textproduct: State College

WHAT HAS CHANGED

* 30% severe wind probability added to NW PA for Wednesday

KEY MESSAGES

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

DISCUSSION

KEY MESSAGE 1:

Moderate to strong instability (2-3000 J/kg MLCAPE) analyzed over most of CPA this afternoon with very rich Td's in the 70-75F range and 30-40kt of deep layer shear increasing from SW to NE. So in other words, a convective environment that is highly favorable to support robust storm organization. That said, nebulous large scale forcing continues to foster high uncertainty regarding overall convective development and evolution this afternoon/evening into tonight. Recent high- resolution guidance may be converging on isolated to widely scattered thunderstorms developing near the I80 corridor and arching to the southeast into the Lower Susq/Delaware Valley.

Perhaps after late night lull, CAMs are suggesting a cluster of storms progressing southeast from Lake Erie Wednesday morning. This could delay onset of additional storms expected to develop later Wednesday afternoon into the evening. The environment will again be supportive of organized storm modes including supercells and bowing segments with elevated tornado potential within corridor of large SRH.

Previous Discussion:

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 multi-day severe storm and excessive rain risk across portions of CPA today through Thursday.

A surge of late summer heat (max temps 80-90F range) with dewpoints in upper 60s to mid 70s will result in a very moist boundary layer with impressive MLCAPE values in the neighborhood of 1500-3000 J/kg given mid level lapse rates around 7 C/km. Forecast soundings also show robust deep layer/effective shear values and appreciably curved low level hodographs. As such, a couple of supercells will be possible.

From a large scale lift perspective, rising motion may be at least partially suppressed by the right exit region of an upper level jet max through much of today until at least mid afternoon. Convection initiation is more likely in the 4pm-9pm time frame as the boundary layer RH increases and the right entrance region of the jet max approaches.

Increasing MUCAPE values late tonight through Wed morning suggest storms/severe risk likely won't be tied to the typical diurnal cycle. Any preexisting MCS upstream could be maintained by the unstable atmosphere with us Wednesday morning. Hires model guidance also shows an easterly flow across our eastern zones associated with a back door cold front. Along that boundary, models depict very large hodographs associated with a strongly veered wind profile, with 0-3 km SRH on the order of 350 m2/s2 and 0-3km wind shear approaching 40 kts. If supercells can ride southeastward on the unstable side of the boundary and ingest this highly sheared inflow, a couple stronger tornadoes can not be ruled out Wed afternoon into Wed evening.

The severe risk will be focused more across the southern tier of central PA by Thursday, and south of the PA-MD border on Friday. An upper trough passage suggests some lingering shower activity into the weekend, otherwise it looks mostly dry.

The late summer heat is forecast to peak today and Wednesday with very warm nights +10-20F above the historical average and max heat index values just short of Heat Advisory criteria (100F). The exception will be across the eastern zones Wed when the back door cold front could hold max Ts down a few to several degrees. 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.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Widespread convection is continuing to impact airfields across central PA. These thunderstorms have a history of producing frequent lightning, gusts of 40kts or greater, and heavy downpours. These storms will continue to pose a threat to the area through 06Z tonight. The environment is still promotive of strong updrafts and heavy rain. Expect IFR countdowns if a storm moves directly over an airfield.

Evening convection will wane overnight, giving way to the potential for overnight fog and low stratus across the region as winds diminish and mainly mid/high level clouds remain. Farther east (KMDT/KLNS), a backdoor cold front moving in overnight will result in winds trending easterly & ceilings lowering to MVFR-IFR. Areas of morning fog and low stratus will gradually mix out to VFR through Wednesday morning and the early afternoon hours, with slower improvements expected at KMDT/KLNS in the vicinity of the front. Otherwise, PROB30 groups for -TSRA were added to most terminals (exception being KIPT at this time) given a signal on CAMs for a potential line or cluster of showers/storms which may move E/SEward across the CWA, though timing/certainty remains low given its dependency on upstream evolution.

Outlook...

Wed-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.

Sun...VFR favored; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.