening to 4 PM EST
Saturday for PAZ004-005.

&&

$$
SYNOPSIS...La Corte
NEAR TERM...La Corte
SHORT TERM...La Corte
LONG TERM...Watson
AVIATION...La Corte



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