textproduct: State College

WHAT HAS CHANGED

* Bumped PoPs up for isold-sct SHRA/TSRA this PM in the north/east * Expanded the Heat Advisory into Somerset County for today and Thursday * Expanded the Extreme Heat Warning into Bedford, Fulton, and Blair Counties for today and Thursday

KEY MESSAGES

1) Significant, long duration, and dangerous heat wave expected for the rest of the week, and into Independence Day for the southeastern cities.

2) Isolated to scattered showers and thunderstorms today, mainly over the northeast.

DISCUSSION

KEY MESSAGE 1: Significant, long duration, and dangerous heat wave expected for the rest of the week, and into Independence Day for the southeastern cities.

Surface dewpoints are remaining elevated even as peak daytime heating begins early this afternoon. Drier air is struggling to mix down to the surface and as a result high heat index values are being observed even in the higher terrain. This has prompted the expansion of the both the Heat Advisory and Extreme Heat Warning for this afternoon and tomorrow. The messaging will remain the same over the next few days for dangerous heat impacting all of Central Pennsylvania.

The big upper ridge and the sfc high will be in place for today and Thursday. Starting on Friday, the upper ridge gets a little less-strong, and the sfc pressure pattern gets a little muddied. Clouds and better chances for SHRA/TSRA will ensue. That should help nudge temps down. While we will still be in a general ridge, the sfc pressure dips a bit and a baroclinic zone slowly dips in from the north. By Sunday, the flow aloft is zonal with the baroclinic zone parallel to the flow aloft (W-E) providing a focus SHRA/TSRA for the end of the weekend and into next week. Time will tell if we will be under the gun for any flash flooding worries Sun and beyond.

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KEY MESSAGE 2: Isolated to scattered showers and thunderstorms today, mainly over the northeast.

Cooling aloft has allowed a few showers with embedded thunder to form over NY and drop into the northern mtns early this morning. These are well away from the severe storms that affected northern NY earlier in the night, but still connected with the same weak forcing aloft. There is a little capping inversion overhead that disappears around 9-10kft as temps cool off rapidly above that level, more so in the northeast than the rest of the CWA. However, this early morning convection does not have the extreme CAPE that any afternoon/evening storms will have. The high heat and humidity and the relatively cool temps above the cap in the mid levels will mean that any storms that do form later today could make very strong up/downdrafts and produce some isolated severe wind gusts/damage. SPC DAY1 MRGL risk remains unchanged from where the DAY2 outlook had it - over the NErn 5-6 counties. Breaking the cap will be necessary to realize the 2000-3500J of CAPE that are there for the taking. Severe 1"+ hail is probably not a big issue for us today since it is so warm. But, the WBZ is only 12kft aloft. As we lose the daylight, the risk for storms goes down quickly. Did expand the 20+ PoPs into Lebanon and Lancaster Cos to match up with LWX and PHI. But, the cap gets stronger the farther south you get in the CWA as the 7H temps will be near +12C and the 5H heights will be around 595dam. Old rule of thumb that says a 7H temp of +10C is likely too strong to break (over PA) on most days is a pretty good rule. However, the extreme heat we will experience today could blow right through that warm capping layer aloft.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Fair weather VFR cumulus clouds continue to dot the state this afternoon. These will persist until the evening when the loss of daytime heating and ridging building in clears most of them out. Otherwise, westerly winds of 5-10 kts can be expected to the remainder of afternoon. Observations today trending towards less vertical mixing have removed wind gust mentions from the TAFs. Thunderstorm chances remain low (10-20% chance) despite the heat & humidity due to mid- level subsidence and dry air aloft (mean 700-300 mb RH around 20%) and building ridge, with convective activity continuing to favor Upstate NY and New England.

For tonight, winds will trend light and variable with mainly clear skies precluding any prevailing MVFR conditions. Patchy overnight fog may once again develop given the humidity, though largely rain- free conditions and plentiful sunshine during the daytime will continue to mitigate the overnight fog potential, relegating most to the valleys. As the ridge builds in tomorrow, skies remain mostly clear in the morning with westerly flow once again at 5-10kts. Rain chances look low once again tomorrow (10-20% chance) given the synoptic scale subsidence and dry air aloft.

Outlook...

Thu-Sat...Patchy AM fog; otherwise VFR. Hot & humid with PM t-storms psbl. High density altitude concerns possible.

Sun...Restrictions possible in afternoon t-storms

Mon...Restrictions possible in afternoon t-storms

CLIMATE

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

The record maxes for the next few days are in jeopardy:

Station/Date...07/01...07/02...07/03...07/04...07/05. MDT.............100.....104.....107.....104.....102.. IPT.............102......99.....100.....100.....100.. AOO..............94......97......97......97......94.. JST..............98......98.....100.....100.....101.. BFD..............88......90......91......91......91..

CTP WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 8 PM EDT Friday for PAZ004>006-010>012-017- 018-024-037-041-042. Extreme Heat Warning until 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. Extreme Heat Warning until 8 PM EDT Thursday for PAZ025-034- 035. Heat Advisory from 8 PM Thursday to 8 PM EDT Friday for PAZ025- 034-035. Heat Advisory until 8 PM EDT Thursday for PAZ033.