textproduct: State College

WHAT HAS CHANGED

* Added thunder mentions throughout much of the aviation forecast period * Dropped SVR watch as storms moved off to the south. * Western zones will likely destabilize again later today and this evening.

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

A series of mid level impulses tracking eastward along the northern periphery of an expansive 500mb ridge will continue to interact with anomalous moisture/instability and strong WNW flow aloft to promote severe storm and excessive rain risk today and tomorrow.

The first cluster of storms is already poised to move off of Lake Erie into our NW zones between 7 and 8 AM. This cluster has near continuous lightning and was severe warned along the lake shore by the Cleveland WFO. Inland from the lake shore, the sfc boundary layer is still rather stable this morning with near 150 J/kg of SBCIN, so this will at least somewhat offset the severe wind potential across our NW zones. However the wind threat could increase to the south and east IF the cluster is maintained until mid-morning, by which time morning sunshine will have eroded the sfc stable layer. Climatologically, however, these morning MCSs rarely survive the nocturnal to diurnal transition, and several CAMs do show it falling apart before the boundary layer destabilizes.

The mesoscale severe weather setup will be complex this afternoon into this evening, with key players being the presence of a north-south stationary front / backdoor cold front across the eastern zones, and whatever remains of the cold pool from the morning MCS. These boundaries will likely be the focus for new convection initiation this afternoon. Forecast vertical soundings depict large, clockwise turning hodographs associated with a strongly veered wind profile, with 0-3 km SRH on the order of 200-350 m2/s2 and 0-3km wind shear approaching 40 kts. Combined with MLCAPE on the order of 1500-3000 J/kg, this setup will likely yield both supercells and bowing line segments. If supercells or southeast-bulging bow echos remain on the unstable side of the stationary boundary and ingest this highly sheared inflow, a couple stronger tornadoes and large hail storms can not be ruled out this afternoon into this evening. There will be a large CAPE gradient across the stationary front, with limited severe potential to its east.

Increased southwest to westerly flow on Thursday will push the backdoor cold front out of the region, ushering in higher temperatures and dewpoints. The Heat Index will make another run at 100-104F across the southern zones, similar to what we saw on Tuesday. We issued a Heat Advisory valid 11a-8p Thu for this likely threat.

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 additional showers and storms on Friday, but fortunately the Labor Day weekend is trending drier. The upper ridge is forecast to retrograde by the end of the week which allows for a return to more seasonal temperatures.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Some of the least certain terminal forecasts I have ever made, even during lake effect snows. The possibility, but not certainty, of convection exists throughout the entire TAF period for almost all sites. Pointing out a few more-likely times are the best I could do at this point. After a patch of SHRA/TSRA affects IPT this aftn, it could slide S into MDT/LNS, but it is not more than a 40% chc of getting to those two sites. Most guidance then develops convection over western PA and sliding into the airspace over Central PA before 22Z. That possible area of SHRA/TSRA could (50%) last into the late evening while inching eastward. A significant push of forcing moves in from NY toward 03Z. The batch of convection it will fire up has been fairly consistent with previous guidance and would drop into a pool of very low stability and high moisture. These SHRA/TSRA would certainly drop into BFD and then IPT-UNV. They could survive into the southern half of the airspace for the post- sunrise hours. I would have put more/lower visby fog into the terminals if I thought it would clear out after the sun sets. Still mentioned a period of fog for the western terminals later tonight.

Outlook...

Thu...Scattered-Numerous SHRA/TSRA with brief restrictions possible; morning fog.

Fri...Scattered SHRA/TSRA SW. VFR elsewhere after morning fog.

Sat-Mon...VFR favored; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM to 8 PM EDT Thursday for PAZ026>028- 035-036-056-057-059-063>066.