textproduct: State College

WHAT HAS CHANGED

* Decrease in severe coverage this evening with tranquil weather outside of fog overnight into tomorrow morning. * Updated Heat Key Message for tomorrow's heat index forecast, lower confidence in Heat Advisory at this time.

KEY MESSAGES

1) Periods of showers/scattered thunderstorms through Saturday will bring localized flash flooding risk across central Pennsylvania. Strong-to-severe storms also possible Friday afternoon/evening.

2) Very warm temperatures and humid conditions will lead to heat index values approaching 100 degrees across portions of the Lower Susquehanna Valley on Friday.

DISCUSSION

KEY MESSAGE 1: Periods of showers/scattered thunderstorms through Saturday will bring localized flash flooding risk across central Pennsylvania. Strong-to-severe storms also possible Friday afternoon/evening.

Severe and flash flooding risk for today has decreased with the loss of daytime heating with very little precipitation across central Pennsylvania as of 11 PM EDT. Overall trend is that showers approaching the Commonwealth will continue to decrease in coverage/intensity with mainly dry conditions expected overnight into Friday morning. Main concern overnight will be the potential for fog/low stratus development with many stations across central Pennsylvania already reporting clear skies and calm winds in their most recent observations. This concern will be highest in areas that received heavy rainfall across southern Pennsylvania. Updated fog forecast does outline this potential with valley fog expected across the northern half of central Pennsylvania. Any fog formation likely mixes out quickly after sunrise and is out by 9AM EDT.

First glance at the first couple of hours of 00Z 8/7 HREF ensemble guidance outlines an uptick in shower activity across northern Ohio overnight compared to previous runs. Thus, have increased PoPs slightly earlier across NW PA closer to sunrise compared to previous forecast. Increasing humidity throughout the day with ample instability (~1000-2000 J/kg across N/W PA and 2000+ J/kg across S/E PA) will likely allow for slightly more widespread coverage with more height falls observed in recent model guidance compared to this afternoon and evening. In terms of a severe threat, expect scattered thunderstorms to develop with potentially a slight uptick in coverage compared to this afternoon. Isolated instances of strong-to-damaging winds appear to be the main hazard with these, mainly in the form of wet microbursts given the instability, moisture, and lift (trough approaching the area) in play.

The overly moist air mass, with PWATs exceeding the 90th percentile across all of central Pennsylvania, will brings another round of flash flooding concerns on Friday and Saturday, mainly during the afternoon/evening hours. This flooding threat could be minimized by more progressive storm motions; however, given the amount of moisture, any shower/storm will likely bring efficient rainfall to the region and any location (especially urban and poor drainage locations) that receive multiple rounds of heavy rainfall could experience localized flash flooding. Recent HREF PMMs does paint a corridor 2-3" rainfall amounts along the Pennsylvania Turnpike Friday afternoon/evening, which tracks with the ample moisture available. Do want to highlight that while this signal exists, location of this corridor remains the main point of uncertainty at this time so remain weather aware and have multiple ways to receive warnings tomorrow afternoon/evening. Heavy rain signal on Saturday is more muted; however, if this heavier corridor of rain plays out as predicted, some locations could need very little rainfall for additional flooding concerns on Saturday.

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KEY MESSAGE 2: Very warm temperatures and humid conditions will lead to heat index values approaching 100 degrees across portions of the Lower Susquehanna Valley on Friday.

Increasing heat and humidity throughout the afternoon hours ahead of the aforementioned showers/storms will lead to another hot day across portions of the Lower Susquehanna Valley and surrounding locations. Recent model guidance does indicate a slight decrease in temperatures tomorrow with increasing cloud cover during the late morning/early afternoon hours across the region which could limit peak heating slightly. That being said, moisture will be plentiful with dew points expected to reach into the mid-70s during the afternoon hours ahead of forecast precipitation. Recent HREF guidance of heat index values exceeding 100F appear to be more confined to the south and east of Lancaster County. Highest probabilities max out around ~30-40% for far southern Lancaster County. For reference, similar guidance was highlight probabilities closer to ~60-70% for similar locations this afternoon/evening. Given the uncertainty, have decided to let the upcoming midnight shift handle heat headlines for Friday to get another run of HREF/NBM model guidance to assist in this decision. Whether or not a heat headline gets posted, hot and humid conditions are expected for the Lower Susquehanna Valley tomorrow before a slight decrease in heat/humidity for the upcoming weekend.

AVIATION /02Z FRIDAY THROUGH TUESDAY/

Near term forecast will focus on scattered convection through the evening hours. Roughly a 10-20% coverage that will diminish as the sun sets. However, the proximity to a few of the TAF sites will allow for a VCTS or -TSRA in the next hour or so.

The focus overnight shifts to FG and BR with plenty of low level moisture and light winds. Tried to highlight the airfields that received rain this afternoon with a reduction to IFR or LIFR. Otherwise, MVFR to IFR was noted.

Any fog will dissipate during the morning hours with a repeat of the past few days through the afternoon. -SHRA and -TSRA will develop, but confidence in any one location receiving precipitation at a given time is a lower confidence (<20%) forecast, so placed VCTS in most locations.

Outlook...

Sat...Pockets of fog in the morning. Scattered showers/isolated t-storms with heavy downpours and localized restrictions.

Sun...Mostly dry, some potential for isolated showers across far W PA. Restrictions possible.

Mon-Tue...Scattered showers/storms. Restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.