Flash Flood Watch 12, NWS State College, PA

Area Forecast Discussion
National Weather Service State College PA
803 AM EDT Fri Jul 27 2018

.SYNOPSIS...
An upper low and associated cold front over Lake Erie early this
morning will push southeast across the region today. A broad
area of surface high pressure will build across Pennsylvania
this weekend. Then, a developing area of low pressure over the
Lower Mississippi Valley will track northeast into the Great
Lakes early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Pre-frontal trough across our northern and western mtns attm was
responsible for a relatively weak/broken line of mainly light
showers. Some of the rain could be briefly moderate to heavy in
intensity.

The actual cold front is further to our west across Lake Erie.
PWAT will generally be in the 30-35mm range, but it does ramp
up to 40-45 mm for a few to several-hour period later today and
this evening ahead of the front.

SPC has approx the SE 1/3 of the CWA in a SLGT risk for severe,
while a MRGL risk extends back to the Ncent mtns and Laurel
highlands.

The slow moving front will be the focus for strong to locally
severe storms later today/through mid evening across parts of
central and mainly southeast PA as it continues its slow SE
progress. Cloud cover will likely limit heating/destabilization
over portions of the central mountains where the front will be
located early in the day. However, more sunshine/higher cape
over the southeast counties, combined with a core of strong mid
level winds, will result in an afternoon and evening severe
weather threat over that part of the state.

As the aforementioned Cfront slows later this afternoon (thanks
to a backing upper flow and distinct shortwave feature moving
through), there will be another potential for locally very heavy
rainfall rates and the potential for additional Flash Flooding
over our SCent and SE zones.

High today will be more comfortable and in the 70s across the
northern and western mtns, while the Susquehanna Valley and
adjacent parts of South-Central PA will see near normal temps
in the lower to middle 80s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
A cold front will be moving off to our east early in the period
with lingering showers Friday evening tapering off to just an
isolated shower or two by midnight across the far south and
east. Some clearing will develop behind the Cfront later at
night, especially across the northern and western mtns.
Broad-brushed in some 1-2SM patchy fog throughout the valleys,
however, the fog could become locally dense during the predawn
hours across the north and west.

Min temps early Saturday will vary from the low to mid 50s over
the NW and low to mid 60s in the southeast.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Blended MOS suggests a shower possible over northern areas
during the day Saturday, but the ensembles show near to below
normal PWATs so the threat for widespread activity should remain
low.

The large scale pattern favors a broad upper trough over the
NERN US for Sunday through Monday before the trough retrogrades
into the midwest in favor of building heights along the eastern
seaboard/western Atlantic from Tuesday through the end of the
week.

Sunday and Monday will be mostly dry but the expected weak
troughing forces the mention of at least some scattered showers
or thunderstorms, especially in the afternoon.

Later in the forecast period, we will see a renewal of a southerly
more humid airflow over the eastern US up into the local area.
The GEFS shows PWATs rebounding to above normal once again by
Tuesday and continuing into at least Wednesday evening. At this
point the pattern does not have the look of what we saw recently
with the stagnant deep southerly flow and potential for
repeated torrential downpours. It looks like a more typical
summer pattern with humid days under broad SW flow aloft
favoring mainly diurnal showers and thunderstorms. The
potential for flooding will have to still be monitored, mainly
because of the recent widespread huge amounts of rain that will
still be draining into the region's streams and creeks.

With the expectations of a fair amount of clouds and scattered
showers and thunderstorms from Tuesday onward, daytime highs
will actually average near to slightly below normal.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Hires models show band of intermittent showers lingering through
this morning along KJST-KUNV-KIPT line. Local IFR conditions in
fog/low stratus should improve to VFR by 14Z.

T-storms with brief heavy downpours will re-fire this afternoon
along/ahead of a cold front and pre-frontal trough advancing
eastward across PA. Anticipate most likely t-storm impacts
across the southeast 1/2 of the airspace from 18-24Z. Strong to
locally severe wind gusts are possible. Clearing skies and light
winds tonight should allow some fog/low cigs to redevelop into
Saturday morning.

.Outlook...

Sat...AM fog, otherwise VFR. Isolated shower psbl nrn 1/3.

Sun...AM fog, otherwise VFR. Isolated shower/t-storm srn 1/3.

Mon-Tue...Showers and thunderstorms likely.

&&

.HYDROLOGY...
Falling stages observed at all river points above flood stage
early this morning.

Locally heavy rainfall is possible this afternoon/evening in
areas that received very heavy rain over the past 7 days.
Depending on where the rain falls, small streams could see a
brief/sharp rise into early tonight.

Dry weather over the weekend will allow rivers and streams to
recede. However, additional rainfall next week will bring a
renewed risk of flooding/rising rivers and streams.

&&

.CLIMATE...
*Wettest July on record at Harrisburg and Williamsport

Total monthly precipitation of 11.66 inches at Harrisburg and
11.73 inches at Williamsport ranks as the wettest July on record
at both sites. (Data through 7/26).

The previous wettest July at Harrisburg was 1969 with 9.72
inches. The previous wettest July at Williamsport was 1992 with
9.65 inches.

&&

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

&&

SYNOPSIS...Dangelo/Lambert
NEAR TERM...Dangelo/Lambert
SHORT TERM...DeVoir/Fitzgerald/Lambert
LONG TERM...Fitzgerald/Lambert
AVIATION...Steinbugl
HYDROLOGY...Steinbugl
CLIMATE...Steinbugl

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