textproduct: State College

WHAT HAS CHANGED

* Adjusted POP/Wx to better align with Day 2-4 SPC SLGT risk

KEY MESSAGES

1) Late summer surge of heat and humidity brings multiday severe thunderstorm and excessive rainfall threat to kickoff September

DISCUSSION

KEY MESSAGE 1:

A series of mid level impulses tracking eastward along the northern periphery of an expansive 500mb ridge will interact with anomalous moisture/instability and strong WNW flow aloft to promote a multiday severe storm and excessive rain risk across portions of CPA Tuesday-Thursday. A surge of late summer heat (max temps 80-90F range) with dewpoints in upper 60s to low 70s will result in a very moist boundary layer with impressive MLCAPE values in the neighborhood of 2-3k J/kg given mid level lapse rates around 7 C/km. Forecast soundings also show robust deep layer/effective shear values and appreciably curved, sizable low level hodographs. As such, a couple of supercells will be possible. Severe wind and hail are the main threats, though a tornado or two may also occur. On balance, the environmental parameters are quite bullish for severe storm potential. However, timing (time of day), speed, and location of each mid level impulse will be key factors that could modulate the bullish severe setup to some extent. Increasing MUCAPE values late tonight through Tuesday morning suggest storms/severe risk likely won't be tied to the typical diurnal cycle. The severe risk will shift to the south of CPA by Friday, but an upper trough passage suggests some lingering shower activity into the weekend.

The late summer heat is forecast to peak Tuesday and Wednesday with very warm nights +10-20F above the historical average and max heat indices approaching advisory levels/century mark across parts of south central PA. The upper ridge is forecast to retrograde by the end of the week which allows for increased troughing and a return to more seasonal temperatures for early September.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Isolated showers and thunderstorms continue to plague the airspace beginning at the 00Z TAF issuance. Evening convection is expected to continue, but confidence in exact placement and timing of these showers remains low. Coverage of convection was initially expected to be greater at this time, and a lack of storms would suggest weak forcing at the surface. A few mentions additional overnight convection have been made, but near term amendments may be needed for additional overnight storms. Additionally, the initial probabilities for fog and low stratus have come down as well. The most likely places to see fog development tonight would be IPT/MDT/LNS, but confidence remains low (30-40%) due to lack of prior rain near the surface. BFD could see brief periods of MVFR low ceilings tonight, but IFR seems unlikely.

Any fog or stratus that develops overnight will quickly mix out in the early morning hours on Tuesday after sunrise. Attention will then turn towards afternoon convection. Scattered showers and thunderstorms are likely for the afternoon on Tuesday, but there remains uncertainty as to the exact timing and placement. Some guidance would suggest storms in the northwest as early as 15Z and others suggest convection may not arrive until 23Z. Overall, expect periods of active convection across all terminals in central PA on Tuesday bringing reductions to visibilities and ceilings throughout the afternoon.

Outlook...

Tue-Fri...Scattered SHRA/TSRA with brief restrictions possible; morning fog possible.

Sat...SHRA/TSRA trending less likely as cold front moves southward across the Commonwealth; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.