textproduct: State College

WHAT HAS CHANGED

* Upgrade to level 2/4 excessive rain and flash flood risk for south central and southeast PA Thursday afternoon & evening

KEY MESSAGES

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

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 indicates a forward-propagating tstm cluster moving across southern PA/northern MD and the DelMarVa area tomorrow afternoon before tracking offshore the Mid-Atlantic coast by late evening. SPC upgraded to a level 2/5 SLGT risk which clips southern York and Lancaster Counties. The remainder of the Lower Susquehanna Valley is in a level 1/5 MRGL risk with potential for strong to perhaps locally damaging wind gusts dependent/contingent on location of strongest storms. The best CAPE and shear overlap is forecast farther to the south over the DelMarVa. HREF shows 2+ inch pwats available this area to drive very intense rain rates and increased flash flood risk. WPC will upgrade the excessive rainfall outlook to level 2/4 for Thursday in their updated forecast later this afternoon.

AVIATION /00Z THURSDAY THROUGH MONDAY/

SCT-BKN low clouds will linger through this evening, particularly along and east of the Alleghenies where a corridor of moisture at 925 mb (RH values around 75-90%) will persist. This moisture will trend toward eastern Pennsylvania overnight as winds veer aloft over the western 2/3rds of the Commonwealth & allow for drier air to filter in. Areas of mist/fog will likely develop where low-level clouds clear out (KBFD/KJST/KAOO/KUNV), with cloud ceilings expected to develop and lower to MVFR-IFR where the moisture lingers (KIPT/KMDT/KLNS). Furthermore, spotty showers will be possible along and north of the PA/NY border (less than PROB30 for impacts at KBFD/KIPT at this time), with scattered shower/thunderstorm activity along the Mason-Dixon border overnight (PROB30s for -SHRA/SHRA at KMDT & KLNS between 10Z-14Z Thursday).

Gradual improvements to VFR are expected through the morning as overnight mist/fog mixes out & low ceilings over eastern Pennsylvania lift through the early afternoon. A shortwave approaching from the Ohio Valley will spur on more numerous showers and thunderstorm activity during the afternoon, especially over southern/central portions of the state where better instability (MLCAPE around 1000-2000 J/kg) and forcing ahead of the shortwave overlap. Ample moisture (precipitable water values approaching 2") suggests that heavy rain may accompany this convection, with visibility restrictions to MVFR-IFR likely in the stronger showers/storms. Water-laden updrafts and modest low-level lapse rates (6-7 C/km) may also support locally gusty winds with these storms.

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

None.