textproduct: State College

WHAT HAS CHANGED

Increasing confidence in a long lasting heat wave Tuesday into Independence Day/4th of July

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

Confidence and predictability remains high for a large and anomalous 590+dm upper level ridge to deliver a significant, multiday, and potentially dangerous heat wave from the last day of June (Tuesday 6/30) into Independence Day/4th of July holiday weekend.

Widespread temperatures in the 90s to 100F, combined with high humidity, will result in fcst max heat indices in the 100-110F range with the highest values in the lower elevations to the east of the Allegheny Mountains. This equates to widespread Major to Extreme HeatRisk and high probability of heat WWA headlines from Wednesday through Friday or Saturday. (Consideration was given to an extreme heat watch with this cycle per coordination with PHI). Overnight lows will also be quite warm, bringing little to no relief from the heat in the nighttime hours.

The greatest forecast challenge (highest uncertainty/lowest confidence) centers around ring of fire convection that could spill southeastward around the northern periphery of the upper ridge into the area late in the week.

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In the shorter term, low stratus and fog early this morning will give way to mostly cloudy conditions into the afternoon with spotty showers/thundershowers possible. HREF/REFS are keying on southwest PA into the Laurel Highlands with the highest precip probs between 8AM-8PM. Some clearing expected overnight which allows for areas of fog to develop again into early Monday morning.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Fog/low cloud begin to mix out fairly quickly after sunrise, with the bulk of IFR/LIFR mentions out by 15Z Sunday with light winds below 10 knots. Main concern in the later half of the TAF package will be some potential for SHRA/TSRA across mainly the southern half of the forecast area after 18Z. The 00Z TAFs go against GLAMP guidance to include some mentions at all airfields outside of BFD/IPT with low-to-moderate (30-50%) confidence given the scattered nature expected of SHRA/TSRA. The most recent HREF 1-hr lightning guidance does indicate lower (less than 30%) probabilities for TSRA, so have kept mentions out at this time.

Outlook...

Mon-Thu...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

None.