Area Forecast Discussion
Issued by NWS State College, PA

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982
FXUS61 KCTP 281246
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
846 AM EDT THU JUL 28 2016

.SYNOPSIS...
The stalled surface front over the Mason Dixon line will move
northward today and tonight as a disturbance approaches from the
Ohio and mid Tennessee River Valleys. This will produce a soaking
rain over portions of central and south central PA tonight into
Friday morning. Behind this system slightly cooler air aloft
moves over the region to usher in the weekend. But the ridge may
begin to build to our west again early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

QUICK UPDATE: updated grids for the rainfall coming in ahead of
the MCS in Ohio and the enhanced clouds. Updated PoPs for the
region and more rainfall potential for west/southwestern areas.
Updated and nudged all grids through 8 PM tonight too.

Used current satellite and HRRR for all nudging.

Previous discussion: Moisture and
clouds continue to slowly increase from the southwest early this
morning...as quasi stationary front near the Mason Dixon line
begins to lift northward in response to approaching upstream
troughing. Isolated showers are now lined up along the MD/WV/PA
border and extend into portions of the Laurel Highlands and South
Central Mountains...a sign of things to come later today and
tonight.

Deep layer moisture will continue to increase from south to north
today as southerly component to 5h flow increases. Impressive
moisture flux late this afternoon and evening will yield some
2.0"+ PW by this eve (00z Fri) and set the stage for some decent
rainfall over the southern third to half of central PA overnight
into Friday morning. Mins will generally range through the 60s
across northern and central sections...except for the southeast
which should bottom out around 70F.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Impressive boundary layer convergence on the nose of a potent
40kt 850 mb jet will promote a well organized convective complex
that traverses the southern third to half of central PA...with
high confidence in a stripe of 1-2"+ rain totals by 15z Fri.
Latest guidance has shifted the ribbon of max qpf slightly
northward in proximity with the LLJ...and parts of the mid Susq
River Valley may get in on the soaking rainfall late tonight and
Friday morning. Areas well north of I80 may not see much...but
pops still increase to high chc/likely late tonight and Friday
with this wave. Flooding threat remains quite low, but WPC has
placed my far southern and southeastern counties in SLGT risk
(5-10%) of exceeding flash flood guidance. PHL and now LWX have
issued Flash Flood Watches just south and east of my area, and day
shift Thu may want to consider shifting north should latest
northward model trend continue.

For the remainder of Friday...expect only residual light scattered
showers over mainly the higher terrain of the west and north as
cooler air and slightly cyclonic flow come in the wake of the
exiting shortwave.

With abundant cloud cover and showers on Friday...highs will be
cooler than recent days...and range from the upper 70s north to
the lower to middle 80s southeast. Again...the bulk of the rain
will come/wrap up by mid morning Friday...but far eastern sections
may linger into Friday afternoon.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
In the medium to long-range period...conditions slowly revert to
near normal in temp and precip as several chunks of short wave
energy in the westerly flow aloft helps to carve out a mean 500 mb
trough axis over the Glakes and Ohio Valley region during the Thu-
Fri timeframe.

The peak in unsettled weather (with perhaps some periods of down-
right rainy conditions) occurs Thursday night and Friday as a
well-defined wave of sfc-850 mb low pressure lifts ENE out of the
Ohio Valley and heads across the I-95 Megalopolis.

Followed a blend of EC/GEFS/superblend and NBM guidance for that
particular 24-36 hour period to arrive at likely pops for rain and
scattered TSRA. Some uncertainty in timing and areal coverage of
the convection led us to undercut the NBM by 10-20 percent for
now, recognizing issues with large scale model convective
parameterization schemes (and subsequent exaggeration of the areal
coverage and broader qpf amounts) means that we`ll likely see more
concentrated areas/bands of heavy convection, compared to the
model depicted, larger cold season stratiform look to the precip
shield.

Portions of Southern and Central PA...close to the track of the
1006-1008 mb sfc low will likely see some localized heavy 1-2
inch,12-24 hour rainfall amounts by Friday afternoon...with even
the potential for some pinpoint heavier amounts from embedded
TSRA, as a tongue of 2 inch pwat air lifts north and wraps
cyclonically into sern PA and the Lower Susq Valley.

For later Friday through the upcoming weekend, a large area of
high pressure (1020-1024 mb) near the border of southern Quebec
and Ontario will try to push drier air south into the Mid Atlantic
Region.

Another...apparently weaker wave of low pressure at the sfc and
aloft lifts NE out of the Ohio Valley over the weekend and brings
an enhanced chc for showers. Slight diffs in timing/location exist
between the EC/GEFS and NBM guidance. However...the target period
for the best chc of showers and isolated to sctd TSRA appears to
be late Sat into Sunday.

Afterward, heights begin to rise from the southwest, accompanied
by a ridge of surface high pressure building east into PA with
mainly dry conditions and a rebound in temps for early next week.

&&

.AVIATION /13Z THURSDAY THROUGH MONDAY/...
12Z TAFS sent.

Main change to 12Z package was to bring showers in faster. This
mainly for the southern part of central PA, given current radar
and satellite pictures.

Earlier discussion below.

Main change was to back off on fog, given slight breeze in some
spots, and dewpoints not that high.

Isolated showers still across the far south.

Clouds will be on the increase today. Most of the day still
looks dry with VFR conditions.

Showers and storms will overspread the area from south to
north late today into early Friday. MVFR and IFR conditions
will develop overnight. Some improvement on Friday, as weak
low moves east of the area.

OUTLOOK...

FRI...Some restrictions in SHRA/TSRA and patchy fog...mainly
early.
SAT-MON...Wide range of conditions expected with a chance
of showers and storms.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...DeVoir
NEAR TERM...Grumm/DeVoir
SHORT TERM...DeVoir
LONG TERM...Lambert/Gartner
AVIATION...Martin



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