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WHAT HAS CHANGED

Flood Watch issued for Schuylkill, Lebanon, and Lancaster Counties

KEY MESSAGES

1) Scattered strong to severe thunderstorms possible this afternoon into tonight

2) Flooding downpours possible this afternoon into tonight in the Lower Susquehanna Valley

DISCUSSION

Noticeably warmer (+5-10F vs. last night) and more humid overnight/early morning across CPA with a couple of showers and storms over the northwest and southeast portions of the forecast area. Cluster of showers/storms across the NW Alleghenies are being driven by a lead shortwave trough crossing Lake Erie, while the more spotty activity over the mid to lower Susq Valley is fueled more by low level WAA and SWly LLJ. Some valley fog is evident on IR satellite in the interior central valleys, but coverage is significantly less than Wednesday morning.

KEY MESSAGE 1: Scattered strong to severe thunderstorms possible this afternoon into tonight

Scattered thunderstorm development is expected Thursday afternoon ahead of a cold front across CPA. SPC has continued to expand the level 2/5 severe t-storm risk which now reaches farther west to the eastern slopes of the Allegheny Mtns.

Moderate destabilization is fcst into the afternoon along/ahead of a weak surface trough/front extending from central NY southward through central PA. Scattered thunderstorm development is expected by mid to late afternoon, with sufficient deep layer flow/shear for some storm organization. The strongest cells or clusters will be capable of producing scattered damaging winds and at least isolated hail. Modestly curved low-level hodographs may also support some brief-tornado potential. The severe threat should persist into the evening before storms weaken/exit the eastern part of the CWA.

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KEY MESSAGE 2: Flooding downpours possible this afternoon into tonight across portions of the Lower Susquehanna Valley

A healthy influx of moisture via southwest flow will increase PWATs to ~1.5" across the Lower Susquehanna Valley by this afternoon. CAMs indicate potential for 2-4" spot rain amounts (via HREF PMM) within zone of low level convergence along weak pre frontal/lee sfc trough. A couple rounds of heavy t-storm downpours are possible with rain rates 1-2"/hr that could result in scattered areas of flash flooding. Based on the +RA signal in the hires ensembles and coordination with WFO PHI, we issued a [flash] flood watch for Schuylkill, Lebanon, and Lancaster Counties.

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Variable sky conditions for the lunar eclipse tonight: Forecast viewing conditions for the lunar eclipse range from fair to poor Thursday night across central PA. The eclipse will begin at 10:34 pm EDT and last for about 3 hours, ending at 1:52 am EDT Friday. The worst view is most likely over the eastern half of CPA due to exiting storms and associated cloud cover. The eclipse will take place during August's full moon, commonly referred to as the Sturgeon Moon. It will also be the last full moon of the summer with the next full moon set to rise a few nights after the autumnal (Fall) equinox on September 22nd.

A drying trend is forecast Friday into the last weekend of August, thanks to high pressure building into the area behind the cold front. This will likely setup a couple of foggy mornings (Friday and Saturday). Rainfall looks fairly limited heading into the first week of September; however despite the start of meteorological Fall the summer is not over with temps expected to rise above the historical average into the 80-90 degree range.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Morning showers from a lead shortwave and warm air advection hanging around BFD and LNS respectively this morning but are likely to move out by mid morning. VFR is favored region wide during the morning with some MVFR ceilings possible (30-50% chance) at BFD late morning as low level clouds begin to build in.

Kept all sites with thunder Probs this afternoon, confidence is highest at MDT and LNS with higher storm coverage expected in eastern PA. Brief drops to IFR visibilities in these downpours along with gusty winds, possibly in excess of 40kts, may accompany these storms.

With high pressure building in the wake of the storms this afternoon, tonight into tomorrow morning looks to see fog and low ceilings building in, possibly into the LIFR category. Region-wide restrictions into Friday morning are possible.

Outlook...

Fri...Fog and a few lingering showers in the morning, becoming VFR for the afternoon.

Sat-Mon...AM Fog; otherwise VFR.

CTP WATCHES/WARNINGS/ADVISORIES

Flood Watch from noon EDT today through late tonight for PAZ058-059-066.