textproduct: State College

WHAT HAS CHANGED

* Severe Weather Watch #665 in place for Northwestern PA for severe storms this evening

KEY MESSAGES

1) 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.

2) Showers confined to southern tier of PA Thursday. Dry conditions Friday before unsettled weekend.

DISCUSSION

KEY MESSAGE 1: 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.

Increasingly southwesterly flow at the surface and aloft will carry warm and humid air into the region today, in advance of a cold front moving southeast across the Upper Great Lakes. An axis of high PWAT air (1.75"+) 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 surge of moisture, coupled with clearing in West/Central PA, has allowed for sufficient instability to develop for storms to feed on in this evening. Latest RAP Mesoanalysis has 2000+ SBCAPE J/kg nosing its way into Western PA and 1000+ J/kg in the western half of the state. Lapse rates aloft aren't very impressive though, leading to tall but skinny CAPE profiles expected this evening. This favors heavy, efficient precipitation and coupled with Corfidi Vectors expected in the 10-15kt range could lead to training storms. Isolated flash flooding in Northwest PA is possible as a result, but fast enough storm speed should preclude a more widespread threat.

The main deep layer shear and upper level forcing though remains in Southern Michigan as of 18Z and is still a few hours away from Northwest PA. This leads to uncertainty regarding storm coverage and degree of organization. Should these features align with the best CAPE this evening, greater coverage and more organization of storms is more likely. A jet developing at 850mb this evening, coupled with forward storm speed and precipitation loading in downdrafts, will allow for damaging wind gusts to be the main hazard. Low level shear from this 850mb jet will allow for 0-1km and 0-3km shear to be in the 20-30kt range this evening. Coupled with cyclonic low level hodograph curvature, a transient tornado threat may emerge from embedded supercell structures and/or meso-vorticies depending on the dominant storm mode. Day cloud phase has shown an area of enhanced cumulus clouds this afternoon in the northern PA counties. This indicates a potential low level boundary present, which may act as a focus for storms and source of extra vorticity for storms to ingest. Storms are anticipated to weaken as they push further south and east tonight with increasingly unfavorable instability. However, they will still be capable of producing localized heavy downpours.

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KEY MESSAGE 2: Showers confined to southern tier of PA Thursday. Dry conditions Friday before unsettled weekend.

The most recent WPC ERO has pushed most of the Day 2 Marginal to our south. Some lingering precipitation in the early morning may occur in the southern tier as the previous night's wave pushes through. The main shot of precipitation tomorrow is most likely in the southern tier, particularly in the southwest, as the cold front moves through in the afternoon. Most 6hr HREF PMMs stay below 1" tomorrow afternoon and evening, but some locally heavy rain remains possible.

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 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 /00Z THURSDAY THROUGH MONDAY/

00z update... A band of showers and embedded thunderstorms is crossing northern and western PA early this evening, and it is still expected to begin losing steam with southeastward extent by 02-03z. Near-term TSRA in the vicinity at KBFD necessitates some early mention of them at that site. Otherwise, confidence remains on the low side (20-40%) that they will impact other terminals. Thus, we'll continue to employ the strategy that amendments will be issued as necessary.

Overnight into early Thursday, there is a high probability (70-80%) of low cloud development across the central and western portions of the Commonwealth. We have high enough confidence to go with IFR restrictions at KBFD and KJST, with MVFR-fuel alternate restrictions anticipated at KAOO, KUNV, and KIPT. At this juncture, we expect KMDT and KLNS to stay VFR.

By late morning-midday Thursday, cloud bases should be in the process of lifting, with improving categories across the board by/during the afternoon.

Outlook...

Fri...VFR conditions expected.

Sat-Mon...Primarily VFR conditions, with some potential for afternoon SHRA/TSRA on at least a hit and miss basis.

CTP WATCHES/WARNINGS/ADVISORIES

None.