Area Forecast Discussion
Issued by NWS State College, PA-- Remove Highlighting --
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FXUS61 KCTP 200549
Area Forecast Discussion
National Weather Service State College PA
1249 AM EST Fri Jan 20 2017
High pressure overhead will retreat and a warm front will push
rain across the state on Friday. Relatively mild conditions
will persist for the next several days. A very complicated storm
system may bring a prolonged period of rain from Sunday into
early next week. Snow or mixed precipitation is also possible
from that same storm early next week.
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
H5 ridge axis lies over the Allegheny Plateau late this evening.
High clouds area increasing over eastern OH in the veering WSW
flow aloft...while stable air along and east of the plateau
promotes fog formation where low clouds had yet to fully scour
The overnight will see an increase of clouds throughout, with
pockets of near or slightly below freezing temps over mainly
eastern and northern areas. Onset of precip expected after temps
warm above freezing Fri morning. Mins will range overall from
near 30F in northern cold spots to the mid 30s southwest
.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/...
Warm frontal precip moves in Fri morning just after sunrise in
the Laurels and spreads a band of 3-6hr light rainfall across
the entire area during the daylight hours. Timing is very solid
amongst all models and forecasts. Those places which dip below
freezing overnight will have a few hours of daylight to warm.
Also, the wet-bulb effect could present a minute chance of it
freezing up - mainly over the NE - but again, those areas should
have the chance to warm up enough to keep the fzra threat low.
Maxes/diurnal swing will be held down once again by the cool
air trapped and not quite get into the 40s over a good portion
of the area - with mid 30s the expectation in the NE and the
central mtns. Even MDT and LNS may not eclipse 40F.
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
The upper ridge is forecast to re-establish itself over the
eastern U.S. over the upcoming weekend, bringing mild and mainly
dry conditions Saturday, through at least the mid morning hours on
Daytime temperatures across most of Central Pennsylvania will be
well above normal, but the departures for overnight lows should be
greater by several degrees more throughout the entire extended
The ridge axis will shift to the Hudson Valley and Eastern
Seaboard early Sunday as a sfc backdoor cold front drifts swwd
into PA and stalls out as a potent and moisture laden nearly
stacked low lifts NE from the deep south to the Delmarva region by
Temps could be marginal at the start of pcpn overnight Sunday for
some ice or snow, and soundings via the GFS/GEFS and EC appear
cold enough for Sunday night through Monday night for a mix of
rain/snow (or even periods of all wet snow at times across the
northern mtns of PA). However, the bulk of pcpn across the
Central Ridge and Valley Region looks to be rain at this point
through Monday afternoon.
The trend in most operational models and ensembles has been
further to the south and east with this particular late
weekend/early week system. This places parts of the fcst area in
the sweet spot for potentially several inches or more of heavy,
wet snow (especially later Monday/Monday night across the nrn and
wrn mtns of PA at elevations of 1400 ft or higher), as the mean
850 mb low in the GEFS and EC tracks NE along the I-95 corridor in
VA and adjacent SERN PA/Southern NJ.
U-wind anomaly at 850 mb peaks at -4 to -5 sigma during much of
Monday as a 45-55kt easterly llj develops between this approaching
low, and the aforementioned colder airmass pushing swwd from SERN
Canada and the New England states. Rainfall amounts are expected
to be in the 1 - 1.5 inch range across roughly the SE half of PA
with lesser amounts to the NW.
Another short wave ridge will slide east from the Ohio Valley
and move over the Commonwealth Wednesday, accompanied by light
wind and fair/generally dry conditions.
.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
06Z TAFS sent.
Some spots have low cigs and dense fog already, as strong
Earlier discussion below.
Satellite showing a few breaks in the low cloud cover at BFD at
03Z, which has resulted in fog development. GLMP MELD suggests
some improvement is possible at BFD after about 07Z, as a
southeast breeze develops, causing fog to disperse a bit.
However, confidence in significant improvement is low and will
continue to forecast IFR/LIFR conditions through dawn at BFD.
Elsewhere, VFR conditions currently observed across southern PA
at 03Z. However, a calm wind and mclear skies have allowed temps
to fall close to the dewpoint late this evening. Thus, feel
radiation fog will form over much of central/southern Pa between
04Z-07Z. Can`t rule out a brief dip to IFR visibilty before
thickening mid and high clouds arrive late tonight, diminishing
the fog threat. Based on latest SREF and downscaled NAM, believe
IPT stands the best chance of significant vis reductions
An approaching warm front will bring a period of rain and low
CIGs to the entire region on Friday. Based on current model
data, arrival time of rain should range from 12Z-13Z at JST to
17Z-18Z at IPT. The rest of central PA should see rain/low CIGs
develop in between those times. The steady rain will exit the
area between 20Z-00Z. However, low clouds/drizzle are expected
to linger into the evening over most of central PA.
Sat...Low CIGs likely, mainly in the morning.
Sun...IFR/MVFR CIGs likely. Rain advances fm S-N.
Mon...Rain/low CIGs likely with snow possible central Mtns.
Tue...AM rain/snow and low CIGs possible.