textproduct: State College

WHAT HAS CHANGED

*Flood Watch expanded across the southern tier of central PA

KEY MESSAGES

1) Locally significant flash flooding possible across portions of south central PA this afternoon and evening

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

DISCUSSION

KEY MESSAGE 1: Locally significant flash flooding possible across portions of south central PA this afternoon and evening

Showers and thunderstorms are increasing in coverage this afternoon across southern PA. Rainfall rates of 2-3"/hr are likely at times, leading to 2-4" of rain with locally higher amounts possible.

Synoptic ascent associated with shortwave lifting E/NE out of KY is working in tandem with RRQ of a modest jet streak aloft robust thermodynamics reflected by PWs measured via the 12Z soundings that are near previous daily records (2.17" at IAD) and a ribbon of MLCAPE already reaching 1000+ J/kg (2000 J/kg within clearing sky conditions). MRMS hourly rainfall have been above 1 inch in some areas, indicative of the impressive heavy rain environment in place. Earlier this morning, rainfall rates of ~1.5"/hr where achieved via ground truth report from broadcast media partner - so rates >2" should be easily attainable given the time of day and copious amount of moisture available. WPC excessive rainfall outlook continues to favor a "high-end" level 2/4 flash flood risk over the southern tier of central PA where locally significant/urban flash flooding is possible.

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 /00Z FRIDAY THROUGH TUESDAY/

Convection will downtrend this evening as trough axis over central Pennsylvania progresses eastward & instability wanes with the loss of sunlight. A decaying line of showers and embedded thunderstorms may bring brief wind gusts (20 kts) to KUNV/KIPT around 01Z as its outflow boundary moves through, with lower confidence in occurrence of lighting or visibility restrictions given the weakening trend. Otherwise, VFR ceilings are expected for tonight, with areas of mist/fog possible due to recent rainfall and ample moisture in place. Higher confidence in overnight restrictions from mist/fog are over the Allegheny Plateau (KBFD) and portions of the Poconos & Lower Susquehanna Valley (more so KLNS than KMDT), with patchier fog across the central mountain terminals (KJST/KAOO/KUNV/KIPT). A few showers may develop within this moist environment tonight, though sparse coverage (<30%) precludes TAF inclusions at this time.

VFR is favored on Friday, though another shortwave moving across the Mid-Atlantic may bring intermittent visibility restrictions due to showers/thunderstorms. A weaker wave of showers and a few thunderstorms is expected to cross our southern/central (KJST/KAOO/KUNV/KMDT/KLNS) terminals during the morning & early afternoon, with lower confidence in lightning & restrictions from this activity. Higher confidence in scattered thunderstorms, visibility restrictions, and locally gusty winds will be from the activity that develops during the afternoon around KBFD, which will progress south and east across the Commonwealth into the evening. Convective activity will once again trend downward around sunset, with lower certainty with regards to impacts for KMDT/KLNS downstream of the initial storms.

Outlook...

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

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

CTP WATCHES/WARNINGS/ADVISORIES

None.