Flood Watch 13, NWS State College, PA

Adams County, PA, Bedford County, PA, Blair County, PA, Cambria County, PA, Centre County, PA, Clearfield County, PA, Clinton County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Lycoming County, PA, Mifflin County, PA, Montour County, PA, Northumberland County, PA, Perry County, PA, Schuylkill County, PA, Snyder County, PA, Somerset County, PA, Sullivan County, PA, Union County, PA, York County, PA

Area Forecast Discussion
National Weather Service State College PA
639 AM EDT Thu Aug 20 2026

.WHAT HAS CHANGED...
* SPC upgraded southeast PA to a Slight Risk, highlighting
  potential for damaging winds and a tornado or two.
* Increased rainfall totals today, including potential for a
  swath of 1-2"+ and an associated risk for isolated flooding.

&&

.KEY MESSAGES...
1) Locally heavy rain and a few strong thunderstorms possible
across south central Pennsylvania today.

2) An unsettled weekend will be bookended by dry and sunny
weather on Friday and early next week.

&&

.DISCUSSION...
KEY MESSAGE 1: Locally heavy rain and a few strong thunderstorms
possible across south central Pennsylvania today

A low pressure system centered in the Ohio Valley this morning
will drift east today, bringing a period of unsettled weather
across the Commonwealth. Aloft, an embedded shortwave trough
within predominant northwesterly flow will provide sufficient
lift and diffluence aloft to generate an extended period of
heavy rain this morning across Central PA. Light showers before
daybreak will be replaced by a more widespread slug of moderate
to locally heavy rain into the early afternoon. CAMs depict the
potential for localized rainfall amounts approaching or
exceeding 2 inches in a swath draped southwest to northeast
along or perhaps just south of the I-80 corridor.

As the center of low pressure moves east along the Mason-Dixon
Line, southerly flow ahead of the system will advect a very
moist airmass (basically a warm front) northward through the
Lower Susquehanna Valley and southeast PA. That moisture,
combined with increasing diurnal heating (storm fuel), and a
favorable shear environment will likely lead to strong to severe
thunderstorm development this afternoon. Ongoing clouds/showers
north of the PA Turnpike should limit the severe threat there
today, but portions of Adams, York, and Lancaster County will
likely receive one or two rounds of strong storms capable of
producing all hazards.

Bulk shear vectors will generally be westerly and perpendicular
to the boundary across southern PA. This should favor a discrete
mode at least initially and then storms are likely to grow
upscale into a bowing segment as they move east into southeast
PA and New Jersey. The Storm Prediction Center brought the
Slight Risk of severe weather northwest into York and Lancaster
this morning along with an upgrade to a 5% risk of tornadoes in
far eastern Lancaster County and a 15% wind area. The
environment is certainly primed for severe weather and residents
in southeast PA should be prepared to take shelter if Severe
Thunderstorm or Tornado Warnings are issued this afternoon.

Rain exits the area by around 00Z with clouds gradually
clearing out later tonight. Widespread rainfall today followed
by ideal radiational cooling conditions should allow for
widespread valley fog to develop into early Friday morning.

------------------------------------------------------------

KEY MESSAGE 2: An unsettled weekend will be bookended by dry and sunny
weather on Friday and early next week.

By Friday, the aforementioned front will drift south but remain
very close to the Mason-Dixon line. As such we can't rule out a
spotty shower/t-storm in far southern PA Friday afternoon.
However, most locations will stay dry and Friday looks like a
fairly pleasant day with plenty of sunshine.

An upper trough carving out over the Great Lakes will bring
increasingly unsettled weather conditions over the weekend.
There is still considerable uncertainty with timing, intensity,
and coverage of storms, but it is not expected to be a washout
for anyone. This system will also deliver some relatively
cooler air both at the surface and aloft. This setup could prove
favorable for clusters of gusty t-storms with small hail both
Saturday and Sunday. A cold front will move across the region
Saturday and Sunday, helping focus severe weather potential. The
best chance for strong storms will be west of I-99 on Saturday
and then across eastern PA on Sunday.

In the wake of the cold front, the start of next week looks
quite nice. A decaying secondary cold front could bring some
light showers north of I-80 on Monday, but dry conditions
prevail otherwise. Mostly sunny skies, slightly below normal
temperatures, and lower humidity will provide some very pleasant
late August weather. Upper troughing will continue to support
near to below average temperatures and above average
precipitation into early September.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
A swath of steady rainfall will overspread central PA this
morning, with KUNV/KIPT looking to be within the core of this
swath (meaning higher confidence in MVFR-IFR visibility
restrictions), with KBFD/KJST/KAOO more-so on the periphery
(more intermittent MVFR-IFR restrictions, especially where rain
is more showery). Rain at KMDT/KLNS looks to be more
showery/convective in nature, with a few strong to severe
thunderstorms possible in the afternoon. Rumbles of thunder may
occur in the steadier, more stratiform swath of rain where
MUCAPE may rise to 100-250 J/kg (primarily KUNV/KJST/KAOO).

Winds will tend to be variable near the system's track over
southern PA, with winds generally N/NE north of the system &
S/SE to its south. Expect some fog and stratus to develop
tonight in the wake of the showers and storms, though confidence
in widespread IFR remains low.

Outlook...

Fri...AM fog, otherwise trending VFR.

Sat-Sun...Scattered to numerous showers/t-storms likely. Breezy.

Mon...VFR returns for early next week.

&&

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

&&

WHAT HAS CHANGED...Banghoff
KEY MESSAGES...Steinbugl/Banghoff
DISCUSSION...Steinbugl/Banghoff
AVIATION...Bauco/Teare

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