textproduct: State College

WHAT HAS CHANGED

* Risk for severe storms over the southern half of Pennsylvania has increased with a Severe Thunderstorm watch in effect until 00Z. * Reiterating bearish trend for max temps Friday and Saturday

KEY MESSAGES

1) Scattered storms with isolated severe wind gusts and hail will be possible over the southern tier of CPA this afternoon/evening

2) Major cooldown to close out the week and start Memorial Day weekend with widespread soaking rainfall over 1 inch Friday and Saturday

DISCUSSION

KEY MESSAGE 1: Scattered storms with isolated severe wind gusts and hail will be possible over the southern tier of CPA this afternoon/evening

The surface cold front currently over the northern tier of Pennsylvania will advance south and east across CPA today before stalling out to the south of the MD line tonight into Thursday. Convective initiation is already being observed over southern PA along the MD/PA boarder ahead of the front. Moist and unstable air ahead of the cold front combined with increasing low level convergence will lead to additional thunderstorm development as we move through the afternoon across the southeast 1/2 to 1/3 of the forecast area. The atmosphere is continuing to destablize with CAPE between 1500 and 2000 j/kg over the lower Susquehanna Valley and Seven Mountains. Steep low level laps rates along with sufficient 0-6km shear and previously mentioned instability should support a few stronger storms with potential for severe wind gusts and isolated hail. SPC has upgraded portions of our CWA to a D1 Slight risk over the south, and a Severe Thunderstorm Watch is now in effect until 00Z tonight. HREF/REFS also indicate potential for locally heavy rainfall >1" near the MD line as showers and storms likely persist through the evening into the overnight period across the southern tier of CPA.

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KEY MESSAGE 2: Major cooldown to close out the week and start Memorial Day weekend with widespread soaking rainfall over 1 inch Friday and Saturday

Relief from the early season heat wave arrives in earnest on Thursday with max temps 20-30 degrees lower vs. Wednesday across the LSV. Strong high pressure slides from the Lower Great Lakes to off the New England coast to close out the week. Model data shows the high channeling southward down the east-side of the Appalachians, setting up a cold air damming (CAD) event heading into Memorial Day weekend.

Increasing CAD signal strength enhanced by moderately strong east/southeast low level flow and soaking rainfall has prompted a considerable bearish/lower adjustment to max temps Friday and Saturday. In fact, highs on Saturday are near daily record lows at several locations (mini/max). Guidance may be too optimistic in rebounding temps Sunday into Monday, but opted to keep baseline NBM for now. That said, the trend will be to moderate warmer with steadier rain giving way to more showery conditions for the unofficial start of summer.

NBM/WPC QPF continues to show widespread 48hr rainfall totals in the 1-2 inch range Friday-Saturday. The cool/stable air will cap rates with longer duration helping to mitigate flooding risk in most areas. The forecast rainfall is much needed over the far south central/southeastern counties who remain under D1-D2 moderate to severe drought conditions.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Widespread showers and thunderstorms are ongoing across Central PA. Bteween now and around 00Z Thursday is the best chance for convective restrictions to impact southern sites. These include heavy rain, brief convective wind gusts, lightning, and visibility drops. The immediate convective impacts are at AOO with MDT and LNS experiencing thunder likely around and after 20Z. MVFR restrictions due to convection are likely with brief IFR possible as well.

Trailing rain showers are expected during the overnight with MVFR ceilings possible at JST and AOO, will assess possibility of IFR in subsequent TAF packages. These showers will also impact MDT and LNS but VFR, borderline MVFR, ceilings expected. These showers may impact UNV and IPT but confidence is lower at these sites with mostly VFR expected to prevail through the TAF period.

Southeastern sites Thursday morning into the early afternoon are likely to see deteriorating conditions due to lowing cloud decks and possible rain showers. MVFR is expected in the morning with IFR not out of the question in the later morning.

Outlook...

Fri...Restrictions likely with rain moving in from south to north, lasting into Fri night.

Sat-Sun-Mon...Restrictions likely with rain.

CTP WATCHES/WARNINGS/ADVISORIES

None.