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 /12Z MONDAY THROUGH FRIDAY/

Some periods of MVFR will occur through the mid morning hours today as several areas of light showers (falling from a mid level cloud deck) moves SE through the CWA. Afterward, we'll be left with varying amounts and base heights of mainly mid-level (altocu) clouds (somewhat similar to Sunday).

The bulk of the late morning through this afternoon and evening will be dry as a relatively flat ridge of high pressure aloft and the thermally indirect, descending branch associated with the right exit region of a weak jetlet moves ESE across the region.

A few stray showers or even an isolated low-topped TSRA or two with brief MVFR cant be ruled out this afternoon/evening as a result of broad diffluent flow aloft ahead of the aforementioned ridge and pockets of MU CAPE in the 1000-1500 j/kg range.

For late tonight/early Tuesday, a somewhat rare increase in late night CAPE will occur across NW PA that could support TSRA approaching KBFD after the end of this 06Z TAF window. A trend toward MVFR will likely develop tonight in the increased Westerly 925-850 mb flow.

Winds will generally be less than 5 knots throughout this TAF cycle.

Outlook...

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

CTP WATCHES/WARNINGS/ADVISORIES

None.