textproduct: State College

WHAT HAS CHANGED

* Included near-term potential of showers and thunderstorms producing isolated instances of flash flooding in the Laurel Highlands, see Key Message #2. * Confidence is still high in long-duration heat this week beginning Tuesday and continuing into the holiday weekend.

KEY MESSAGES

1) Significant, prolonged, and dangerous heat wave expected from the last day of June into Independence Day/4th of July holiday weekend

2) Slow-moving showers and storms across the Laurel Highlands this afternoon could cause isolated instances of flash flooding.

DISCUSSION

KEY MESSAGE 1: Significant, prolonged, and dangerous heat wave expected from the last day of June into Independence Day/4th of July holiday weekend

Widespread temperatures in the 90s to 100F, combined with high humidity, will result in forecast max heat index values in the 100-110F range with the highest values in the lower elevations to the east of the Allegheny Mountains. This equates to widespread Major-to-Extreme HeatRisk from Wednesday through Friday or Saturday across central Pennsylvania. Additional watch considerations were given this cycle; however, based on collaboration with neighbors decided to hold off on headlines to get another cycle to gauge which zones will be closer to upper- end Heat Advisory criteria versus lower-end Extreme Heat Warning criteria. For zones close to the Advisory/Warning threshold, main uncertainties with heat index values reaching above or remaining below 105F will be dewpoints, which typically trend below NBM model guidance in these types of set-ups with drier air mixing down to the surface. Overnight low temperatures throughout this timeframe will remain quite warm, bringing little to no relief from the heat in the nighttime hours.

The greatest forecast challenge (highest uncertainty/lowest confidence) centers around ring of fire convection that could spill southeastward around the northern periphery of the upper ridge into the area on Friday; however, EC/GFS model guidance have both started to show some precipitation mentions in more recent model guidance, so this timeframe will bear watching as we get closer to the end of the week.

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KEY MESSAGE 2: Slow-moving showers and storms across the Laurel Highlands this afternoon could cause isolated instances of flash flooding.

Current radar as of 2:30 EDT outlines scattered showers and thunderstorms across portions of western Pennsylvania in an environment with ample moisture in play. Recent HREF ensemble guidance outlines PWATs this afternoon/evening slightly above 1.50" which appears to be in the 80th-90th percentile for June 28th across the southwestern Pennsylvania. Storm motions have overall been very slow, thus any area that receives multiple showers and storms this afternoon could experience isolated instances of flash flooding. Recent WPC EROs outline this potential with the bulk of activity occurring to the west/south of central Pennsylvania; however, hydrologically sensitive areas in southern Somerset County could also run into these instances of flash flooding. Further north, coverage is expected to be slightly less and many areas will likely not receive multiple rounds of heavy rainfall to warrant much in the way of flooding concerns this evening.

Rain chances diminish after sunset, with fog formation likely to occur overnight into Monday morning. Fog is not expected to be as expansive as last night; however, could see some locally dense fog across the higher elevations of Somerset County where the aforementioned heavy rain is expected today. Any fog is expected to mix out quickly after sunrise Monday morning, but could still be around for the Monday morning commute.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Daylight has permitted low stratus to lift throughout today, with gradual improvements to MVFR-VFR continuing this afternoon. A weak low pressure system persists across the Mid-Atlantic, forcing scattered showers across the Commonwealth (especially where SPC Mesoanalysis shows overlap of PWATs AoA 1.5" + MLCAPE upwards of 500 J/kg over SW PA). Tall/narrow CAPE will favor rainfall efficiency given deep moisture in place with low chance for lightning (less than PROB30, though non-zero in the heaviest showers). Visibility restrictions to MVFR-IFR are possible in the heavier showers.

Showers will taper off this evening & cloud cover will trend SCT-BKN, bringing a chance for radiational mist/fog tonight given low-level moisture from recent rainfall and light/variable winds as high pressure builds across the Northeastern US. A blend of the GLAMP and HREF models were used for visibility, with higher probabilities (50-60%) of overnight FG around the Appalachians (KJST/KAOO). After sunrise on Monday, daylight will once again mix out the lower stratus, giving way to regionwide VFR by 15Z.

Outlook...

Mon-Fri...AM fog; otherwise VFR. Trending hotter with isold PM t-storms psbl. High density altitude concerns possible.

CLIMATE

Triple digit max temps are forecast in the Lower Susq Valley (LSV) on Thursday and Friday bookended by upper 90s Wednesday and Saturday.

The last time Harrisburg observed consecutive days with maxT greater than or equal to 100F was back in 2011 on July 21-22.

The record number of consecutive days with maxT >= 100F is 3 and has occurred 5 times: (all in the month of July)

1999-07-16 to 1999-07-18 1999-07-04 to 1999-07-06 1991-07-19 to 1991-07-21 1966-07-02 to 1966-07-04 1936-07-09 to 1936-07-11

The last time Harrisburg hit the century mark was July 16th in 2024. Prior to that, it was July 19th in 2020.

Harrisburg averages 1 day annually with a maxT >100F. The most number of days observed in a year is 6 in 1999.

CTP WATCHES/WARNINGS/ADVISORIES

None.