Area Forecast Discussion
Issued by NWS State College, PA-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KCTP 221951
Area Forecast Discussion
National Weather Service State College PA
351 PM EDT Sat Apr 22 2017
A frontal boundary will remain stalled out just to the south of
the state as a few weak waves of low pressure ripple northeast
along it. An upper level cut off low will form over the
Tennessee Valley late this weekend, and could end up bringing a
storm slowly up the East Coast in the first part of the
.NEAR TERM /UNTIL 8 PM THIS EVENING/...
Radar mosaic shows several bands of light-mdt rain showers
streaming to the NE, and covering approximately the SE third
of our forecast area as of 1730Z.
The showers were being generated by a broad area of mid-level
FGEN forcing and uvvel beneath the thermally direct/RE region of
a 95-105 KT upper jet core draped from the Ohio River Valley to
central New York at midday.
High res models agree very well with what we`re seeing on
regional radar loops late this morning, that being that the
NW edge of the showers will be gradually shifting to the SE
through the midday and afternoon hours, and becoming confined to
the region near or to the SE of I-81 after 18Z as another mid
and upper level disturbance (and weak wave of 850mb low
pressure) arrives from the Middle Ohio River Valley and Central
Appalachians as it rides NE along the 850 mb front that`s
stretched out along the I-81 corridor.
Expect additional rainfall (from the current departing area of
showers, and the mid to late afternoon arrival of the second
area) to average between 0.10 and 0.25 of an inch across the SE
third of the CWA for the rest of the day, with nothing more
than a a few sprinkles near and just to the north of a line from
KJST and KAOO to KSEG from the gradually lifting and SE
drifting mid level cloud deck. Breaks of sunshine will occur
through the dual stratocu and altocu deck across the NW half of
Temps will hold between 45-50F in the higher elevations. We may
reach the upper 50s in some locations throughout the Central
Valleys and Mid Susq valley - with the best chance for that
probably in the EC zones. the more persistently rain-cooled air
across the SE will keep temps pretty steady between 52-55F.
.SHORT TERM /8 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Rain is expected to press to the south of the Mason-Dixon line
very early this evening as deepening dry air advects in from the
north-northwest ahead of a ridge of high pressure.
It should remain dry - even in the south - for the rest of the
weekend with POPs less than 15 percent across the southern tier
Temps will get pretty chilly Sunday morning, especially in the
mountains. Much of the valley areas in central and Southern PA
will see the mercury fall into the U30s and L40s. However, it
will probably dip into the mid-upper 20s in the perennial cold
spots near KBFD - and between 30-35F over the rest of the NW
third to half of the CWA.
Wind will be calm or light/northerly. Much of the area will
clear out. The average date of the last spring freeze has passed
for the SE third of the area, and there could be some frost in
the ridge and valley region (between UNV and MDT). Will hold off
on any mentions of frost in the wx grids/zones for now.
Subsidence will lead to plentiful sunshine on Sunday. Temps
should rebound nicely with almost all of the area back into
upper 50s and lower 60s.
.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
-- Changed Discussion --The period of Sunday night to Wednesday will be dominated by
the flow around a slow moving cut-off low which eventually will
pass to our south and east. This system will bring clouds and
light rain to the parts of our region for several days. The
models are in fair agreement on timing and position of this
low. Bands of precipitation should begin to streak through the
region, mainly the east Tuesday morning. Timing and position of
the low will change whether or not different portions of central
Pennsylvania get precipitation. Have increased POPS for eastern
PA. The gradient will be tight on any precipitation amounts.
Once this system goes by large scale retrogression will put our
region on the western edge of a relatively strong 500 hPa ridge
and it will get warm fast. It should feel like summer by next
The potential for rain will be higher in southeastern PA and
much lower in northwestern PA. Rainfall will likely be very
light and the best chance for measurable rainfall will be in
southeastern PA. Enjoy the mostly cloudy and relatively cool
weather while it lasts.
As the 500 hPa low fills and the attendant moves to our
northeast the chance of rain should fall off Wednesday into
Wednesday evening. Our 850 hPa temperatures will rise rapidly
Wednesday and it should be noticeably warmer Wednesday relative
to Monday and Tuesday.
Thursday into Saturday the 850 hPa temperatures will be well
above normal, mainly in the 14 to 16C range and we will be on
the western edge of a rather impressive early season subtropical
ridge. The GEFS and GEFSBC show a closed 5880 m ridge along the
VA/NC/SC coast with +2 sigma height anomalies by Fri-Sat.
Thursday through at least Saturday should be very warm and
humid. We should have several days with high temperatures well
into the 80s. Any precipitation after Wednesday will likely be
in a more summer-like atmosphere so have kept thunder in all
forecasts beyond Wednesday.
The ridge will likely continue retrograding. If the NEAFSBC and
GEFSBC are correct we could be looking at our first enduring
period of warm weather from this coming thursday into the
-- End Changed Discussion --
.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
Radar loop at 16Z showing several bands of showers covering the
SE third of the state, and shifting gradually to the east.
Expect dry conditions there for a few hours early this
afternoon, before another batch of light to moderate showers
impacts the KMDT and KLNS areas between 18-22Z.
Cooler and moist llvl air in the wake of the slow moving cold
frontal boundary across Central PA will bring some MVFR strato
cu clouds early this afternoon across the NW mtns and Laurel
Highlands (Including KJST and KBFD) as that air is lifted
There should be enough dry air and weak downsloping in the low
levels to preclude significant CIG reductions across the SE half
of the state. However, the periods of showers near and to the SE
of Interstate 81 will be accompanied by brief 4-5SM MVFR vsbys
at KMDT and KLNS during the afternoon hours. Generally VFR Cigs
and Vsbys will occur elsewhere at airfields across Central Mtns
this afternoon including KAOO, KUNV and KIPT.
Any showers should exit southeast Pa by early this evening.
After that, a drier northerly flow should ensure widespread VFR
Sun-Mon...No sig wx expected.
Tue...Rain/low CIGs possible.
Wed...AM Rain/low CIGs possible, mainly eastern Pa.