Area Forecast Discussion

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 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 amounts of alto cu from weak warm advection aloft and remnant cloud cover from overnight/mid morning convection. Increasing chc for late afternoon and evening SHRA/TSRA will also add to the uncertainty of max temps.

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 Friday.

The first feature at the surface and aloft that will spark PM SHRA/TSRA comes in the form of a compact shortwave trough centered over Lake Erie/SW Ontario with its trough axis extending south across Ecent/Scent Ohio and moving east at a little over 30KT.

Meso-B lift and 1-2 deg C of cooling within the 700 mb layer along with westerly 0-3 km bulk shear increasing to around 30 KT will help to increase instability and organized updrafts in TSRA that should push through much of our CWA in 2-3 linear features, with the main severe wx threat coming in the form of localized wind damage associated with mini bows/broken-S signatures.

If these 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 across the Mid/lower Susq Valley late this afternoon/evening and potentially in any part of the CWA on Friday. 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 /18Z THURSDAY THROUGH TUESDAY/

VFR regionwide for the Central Mtns and Susq Valley through 20Z, with earlier SHRA/TSRA impacting the NW Mtns and Laurel Highlands with brief restrictions.

Multicell clusters & quasi-linear segments are the favored form of convection given marginally favorable deep layer shear (20-30 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-numerous -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 until 8 PM EDT Friday for PAZ057-059-063>066.