Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 151322
AFDCTP

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

.SYNOPSIS...

A weak upper level disturbance will swing down across the region
today as a weak 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 /UNTIL 6 PM THIS EVENING/...

Clouds came in and stopped the temperature plunge that occurred
last night with the fresh snow cover and light winds.

Today may prove to be more interesting than previously
anticipated. A fairly potent upper level shortwave/Alberta
clipper is forecast to dive SE out of the Gr Lakes. Forcing in
the favorable left front quad of the upper jet streak results in
a brief period low level convergence/frontogenetic forcing and
perhaps a quick inch or two of snow over my SERN zones where
advisory criteria is small and travel issues common with even
light snowfalls.

Models use the shortwave to develop a weak surface wave over
coastal NC this morning before speeding it well offshore this
afternoon and overnight. The SPC HREFV2 supports a light snow
shield by 15-18z over my southeast counties with a coating to
perhaps an inch or two of snow possible over especially the
southern half of York and Lancaster Counties. This will have to
be monitored for a possible short advisory for the afternoon
travelers.

Moisture will also increase over the northwest throughout the
day as cold front approaches this afternoon. That will bring
increasing snow shower chances as the afternoon wears on. Snow
showers are most likely from the Laurel Highlands to the
Northwest Mountains by this evening.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Best chances for a couple locally 3 or 4 inches of accumulation
are over Warren County from late afternoon through Saturday.
Duration event won`t produce Advisory criteria in 12 hours, so
no headlines issued for Warren County this morning, although
Advisories and Warnings lie just to the west and north where
trajectories favor several more inches of accumulation. Upslope
into the Laurel Highlands will produce 1 to maybe 2 inches in
spots there. Isolated to scattered snow showers and flurries are
expected elsewhere. Most areas east of the Alleghenies won`t
see much if any accumulation from these, but a coating to
locally 1-2" is also expected over the West Central Mountains.
Mins tonight will be milder and range from the mid to upper
teens north to the lower to middle 20s south.

Lake effect snow showers will continue on Saturday over the
Northwest Mountains and perhaps the West Central Mountains, with
improving conditions elsewhere. It will still be cold with
highs ranging from the upper 20s northwest to the lower to
middle 40s South Central Mountains.

&&

.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 /13Z FRIDAY THROUGH TUESDAY/...

* Cloud/Ceiling info will be missing at KBFD until new part can
  be installed *

MVFR ceilings from AOO-JST north to BFD will continue through
much of the morning with AOO/UNV standing the best chance of
seeing their ceiling improve above 3000`.

However a new disturbance aloft will bring localized reductions
from late morning through mid afternoon with occasional light
snow mainly over my SERN zones. Visibilities will
intermittently drop to 1-2 miles in areas of snow with ceilings
staying mainly above 2000-3000` this afternoon.

Late this afternoon into tonight northwesterly flow will bring
snow showers and a reduced conditions to the western TAF sites.
MVFR to IFR is possible at BFD and JST.

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



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