textproduct: State College

WHAT HAS CHANGED

* SPC has added a slight risk (level 2 of 5) of severe weather for Wednesday evening across northwest PA.

KEY MESSAGES

1) Uptick in heat and humidity for midweek.

2) Showers, storms, and gusty winds Wednesday night into Thursday.

DISCUSSION

KEY MESSAGE 1: Uptick in heat and humidity for midweek.

Increasingly humid air will work its way back into the region for Wednesday as southerly return flow develops along the western periphery of high pressure. High temperatures will be around 2 to 4 degrees warmer than they were today, generally ranging from the low to mid 80s. Heat index values may touch the 90F mark in the south central mountains on Wednesday as dewpoints climb back into the mid to upper 60s. Rain-free conditions will prevail for much of the day, though we should start to see thunderstorms develop over northwest PA by the evening.

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KEY MESSAGE 2: Showers, storms, and gusty winds Wednesday night into Thursday.

By Wednesday evening, a low pressure system will be moving across northern Ontario with a cold front extending south through the Great Lakes and Ohio Valley region. The pressure gradient will support an uptick in synoptic scale/gradient winds Wednesday afternoon into Thursday. Expect gusts into the 20s on the hilltops and also in the SE. A gust near 30MPH is possible on the tops of the Laurels Wed night just before the front.

The cold front will spark numerous showers and thunderstorms, with the majority occurring before the front passes later Wednesday night in the north and Thursday morning in the south. There is some potential for severe weather across northwest PA with these storms as confidence in favorable overlap of CAPE and shear continues to increase and SPC has added a slight risk of severe weather to highlight this potential. Most guidance shows roughly 500 to 1200 J/kg of SBCAPE developing out ahead of the front, with the highest values concentrated over our northwestern counties. At the same time, an increase in 0-6 km bulk shear is expected with values between 40 to 50 knots which should help to organize storms as they develop along the front. Storm mode looks to be linear as the deep layer shear should be largely parallel to the front, so damaging wind gusts are expected to be the primary hazard. Model soundings do show some turning in the lowest 0-3 km in association with a 40 to 45 knot low-level jet, and a few transient supercell structures or circulations embedded in linear segments cannot be ruled out. SPC has included a 2% tornado contour that covers much of the slight risk area.

Showers and thunderstorms should weaken as they move south and east through the night into a more stable airmass. However, they will still be capable of producing localized heavy downpours. The current WPC outlook for Thurs paints a SLGT risk of excessive rainfall over portions of Cambria and Somerset Counties. Some CAMs show the potential for up to an inch of rain to fall into Thursday morning in the slight risk area, though most guidance would suggest that up to half of an inch of rain is the more likely outcome. Even though flash flood guidance is relatively low in these areas, the potential for any flash flooding appears to be low at this time unless any areas see training thunderstorms develop.

High pressure builds behind the cold front, giving way to a dry and sunny Friday. There could be a few PM SHRA/TSRA Saturday and/or Sunday afternoons, especially over the western mtns. A more-active (wetter) pattern is in store for Mon-Tues. But, the flat upper ridge over the southern half of the CONUS will make it difficult for deep moisture to make it into PA ahead of any shots of atmos forcing. The second half of the week probably ends dry as the upper ridge pumps up over the central CONUS.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Other than a few high clouds, skies will remain mainly clear overnight with VFR conditions expected to persist until Wednesday evening. Clouds will begin building into the region over the northwest during the morning on Wednesday. Broken to overcast skies will likely be present across the entire region by 18Z Wednesday.

Showers and thunderstorms will likely form ahead of a cold front late Wednesday afternoon into the early evening. A few of these storms will be strong to severe and impacts to visibilities and low ceilings will likely begin at BFD around 00Z Thursday before overspreading the rest of the area. The activity will likely weaken later in the evening as it moves to the east and south, but MVFR to near IFR will be possible as the front arrives across all TAF sites. Some showers and even a few storms will likely linger into Thursday morning, before tapering off as the cold front drops south and east of central PA.

Outlook...

Wed...Primarily VFR, although late-day SHRA/TSRA possible across the northern tier.

Thu...Cold FROPA could bring scattered SHRA/TSRA areawide.

Fri...VFR prevails with drier conditions across all of central Pennsylvania.

Sat-Sun...Primarily VFR conditions, some potential for afternoon SHRA/TSRA but uncertainty remains high.

CTP WATCHES/WARNINGS/ADVISORIES

None.