textproduct: State College

WHAT HAS CHANGED

* Raising POPS and basin avg QPF by a few to several tenths of an inch across the Northwest this morning/early afternoon and across the SE half of the CWA for this afternoon.

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 this 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 today.

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 this afternoon/evening.

Weak llvl boundaries and areas of signif differential heating gradient and topographic effects will be the weather influencers for the midday through early evening hours today. Larger scale forcing associated with the favorable right entrance region of a 50-60 kt upper level jet core will move gradually east across the CWA this afternoon and evening and help to realize the hefty 2000-3500 j/kg CAPE in the form of localized strong pulse/multicell TSRA updrafts. Bands of significantly lower 1-6 hour Flash Flood Guidance values across portions of the Susq Valley and Scent Mtns will have to be monitored closely this afternoon as convection ramps up/moves east and feeds off the best instability across this same area.

Weakening convection across the NW third of the CWA and extensive cloud cover was leading to a notable min in instability with SB CAPE of 500-1000 J/KG, similar to the setup we saw Thursday afternoon/evening.

Previous...

Activity on Thursday was largely driven by the terrain in combination with a very moist airmass. Overall a similar situation today and Saturday. Location of the more active weather on Saturday may be a bit further to the east and south. That is still a ways out.

For the overnight hours into Friday morning, the main concern aside from an isolated shower will be the potential for fog/low stratus development with mainly clear skies and calm winds.

More information below.

This concern will be highest in areas that received heavy rainfall across southern Pennsylvania on Thursday. 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 today.

Current grids only show a few grid points getting close. Also temperatures being push back down by showers and storms, temperatures may not stay up in that range for very long anyway.

More information below.

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 /15Z FRIDAY THROUGH TUESDAY/

For the 18Z TAF package, main changes will be to adjust the timing and duration of convection and the onset of lower cigs/vsbys in fog later tonight/early Saturday.

Included PROB30s with TSRA with visibility restrictions for scattered afternoon thunderstorms at the TAF sites. Similar to Thursday, the storms will be hit or miss, but high PWATs and heavy downpours mean storms may be capable of brief IFR visibility restrictions should they cross paths with a terminal.

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.