textproduct: State College

WHAT HAS CHANGED

* Risk of damaging winds and tornado potential increased over the northern tier

KEY MESSAGES

1) Late summer surge of heat and humidity continues severe thunderstorm and excessive rainfall threat through Thursday

DISCUSSION

KEY MESSAGE 1: Late summer surge of heat and humidity continues severe thunderstorm and excessive rainfall threat through Thursday

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 through Thursday.

After a round of severe storms this morning, robust destabilization is occurring across western and central PA with plenty of heating and very rich dewpoints in the 70-75F range. More thunderstorms are expected this afternoon through the evening in an convective environment highly supportive of organized storms capable of all severe weather hazards. There is also the potential for heavy rain and flash flooding especially were storms repeat over the same area.

Increased southwest to westerly flow on Thursday will push the will continue to pump in the late summer heat and very high dewpoints. Heat advisory remains in effect from 11a-8p Thursday across a portion of south central PA where max heat index values are projected to reach/exceed 100F.

The severe risk will be focused more across the southwest quadrant of central PA Thursday, and south of the PA-MD border on Friday. An upper trough passage favors one final round of showers and storms on Friday, but fortunately the Labor Day weekend is trending dry. The upper ridge is forecast to retrograde by the end of the week which allows for a return to more seasonal temperatures and lower humidity this weekend into early next week.

AVIATION /19Z 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.