textproduct: State College

WHAT HAS CHANGED

* Issued a Flash Flood Watch for all of our CWA through Midnight this Thursday night.

KEY MESSAGES

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

DISCUSSION

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

Main change this forecast cycle was to issue a Flash Flood Watch for the entire CWA based on the current wide distribution of moderate to heavy rain and embedded/training TSRA along with a HREF/NAM/MRMS blend of short range model guidance and remote sensing.

A seemingly endless train of mid/upper level impulses riding ESE along the southern edge of a persistent jet and within a region of broad upper level diffluence and anomalously high PWATs of nearly 2 inches will continue to interact with numerous, converging convective outflow boundaries to produce several more bouts of heavy showers/TSRA today through at least early tonight.

Basin "Average" rainfall across the CWA will likely range between 0.75 to 1.25 inches but as we saw Wed/Wednesday night, training/and back-building TSRA could easily dump over 3 inches of rain in just an hour and lead to significant flash flooding.

The target areas for the heaviest rainfall today and this evening should be the Southwest half of the CWA as a well- defined outflow boundary has pushed into southern PA early today. We do expect the airmass to recover quickly late this morning/early this afternoon as this first MCS moves SE. However, a glance just to the left shows another developing MCS close on its heels over West-Central Lake Erie. This feature is progged by high res near term model guidance to track SE twd the laurel and diminish by 14-15Z. By 19-20Z today, more TSRA fire across Northern PA and a second larger area follows for this evening over a similar part of the state that experience the heavy rain over the past 24 hours.

Additionally, increased southwest to westerly flow today will continue to pump in the late summer heat and very high dewpoints. A 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.

As mentioned above, SPC's severe risk are will encompass primarily the SW half of the state where ML CAPE will be the highest, but the MRGL risk area will cover practically the entire rest of Eastern PA.

An upper level 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 /12Z THURSDAY THROUGH MONDAY/

Highly variable flying conditions expected early today and again late in the day and tonight as a few more distinct, mid and upper level disturbances fuel the development of showers and thunderstorms.

The SE half of the CWA will see a 2-3 hour period of improvement through the mid/late morning hours before another fast-moving area of new TSRA impacts much of the southern half of PA through 16Z with conditions dipping into the MVFR range.

Mainly VFR will return for much of the 16Z to 20Z period before the heat and humidity and interactions of outflow boundaries helps to fire up new convection that expands and continue through early tonight with periods of poor flying conditions and brief, dangerous LLWS caused by convective updrafts and wet microbursts.

The exact timing of when exactly any of this scattered activity could impact a site remains a bit uncertain.

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

Flood Watch through this evening for PAZ004>006-010>012- 017>019-024>028-033>037-041-042-045-046-049>053-056>059- 063>066. Heat Advisory from 11 AM this morning to 8 PM EDT this evening for PAZ026>028-035-036-056-057-059-063>066.