textproduct: State College

WHAT HAS CHANGED

*Flood Watch issued for this afternoon for the Lower Susquehanna Valley.

KEY MESSAGES

1) Torrential downpours and strong to locally damaging thunderstorm wind gusts may impact portions of south central PA this afternoon and evening.

2) Unsettled weather continues into the first half of the weekend.

DISCUSSION

KEY MESSAGE 1: Torrential downpours and strong to locally damaging thunderstorm wind gusts may impact portions of south central PA Thursday afternoon and evening.

Most CAM guidance shows numerous thunderstorms developing this afternoon over south-central PA as a shortwave tracks toward the region from the Ohio Valley. These storms will be in an environment that is highly supportive of very heavy rainfall with PWATs in excess of 2 inches and tall, skinny CAPE profiles. One hour flash flood guidance is as low as 1 to 1.5 inches in some locations, so flash flooding will be possible in areas that see repeated rounds of slow moving thunderstorms. There is growing confidence that southeastern portions of the forecast area will have the best chance at seeing flash flooding, so we have issued a Flood Watch for the Lower Susquehanna Valley that will be in effect from noon through midnight.

While flash flooding will be the primary hazard with thunderstorms this afternoon, there is still some potential for storms to produce a few strong to severe wind gusts. The best deep layer shear is expected to remain to our south, but there could be just enough to support some organization of storms across southern PA. SPC has maintained a slight risk that just barely clips southern York and Lancaster Counties and they have expanded the marginal risk to cover much of south-central Pennsylvania.

KEY MESSAGE 2: Unsettled weather continues into the first half of the weekend.

Additional showers and thunderstorms are expected on Friday ahead of a cold front that will slowly cross the region into Saturday. Isolated flash flooding will be the primary concern as the environment will still be supportive of very heavy rainfall. A few showers remain possible on Saturday across the southern half of the area as the cold front will be very slow to move to our south. Sunday looks like a much drier day as the front finally moves off to the south and a strong ridge develops over the middle of the country.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Seeing improvement in visibilities in BFD and JST this morning with UNV and AOO likely to improve visibilities in the coming hours. Some showers will briefly impact/move nearby JST and AOO this morning ahead of the more widespread convection later today. Guidance depicts lower ceilings at MDT and LNS this morning in the MVFR category lasting possibly to noon.

Restrictions are likely to return in the form of numerous showers and thunderstorms as a shortwave trough moves in from the Ohio River Valley. Greatest coverage of storms is likely in the southern tier of the state, particularly at MDT and LNS in the afternoon. With tall CAPE profiles, minimal capping, and PWATs approaching 2", repeating, scattered, heavy showers and thunderstorms are possible, leading to visibility restrictions as they pass. Isolated instances of gusty winds may also accompany these storms.

In the wake of this convective activity, the moist low levels may lead to mist/fog developing over Southeast PA after 06Z, will watch for visibility restrictions possible. After 06Z, a more broken deck of clouds will begin to move into the western TAF sites which may lead to some MVFR restrictions.

Outlook...

Fri...T-storm impacts likely during the afternoon. AM low clouds and fog possible.

Sat...Reduced chance and more limited coverage of PM showers/t-storm.

Sun-Mon...Mainly VFR/no sig wx.

CTP WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory until 9 AM EDT this morning for PAZ004>006- 010>012-017-018. Flood Watch from noon EDT today through this evening for PAZ056-057-059-063>066.