textproduct: State College

WHAT HAS CHANGED

* SPC upgraded southeast PA to a Slight Risk, highlighting potential for damaging winds and a tornado or two. * Increased rainfall totals today, including potential for a swath of 1-2"+ and an associated risk for isolated flooding.

KEY MESSAGES

1) Locally heavy rain and a few strong thunderstorms possible across south central Pennsylvania today.

2) An unsettled weekend will be bookended by dry and sunny weather on Friday and early next week.

DISCUSSION

KEY MESSAGE 1: Locally heavy rain and a few strong thunderstorms possible across south central Pennsylvania today

A low pressure system centered in the Ohio Valley this morning will drift east today, bringing a period of unsettled weather across the Commonwealth. Aloft, an embedded shortwave trough within predominant northwesterly flow will provide sufficient lift and diffluence aloft to generate an extended period of heavy rain this morning across Central PA. Light showers before daybreak will be replaced by a more widespread slug of moderate to locally heavy rain into the early afternoon. CAMs depict the potential for localized rainfall amounts approaching or exceeding 2 inches in a swath draped southwest to northeast along or perhaps just south of the I-80 corridor.

As the center of low pressure moves east along the Mason-Dixon Line, southerly flow ahead of the system will advect a very moist airmass (basically a warm front) northward through the Lower Susquehanna Valley and southeast PA. That moisture, combined with increasing diurnal heating (storm fuel), and a favorable shear environment will likely lead to strong to severe thunderstorm development this afternoon. Ongoing clouds/showers north of the PA Turnpike should limit the severe threat there today, but portions of Adams, York, and Lancaster County will likely receive one or two rounds of strong storms capable of producing all hazards.

Bulk shear vectors will generally be westerly and perpendicular to the boundary across southern PA. This should favor a discrete mode at least initially and then storms are likely to grow upscale into a bowing segment as they move east into southeast PA and New Jersey. The Storm Prediction Center brought the Slight Risk of severe weather northwest into York and Lancaster this morning along with an upgrade to a 5% risk of tornadoes in far eastern Lancaster County and a 15% wind area. The environment is certainly primed for severe weather and residents in southeast PA should be prepared to take shelter if Severe Thunderstorm or Tornado Warnings are issued this afternoon.

Rain exits the area by around 00Z with clouds gradually clearing out later tonight. Widespread rainfall today followed by ideal radiational cooling conditions should allow for widespread valley fog to develop into early Friday morning.

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KEY MESSAGE 2: An unsettled weekend will be bookended by dry and sunny weather on Friday and early next week.

By Friday, the aforementioned front will drift south but remain very close to the Mason-Dixon line. As such we can't rule out a spotty shower/t-storm in far southern PA Friday afternoon. However, most locations will stay dry and Friday looks like a fairly pleasant day with plenty of sunshine.

An upper trough carving out over the Great Lakes will bring increasingly unsettled weather conditions over the weekend. There is still considerable uncertainty with timing, intensity, and coverage of storms, but it is not expected to be a washout for anyone. This system will also deliver some relatively cooler air both at the surface and aloft. This setup could prove favorable for clusters of gusty t-storms with small hail both Saturday and Sunday. A cold front will move across the region Saturday and Sunday, helping focus severe weather potential. The best chance for strong storms will be west of I-99 on Saturday and then across eastern PA on Sunday.

In the wake of the cold front, the start of next week looks quite nice. A decaying secondary cold front could bring some light showers north of I-80 on Monday, but dry conditions prevail otherwise. Mostly sunny skies, slightly below normal temperatures, and lower humidity will provide some very pleasant late August weather. Upper troughing will continue to support near to below average temperatures and above average precipitation into early September.

AVIATION /12Z THURSDAY THROUGH MONDAY/

A swath of steady rainfall will overspread central PA this morning, with KUNV/KIPT looking to be within the core of this swath (meaning higher confidence in MVFR-IFR visibility restrictions), with KBFD/KJST/KAOO more-so on the periphery (more intermittent MVFR-IFR restrictions, especially where rain is more showery). Rain at KMDT/KLNS looks to be more showery/convective in nature, with a few strong to severe thunderstorms possible in the afternoon. Rumbles of thunder may occur in the steadier, more stratiform swath of rain where MUCAPE may rise to 100-250 J/kg (primarily KUNV/KJST/KAOO).

Winds will tend to be variable near the system's track over southern PA, with winds generally N/NE north of the system & S/SE to its south. Expect some fog and stratus to develop tonight in the wake of the showers and storms, though confidence in widespread IFR remains low.

Outlook...

Fri...AM fog, otherwise trending VFR.

Sat-Sun...Scattered to numerous showers/t-storms likely. Breezy.

Mon...VFR returns for early next week.

CTP WATCHES/WARNINGS/ADVISORIES

None.