textproduct: State College

WHAT HAS CHANGED

* Daytime temperatures have trended lower through Thursday with a cold air damming pattern expected.

KEY MESSAGES

1) Periods of light to moderate beneficial rain will slowly drift south through Wednesday with cloudy/damp/cool conditions.

2) Trending drier later in the week as a coastal storm develops offshore.

DISCUSSION

KEY MESSAGE 1: Periods of light to moderate beneficial rain will slowly drift south through Wednesday with cloudy/damp/cool conditions.

A quasi-stationary area of mid-level frontogenesis that has been the focus for showers overnight along the I-80 corridor will move gradually south today. North of this boundary, dry air will gradually pour into the area as a sprawling high builds into the northeast US. Along and to the south of the aforementioned feature, periods of light to moderate rainfall will continue through much of the day. A compact wave of energy drifting across the Laurel Highlands could generate an isolated rumble of thunder or two. Although a rogue storm could produce some heavier rainfall, rates are expected to remain low enough to preclude flash flooding today. Ponding in low-lying or poor drainage areas cannot be ruled out, but most of the rain will be beneficial.

Temperatures will hold steady if not drop a bit today thanks to persistent easterly flow and a cold air damming pattern. Highs this afternoon will be some -10 to -15F below average for this time of year, generally not making it out of the 50s. Overnight, rain will continue to shift southwest and we could even see rain-free conditions for much of the night. Fog/low clouds are a good bet again tonight, especially along and south of the moisture gradient that will set up across the Commonwealth. By Wednesday morning, dewpoints in the low 40s and perhaps upper 30s will pour into northeast PA and signal the arrival of fall right on time.

On Wednesday, showers and low clouds will linger across southwest PA, while locations farther north will see improving conditions - albeit gradually. Easterly winds will again spell cooler-than-normal temperatures and stubborn clouds. Wednesday will be another cool day with temperatures only a few degrees warmer compared to Tuesday and highs in the upper 50s to low 60s.

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KEY MESSAGE 2: Trending drier later in the week as a coastal storm develops offshore.

Northeast flow Wednesday into Thursday between 1038 mb high in Quebec and a developing coastal low will advect in dewpoints in the 40s and perhaps even the 30s at the sfc Wed-Thu, with even drier air in the 500-850 mb layer. This should bring an end to the rainfall for at least a couple of days. The next feature to monitor will be the coastal storm, which may meander off the New England and Mid Atlantic coast into the early part of next week with this blocking high to its north. Such blocky patterns are notoriously difficult to predict with respect to sfc features and precip potential/placement/timing. Depending on the size of the coastal storm and how much it retrogrades, rain will be possible in at least east central PA between Fri and early next week, but confidence is low. Higher confidence in wind gusts of 15-25 mph and below normal temps through the end of the week.

If the current forecast holds true, eastern PA could be rather cloudy (mainly high clouds) while western PA is sunny at least for the first half of the weekend. The western periphery of the coastal storm's cirrus deck could make for some vibrant sunsets. Temperatures will gradually increase into the weekend, returning to near normal conditions by Saturday and Sunday.

AVIATION /08Z TUESDAY THROUGH SATURDAY/

Periods of rain and showers will continue primarily across the southern 2/3rds of the Commonwealth through Tuesday night, with areas of -RA/RA this morning trending towards -SHRA in the afternoon/evening as high pressure gradually builds in across the Northeastern US.

A period of prevailing MVFR restrictions is likely across all terminals, especially through 18Z Tuesday, as 925 mb RH values approach 100% along a NW-SE gradient. Periods of IFR to briefly LIFR are most favored across the south-central mountains (KJST/KAOO/KUNV) where easterly sfc flow across terrain + steadiest rainfall look to overlap.

The quickest improvements to VFR are most likely at KIPT around 18Z Tuesday as drier air filters in from the northeast, with improvements to VFR occurring elsewhere Tuesday night and perhaps into Wednesday morning (mainly KJST/KAOO, farthest from the moisture gradient, and perhaps KMDT/KLNS as well).

Outlook...

Wed-Thu...Trending VFR across central PA, with slower improvements possible over the Laurels and Lower Susquehanna Valley where -SHRA/BR and low cloud ceilings may linger.

Fri-Sat...VFR favored as a developing coastal low looks to bring subsidence overhead, though -RA and lower cloud ceilings associated with this system may reach our eastern terminals.

CLIMATE

Astronomical fall begins today, September 22, 2026, at 8:04PM EDT. This marks the time of the autumnal equinox when the sun is directly above Earth's equator.

CTP WATCHES/WARNINGS/ADVISORIES

None.