textproduct: State College

WHAT HAS CHANGED

* Issued an Extreme Heat Watch for the Bulk of the Central Ridge and Valley Region of PA along with the Susquehanna River Valley where heat indices could peak between 105 and 110F Wed-Fri afternoons.

KEY MESSAGES

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

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

The main event for this week will undoubtedly be the building heat and humidity with some record highs likely broken.

GEFS Standardized Anomalies show a large area of +2-3 sigma 500 mb heights building east and anchoring itself across PA for the Wed-Fri period, bringing the potential for extreme heat with apparent temps over 105F each of the three afternoons. It's pretty much a slam dunk for Heat Advisory conditions across the Central Ridge and Valley Region and Susq Valley during the mid to late week.

As the pesky quasi-stnry sfc frontal boundary to our south moves northeast as a warm front later today into Tuesday, heat indices could soar to near 100F in the Allegheny Valley of Warren County Tuesday, while most other elevated and forested areas of the county nose into the 90s. They'll be on the cusp of a potential Heat Advisory for Tuesday. Elsewhere, a decent amount of sunshine and a developing light southerly flow will boost afternoon highs into the upper 80s and low 90s, with heat indices ranging through the 90s.

The greatest forecast challenge (highest uncertainty/lowest confidence) continues to center 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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AVIATION /12Z MONDAY THROUGH FRIDAY/

VFR conditions will return area-wide by 15Z Monday as sunlight mixes out the fog this morning. Otherwise, today will feature light winds and regionwide VFR conditions with scattered clouds as high pressure builds in across the Northeastern US. A few showers may develop during the afternoon, but confidence in timing/placement/coverage remains too low for mentions of rain in the TAFs.

VFR will prevail across most of the Commonwealth for tonight, though patchy overnight fog will once again be a consideration (albeit less extensive & primarily concentrated in low-lying areas vs. this morning, given that yesterday's rainfall primed us for fog).

Outlook...

Tue-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

Extreme Heat Watch from Wednesday afternoon through Friday evening for PAZ019-026>028-035-036-045-046-049>053-056>059- 063>066.