textproduct: State College

WHAT HAS CHANGED

* Bumped PoPs up for isold-sct SHRA/TSRA this PM in the north/east

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.

Have tried to keep the temps and dewpoints right as they have been for many days. This has resulted in no changes to the heat indices. So, no changes have been made to the heat headlines with this package. One fly in the ointment could be the few TSRA that may pop up over the NE later today, which may serve to keep the temps down a couple of degrees from current forecast numbers. But, if those storms do make it wet, they could raise the dewpoints locally, and keep the heat index right in the forecast range. See below for more details on convection today.

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 /12Z WEDNESDAY THROUGH SUNDAY/

Intermittent MVFR ceilings are possible early today as diurnal mixing rejuvenates the daytime cumulus deck within this hot/humid air mass, primarily outside of the Lower Susquehanna Valley (KMDT/KLNS) where low-level moisture is less favorable (RHs around 60-80%). Otherwise, westerly winds of 5-10 kts with a few gusts to 15 kts are expected during the day, with VFR cumulus deck in the afternoon trending towards mainly clear skies during the evening as ridging settles in. 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%), with convective activity continuing to favor Upstate NY and New England.

For tonight, winds will trend light and variable with mainly clear skies. 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.

Outlook...

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

Sun...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. Heat Advisory from noon today to 8 PM EDT Friday for PAZ005- 006-010>012-017-018-024-025-034-035-037-041-042. Extreme Heat Warning from noon today 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.