textproduct: State College

WHAT HAS CHANGED

* Confidence in a prolonged heat spell has increased, and heat products have been issued into Saturday for affected areas. * Bumped PoPs up for isold-sct SHRA/TSRA this PM and Wed PM

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

2) Isolated to scattered SHRA/TSRA possible this aftn/evening and again Wed aftn/evening.

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 issue and over-arching message is the excessive heat throughout the remainder of the week and even into the 4th. The big upper ridge over the OH/TN Valleys will heave east and a little north through the next several days. Abundant sunshine and high moisture will bring the word /oppressive/ back into vouge for the next few days when talking about the heat and humidity. As we have been messaging for many days, we expect a spell of 5 days of heat indicies over 100 for at least part of the CWA. The peak of the heat and humidity will be Wednesday and Thursday with nearly identical HIs both afternoons. Much of the area will have heat indicies between 100 and 110 those two days. Due to the high confidence in that part of the forecast, we have issued appropriate excessive heat warnings and heat advisories for the next few days. The humidity creeps into Warren Co today, while the dewpoints stay in the 60s in the SE. The heat expands on Wed with temperatures nearing 100 in the urban areas of east. We've made very minor tweaks to the temps and dewpoints each day in collaboration with the neighbors. They were mainly to nudge Ts and Tds down from NBM just a hair. But, it keeps us right where we were 12 hrs ago w.r.t. heat indicies. The heat will start to abate Fri and a little more Sat. Not much, but likely enough to start tapering off the advys/warnings.

We have tried to reflect the trend in temps later in the week in the timing of each of the hazard segments. Note: After the first pass at issuing the hazards/flags, I realized I had forgotten to have McKean Co in with the wide advy for the bulk of the Alleghenies -- and also forgot to keep the advy for Blair, Bedford and Fulton Cos into Fri. So you may see tweaks from the first issuances of the products - but, they are good, now.

As we have been encouraged to do, we chose to use the low end of the scale to base the WWAs. So, while on the forecast map, >=50% of Cambria Co does not get to 100HI any day, we felt it was wise to include them in the advy due to Johnstown's few pixels that do reach 100. Many of the people most vulnerable to heat stress/illnesses live in the urban area of the Conemaugh Valley. Also, we considered leaving McKean and Potter Cos out of the heat advys, but decided to include them as there were a few pixels of 100HI there, and keeping in mind that NY state's threshold for advy is only 95 (PA is 100). BUF and BGM may yet issue advys for their own areas, and the resulting hole in the north-central PA mtns may look odd. To hit home the message that it will be hot and humid (and dangerously so in many places for a few days), we felt it was not a bad move to include McKean and Potter. Now, Somerset Co is the odd man out for this go 'round. We've left them out of the advys becasue their HI numbers just weren't close enough to make a strong case for inclusion based on forecaster judgement/nudgers. This matches with PBZ and LWX leaving their highest-elevation Cos out of the products - for now.

Height falls hail the approach of a minor cold front which should arrive on Friday or Saturday. Natural fireworks for the 4th. The increased cloud cover and some rain should help knock down the temps, and knock out most worries for heat products. Temps in the SE half of the CWA will come down 3-5F for Sat (from the 100s of Friday). So, it won't be a big change, but probably enough to drop the advys for the NW and downshift the warning to an advy for the SE half. Sunday should be "cold" enough to be done with heat products.

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KEY MESSAGE 2: Isolated to scattered SHRA/TSRA possible this aftn/evening and again Wed aftn/evening.

Changes have been very minor with the main change nudging PoPs up this aftn/evening and Wed aftn/evening with two shortwave troughs rolling over the strong upper ridge and nipping the N and E with some forcing. This forcing, combined with the help/lift of the Central PA ridges, should be enough to trigger a couple of SHRA/TSRA. SPC general thunder supports this idea, but most should be N & E of the CWA. A wild card will be the SE third of the area on Wed, where CAPEs get super high and the possible dip in upper temps may allow isold/sct storms to pop up there. In collab with PHI and LWX, we added 20-30 PoPs there for Wed PM.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Broad high pressure over the Northeastern US will shift towards the Tennessee Valley throughout today, bringing southwest winds of 5-10 kts with a few gusts of 15 to 20 knots possible in the afternoon. Moreover, a VFR SCT-BKN cumulus deck will develop during the morning & early afternoon hours, with a few afternoon showers/thunderstorms possible across northern PA. However, guidance continues to bring higher storm chances (>30% chance) towards Upstate NY & New England where less capping is in place, precluding TAF mentions of showers/storms at this time.

For tonight, VFR will continue to prevail with SCT-BKN cumulus remaining overhead & fog remaining unlikely at our TAF sites. A 20-30 kt LLJ (below LLWS thresholds) will develop during the evening in response to a tightening pressure gradient on the periphery of the high pressure, with 925 mb winds veering from SW to W/WNW through the night.

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 today to 8 PM EDT Thursday for PAZ004. Heat Advisory from noon Wednesday to 8 PM EDT Thursday for PAZ005-006-010>012-017-018-024-037-041-042. Extreme Heat Warning from noon Wednesday to 8 PM EDT Friday for PAZ019-026>028-036-045-046-049>053-056>059-063>066. Heat Advisory from 8 PM Friday to 8 PM EDT Saturday for PAZ019- 026>028-036-045-046-049>053-056>059-063>066. Heat Advisory from noon Wednesday to 8 PM EDT Friday for PAZ025-034-035.