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.

Portions of Central PA may wake up to patchy fog this morning thanks to a very humid airmass and overnight showers/storms. With an upper level ridge in place advecting in a very warm to hot airmass, heat and humidity will continue to build today and Friday, particularly for the eastern two thirds of the forecast area.

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 where HREF probabilities of heat index > 100F are highest. ---------------------------------------------------

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 weak but sufficient shear will lead to multiple rounds of showers and thunderstorms today and tomorrow. 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). Slight Risks with 30% wind probabilities have been issued for both today and Friday across southeast PA.

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.

It is worth noting that recent hi-res convective allowing models actually bring most of the convection to NW PA today and fail to initiate many storms at all in our forecast area on Friday. Possible flies in the ointment to monitor.

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.

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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 /12Z THURSDAY THROUGH MONDAY/

Ceiling/visibility restrictions from overnight mist & fog will trend to VFR regionwide through diurnal boundary layer mixing. While the primary convective threat will be later today, stray thundershowers developing within a moist (PWATs > 1.5") and destabilizing (MUCIN trending to 0 J/kg) environment may yield brief visibility restrictions & gusty winds.

Additional rounds of convection are expected Thursday afternoon/evening as another, more focused shortwave crosses the moist/unstable environment in place. Multicell clusters & quasi-linear segments are favored given marginally favorable deep layer shear (20-25 kts), along with threat for gusty to perhaps severe-level winds given steep low-level lapse rates (8-9 C/km) and moderate-high DCAPE (1000-1250 J/kg). Visibility restrictions will be likely in these storms, especially given high moisture content (PWATs around 1.5") which will bring heavy rain in storms. Convective activity across central PA will downtrend after 00Z as the shortwave progresses east of our area, with patchy fog possible once again Thursday night given moist environment & breaks in cloud cover.

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 this morning to 8 PM EDT Friday for PAZ057-059-063>066.