textproduct: State College

WHAT HAS CHANGED

* Heat Advisory for Warren County noon Tuesday-10PM Thursday

KEY MESSAGES

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

DISCUSSION

KEY MESSAGE 1: Significant, long duration, 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 be the heat wave beginning Tuesday (last day of June) into the Independence Day/4th of July holiday weekend.

For the rest of the afternoon, we can't rule out a localized, terrain induced, southward drifting rain shower or brief thunderstorm over the southern Poconos into the south central Alleghenies. A mainly clear and muggy overnight period should allow for good viewing of the Strawberry Moon and may also allow for patchy fog toward daybreak.

A very strong ridge becoming anchored over the eastern half of the country will support a significant, long duration, and dangerous heat wave impacting central PA. Max heat index values between 95 and 110 degrees are forecast during the afternoon and evening Tuesday through Sunday with warm and muggy conditions offering little to no overnight relief. This equates to widespread major to extreme HeatRisk. We incrementally issued a heat advisory for Warren County with the expectation of future expansion of heat headlines to eventually cover all of central PA in the coming shifts.

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. SPC has shifted the level 1/5 MRGL risk a bit farther to the northeast and essentially out of the forecast area/CWA for Tuesday. However, inherent uncertainty in these types of patterns will keep lower rain/thunderstorm chances in the forecast through the weekend.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Fair weather cumulus clouds are being observed across the commonwealth this afternoon as VFR is expected to prevail across all TAF sites for the next 24 hours. High pressure is building in over the region and dry conditions will persist for today. Afternoon thunderstorm chances will be possible later in the week, but given the dome of high pressure it will be difficult for storms to become widespread. While VFR is most likely to prevail across most of the Commonwealth for tonight, 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). Confidence in fog formation is relatively low, but a few mentions of brief visibility reductions have been made for the appropriate TAF sites. Winds will remain calm out of the ESE for the remainder of today, but as high pressure builds in for Tuesday expect a wind shift to the SW with gusty winds possible during the afternoon on Tuesday.

Outlook...

Tue-Sat...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

Heat Advisory from noon Tuesday to 10 PM EDT Thursday for PAZ004. Extreme Heat Watch from Wednesday afternoon through Friday evening for PAZ019-026>028-035-036-045-046-049>053-056>059- 063>066.