Area Forecast Discussion
Issued by NWS State College, PA

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140
FXUS61 KCTP 281419
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1019 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/...

The lead showers that formed over Cambria and Somerset county have
died off. Radar and MRMS radar show more enhanced activity to our
west over Ohio. This is the MCV with the overnight MCS which
brought heavy rainfall in portions of Ohio. This system will bring
us more clouds and temper the high temperatures.

Best chance showers and thunderstorms today will be over
southwestern areas. Used the HRRRV1 to add texture to the shower
potential in southwestern PA.

The convection over Ohio likely has modified the boundary layer
so have little faith in the CAMS initialized on or prior to 0600
UTC today. The HRRR is the only rapidly updated CAM which can
handle the modifications due to the, hourly updates.

That said HRRRV1 was too hot today, except in SW PA where it has
rain cooled air.

Bottom line, northern PA should be under cirrus clouds and have a
relatively nice day. Central areas should not see widespread
rainfall until the afternoon or evening. But high and mid clouds
will limit sunshine and maximum temperatures. Similar issue in the
southeast. More enduring rain will come in the evening.

Southwest areas will have thicker clouds and scattered light rain
with the potential, increasing in afternoon, for steadier rainfall
and thunderstorms. By 6 PM should be widespread rain over the
this area region.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...

It should rain overnight across most of central and southern
Pennsylvania. The hydrostatic models seem to favor a heavy rain
stripe from southwestern PA into southeastern NY. The GEFS QPF
fields in this region are in the 98th to 100th percentile. The
GEFS is cranking out all the rain it can and thinks it will be a
near record to record rainfall in the model atmosphere.

Experience says if the GEFS has record rain then the real
atmosphere does too. But not in the same spot. So we are confident
in a big rainfall event.

The Convective allowing models (CAMS) seem to favor the heavier
rainfall farther to the south and east. The 3km NCAR models keeps
probabilities of over 3 inches mainly along and south of the
Mason-Dixon line. But, the NCAR ensemble was initialized before
the MCS in Ohio formed and change the atmosphere.

The HREFX is more to the north with the heavy rainfall, but south
of the axis in the GEFS and SREF.

The HRRRV1 and HRRRV3 (see GSD site) seem to favor south-central
PA and portions of Maryland for heavy rainfall. And the HRRRV3-TLE
shows areas in southern/southeaster PA with best chance of over 3
inches of rain. And it and each HRRRV3 member show 1-2 inch and
hour rates during the height of the event.

For the pattern folks, the GEFS develops easterly winds which get
to -3 to -4 sigma on the cold side of the boundary. This and high
PW explains the extreme rainfall from central PA into SE NE as
this feature intensifies. The GEFS also has a good 3 sigma
westerly jet in the warm air. This feature and the high PW air are
probably causing the CAMS to produce training and one can see the
radar forecasts show SW-NE convective bands implying training.

So, we are confident in a good 1-3 inch rainfall even. Mainly
overnight. We are not confident as to exactly where. Debating
which type of watch to issue. Leaning toward Flash flood in
south/south east based on rates in HRRR/HRRR-TLE. Some places will
see over 3 inches as the probabilities in the CAMS imply low
probabilities of 4 and 6 inches.

The intense rain should move out relatively early Friday. A cooler
day due to wet start in south and central areas and lingering
clouds. Most areas will probably see more rain than they have seen
all month to date. Should be nice enough to sneak in some outdoor
activates Friday evening. I hope to go for a 30 mile bike ride.

This will end our enduring heat wave.

&&

.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 /14Z THURSDAY THROUGH MONDAY/...
12Z TAFS sent.

Minor updates for showers and rain coming into southwestern areas.

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
SHORT TERM...Grumm
LONG TERM...Lambert/Gartner
AVIATION...Grumm



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