textproduct: State College

WHAT HAS CHANGED

* WPC added a MRGL Risk (level 1 of 4) for excessive rain on Saturday across Southern PA.

KEY MESSAGES

1) Heat and humidity will peak today.

2) Showers and scattered strong to potentially severe thunderstorms, with the threat for localized gusty to damaging winds for late today and early tonight, mainly across the NW third of PA.

DISCUSSION

KEY MESSAGE 1: Heat and humidity will peak today.

Increasingly southwesterly flow at the surface and aloft will carry warm and humid air into the region today, well in advance of a cold front moving southeast across the Upper Great Lakes.

A gradually narrowing SW-NE axis of very high PWAT air (1.8 - 2 inches) will reach the south shore of Lake Erie during the early afternoon, then sag SE across the NW Mtns early this evening before reaching the Southern Tier of PA before daybreak Thursday. This will be a 0.5 to 0.75 inch increase in PWAT over the upcoming 15 hours which will make it feel much more uncomfortably humid later today and much of tonight.

The late day/nighttime peak in the won't result in high heat indices today with apparent temps only about 3-4 deg F higher than the actual temps.

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 and scattered strong to potentially severe thunderstorms, with the threat for localized gusty to damaging winds for late today and early tonight, mainly across the NW third of PA.

Surface low pressure system will be moving across northern Ontario with a cold front extending south/southwest through the Great Lakes and Ohio Valley.

Tightening llvl pressure gradient will lead to an increase in synoptic scale/gradient winds this afternoon into Thursday.

Wind gusts will increase in into the 20s today on the hilltops and also across the Lower SUsquehanna Valley. A gust near 30MPH is possible on the tops of the Laurels Wed night just before the front.

The cold front will provide enough lift of the airmass, (containing a high shear/relatively low instability environment) to create numerous showers and scattered, embedded thunderstorms, with the majority of this convection occurring before the front passes later tonight 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 with SPC maintaining similar areal coverage of their slight risk of severe weather. Most guidance (including the latest HREF) shows 500 to 1200 J/kg of MUCAPE developing out ahead of the front, with the highest values concentrated over our northwestern counties.

Coinciding with this will be an increase in 0-6 km westerly bulk shear, with values between 40 to 50 knots. This should support some fast-moving discrete cells, growing upscale into some line segments containing mini bows with an enhanced threat for wind damage late today/this evening from Warren to Bradford and points about 40 NM to the South.

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.

Low level moisture lurking just south of the Mason/Dixon line Fri/Fri night will advance NE across the state on Saturday as SFC high pressure moves well off the Mid Atlantic and Southern New England Coast.

This will lead to some showers spreading NE across the region with a few PM TSRA possible 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 /07Z WEDNESDAY THROUGH SUNDAY/

Aside from some spotty very light fog/HZ (particularly at KAOO early today) VFR conditions will prevail for much of today, with high based strato cu and alto cu moving ESE across the Commonwealth.

Showers and scattered thunderstorms with brief restrictions will likely form ahead of a cold front late this afternoon and this evening across the Northern half of Central PA.

A few of the storms will be strong to severe and impacts to visibilities and low ceilings will likely begin at BFD in the 22Z Wed - 02Z Thursday period before spreading gradually south across the rest of the region.

This convection will gradually weaken late this evening/overnight 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. Widespread IFR to MVFR CIGS will develop/advect south across the Northern half of PA following the front later this evening and continue into Thursday morning with localized LIFR expected at KBFD after midnight tonight.

Outlook...

Wed...Primarily VFR, although late-day/evening SHRA and sctd TSRA expected 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.