textproduct: State College

WHAT HAS CHANGED

Heat Advisory issued for Lancaster, York, Adams, Cumberland, Dauphin, and Lebanon Counties for Thursday and Friday.

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

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. High levels of humidity tonight, as well as recent rainfall, will result in patchy fog tonight into early Thursday morning.

There is quite a bit of uncertainty remaining in how high the temperatures and heat index values 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 heat index for Thursday and Friday still get to the upper 90s and low 100s for the southeastern portions of the area on Thursday, particularly the Harrisburg/Lancaster/York areas. Heat Advisories have been issued for Lancaster, York, Adams, Cumberland, Dauphin, and Lebanon Counties for Thursday and Friday. We will continue to evaluate the potential for expanding the advisory either day.

---------------------------------------------------

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

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 tonight through Friday. This will be a typical summerlike 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 will occur on Friday. An approaching upper trough and 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 repeated shots of scattered to numerous SHRA/TSRA over the next 2 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.

---------------------------------------------------

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

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 showers and thunderstorms.

AVIATION /01Z THURSDAY THROUGH MONDAY/

As of 00Z, some isolated showers tapering off remain after the main storms moved out of the region. A cluster of showers and thunderstorms north of Pittsburgh may allow for some convective restrictions in the next 6 hours as they move westward. This solution is most likely at JST and AOO but uncertainty remains on how much coverage they will occupy. Some other overnight convective activity is possible across Central PA but confidence remains low in exact impacts/timing. Something to watch tomorrow will be for multiple rounds of showers and thunderstorms to impact the region. Model guidance depicts an unstable environment with minimal inhibition through the day as multiple shortwave impulses move through the region. As a result, an early round of showers and thunderstorms (12Z-17Z) may develop in addition to later afternoon/evening (20Z-00Z) convective activity. With such a moist airmass in place, visibility restrictions and gusty winds within these showers and thunderstorms are likely.

As for ceilings, BFD is most likely to see IFR ceilings tonight. Prevailing MVFR and low VFR are possible at JST, BFD, UNV, and IPT with the moist airmass lingering. Prevailing visibility restrictions are most likely at IPT, but confidence in <3SM is highest east of the airport at this time. VFR looks to prevail tomorrow, however as mentioned before, multiple rounds of convection may lead to restrictions and lightning.

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 -SHRA and associated restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for PAZ057-059-063>066.