textproduct: State College

WHAT HAS CHANGED

No significant changes were needed with the afternoon forecast package. Did adjust the timing and coverage of showers and thunderstorms slightly to match latest radar observations/trends.

KEY MESSAGES

1) Increasing heat and humidity will bring increased risk for heat-related illnesses Thursday, and especially on Friday.

2) Daily shower and thunderstorm chances continue through the end of the work week. Some strong to severe thunder with damaging winds possible.

3) A refreshing decrease in humidity is a good bet this weekend with high pressure moving in after a cold front.

DISCUSSION

KEY MESSAGE 1:

With an upper level ridge in place advecting in a very warm to hot airmass, heat and humidity will continue to build for Thursday and Friday, particularly for the eastern two thirds of the forecast area. That being said, there is quite a bit of uncertainty remaining in how high the temperatures and heat indices get due to variable cloud cover and the potential for showers and thunderstorms both days (especially Friday). The continued thinking is that the NBM is a bit too high with both temperatures and dew points both days, especially given the recent high bias. Therefore, similar to previous forecast cycles, went a few degrees cooler than the NBM.

All of this being said, the forecast Apparent Ts/Heat Indices for Thursday and Friday still get to near 100 degrees for the southeastern portions of the area on Thursday, particularly the Harrisburg/Lancaster/York areas. Friday should be 1-2F higher. Heat Advisories may be needed for parts of the area both days, but the decision was made to hold off for now due to the above mentioned uncertainties.

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KEY MESSAGE 2:

Underneath a broad ridge of high pressure, a couple shortwaves will drift across Pennsylvania over the next few days. Sufficient instability in the presence of warm & moist air along with variable amounts of shear will lead to multiple rounds of showers and thunderstorms. This will be a typical summer-like pattern with many storms on radar and lingering questions about how strong they'll get. If multicell clusters can develop large enough cold pools, an elevated threat of damaging wind gusts may materialize in the form of an MCS. Such a scenario cloud produce a swath of more significant wind damage (most likely on Thursday).

A cold frontal passage (CFROPA) will occur on Friday. Dynamics in the form of a digging upper trough, high available moisture suggest severe thunderstorms are possible. But, the small ridge that pumps up overhead late Thu could be a setback for how widespread the threat will be.

The biggest question is timing/speed of the cold front. A faster progression could mean the cold front clears our area before peak heating (lower threat of damaging winds), while a later arrival could spell more concern for severe weather. The best mid-and upper-level winds will be well to the north of PA precluding any higher severe risks.

The repeated shots of scattered to numerous SHRA/TSRA over the next 3 days is likely to yield lower FFG thresholds and some flash flooding would become possible, too. But, it has been very dry recently, and everything is growing and thirsty, ready to soak up as much as it can. As always, too much of a good thing can be bad. We'll keep an eye on the soil saturation and other factors that could lead to flooding. Back-to-back-to-back Marginal risks of excessive rainfall are in place through Friday.

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KEY MESSAGE 3:

Following the passage of the aforementioned cold front, much more comfortable conditions will arrive on Saturday and Sunday. Temperatures will remain warm (highs in the 80s and even some lower 90s on Sunday), but dew points will be much lower (in the 50s and lower 60s, instead of the 70s). Then, cooler temperatures arrive by Monday with highs in the 70s to lower 80s. This will be due to another cold front passage on Sunday, which will bring a renewed chance for shower and thunderstorms.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

As of 18Z, mainly VFR conditions are present, with some occasional MVFR to Fuel Alternate restrictions in any showers and thunderstorms, as well as some lingering restrictions behind those showers (especially at KJST). The best chance for thunderstorms prior to late this afternoon/early this evening will be at the eastern terminals (KIPT, KMDT, and KLNS).

As precipitation tapers off, ceilings will likely lower late tonight/early Thursday morning with plenty of lingering low- level moisture present. That being said, confidence in seeing IFR- or- worse conditions remains on the lower side, except at KBFD.

Outlook...

Thursday afternoon...Restrictions possible in scattered -SHRA/-TSRA.

Friday...Restrictions expected as FROPA brings line of -TSRA.

Saturday...Mainly VFR.

Sunday...VFR favored early, with restrictions possible as FROPA brings chance of -SHRA/-TSRA later in the day.

Monday...Additional scattered showers and associated restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.