textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

An Extreme Heat Watch has been issued for all of our New Jersey and Pennsylvania counties, as well as northern Delaware, which is in effect from Wednesday afternoon through Saturday evening.

Confidence is increasing that multiple days of near record breaking heat and humidity will impact the region beginning Wednesday through the holiday weekend. Additionally, there will be a threat of thunderstorms starting Friday and through the holiday weekend.

Updated aviation discussion for the 12Z TAFs.

KEY MESSAGES

1. A few isolated showers and thunderstorms possible through this afternoon. 2. Multiple days of extreme heat and humidity impact the region beginning Wednesday through the holiday weekend. High temperatures will be well in the mid 90s to as high as 105 degrees, with maximum heat index values between 100 and 110 degrees for multiple days.

3. Potential for thunderstorms will return starting Friday and through the holiday weekend, which could impact outdoor holiday festivities.

DISCUSSION

KEY MESSAGE 1...A few isolated showers and thunderstorms possible through this afternoon.

A few isolated showers and thunderstorms are forecast to develop through this afternoon. While some brief heavy downpours are possible with this activity, no severe thunderstorms or hazards other than lightning are expected. High temperatures in the upper 70s to mid 80s and remaining a bit humid. Mostly cloudy skies should begin to scatter out by late afternoon.

KEY MESSAGE 2...Multiple days of extreme heat and humidity impact the region beginning Wednesday through the holiday weekend. High temperatures will be well in the mid 90s to as high as 105 degrees, with maximum heat index values between 100 and 110 degrees for multiple days.

Strong mid-level ridging will develop over the Great Lakes and Ohio Valley early in the new week, then will build into the Northeast and Mid-Atlantic starting on Wednesday and continuing through into the holiday weekend. This will setup a west to northwesterly flow pattern for our region. Hot and humid conditions return with high temperatures well into the mid to upper 90s starting on Wednesday, then upper 90s to low 100s on Thursday and Friday. Combined with surface dewpoints well in the 60s to low 70s, though dewpoints should drop a bit in the afternoon hours during peak heating of the day, the max heat index values will be over 100 degrees, likely exceeding 105 degrees on Thursday and Friday.

While the ridge looks to finally start breaking by Saturday, highs are still forecast to range from the mid 90s to near 100 degrees for the 250th Independence Day. Current indications in the guidance are for further improvements in the heat and humidity by Sunday, however it will likely still be quite hot and humid with highs in the upper 80s to mid 90s possible.

Given the increasingly higher confidence in a significantly impactful extreme heat event impacting much of our area leading up to and during the holiday weekend, an Extreme Heat Watch was issued to highlight the potential impacts. This includes all of our New Jersey and Pennsylvania counties, as well as northern Delaware, and is in effect from Wednesday afternoon through Saturday evening. Delmarva was excluded for now, as the current forecast does not support heat indices reaching at least advisory criteria until Thursday, however this will likely need to be expanded to include the entire CWA in a future update.

01Z/28 NBM probabilities indicate probabilities of high temperatures of 100 degrees or greater of 80-90% for southern New Jersey, the Delaware Valley including Philadelphia, and Delmarva on both Thursday and Friday, even as high as 15% for Mount Pocono. Probabilities of 100+ degree temperatures are lower, but noteworthy on Wednesday (20-50%) and Saturday (50-80%).

The mid-level ridge will break and retrograde back towards the south and west on Saturday, and a mid-level trough will try to slide down into the Northeast. This pattern change will begin to favor slightly cooler temperatures and a break in the extreme heat by Sunday or Monday, as well as the potential for showers and thunderstorms next weekend (see Key Message 3 below).

KEY MESSAGE 3...Potential for thunderstorms will return starting Friday and through the holiday weekend, which could impact outdoor holiday festivities.

The synoptic pattern that will be in place with a ridge axis to our west by the end of the week and into the holiday weekend will be supportive of daily thunderstorm chances returning to the region, including at least some potential for severe thunderstorms. While details of this activity won't become clear until later this coming week, it is important to point out that any holiday weekend festivities could be impacted by thunderstorms (in addition to the extreme heat). Currently, PoPs are highest (20-40%) Saturday through Sunday, and around 10-20% on Friday.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Today...Mainly MVFR ceilings to start, then ceilings will gradually lift and scatter out to VFR by 18Z. Northeast to east winds near 5-10 kts. A few showers possible, but coverage will be too low to include a TAF mention for now. Moderate confidence on timing of improvements.

Tonight...VFR to start with winds becoming mostly calm. Restrictions potentially developing after 06Z in fog and/or low stratus. Low confidence.

Outlook...

Monday through Thursday...VFR. No significant weather.

MARINE

No marine hazards expected through tonight. Some patchy fog possible nearshore.

Outlook...

Monday through Thursday...Overall, no marine hazards expected. Cannot rule out a brief period of southerly 25+ kt gusts or seas approaching 5 feet late Tuesday and/or Wednesday afternoon north of Barnegat Light.

Rip Currents...

For today, winds shift to mainly northeasterly around 10 mph with continuing breaking waves in the surf zone around 2 feet with a 6 to 7 second period and southeast swell. We will continue with a LOW risk for the development of dangerous rip currents for both the Jersey Shore and Delaware Beaches.

For Monday, we're not expecting much change as breaking waves in the surf zone will be around 1-2 feet with east to northeast winds around 10 mph and a southeast swell. We'll continue with a LOW risk of dangerous rip currents for both the Jersey Shore and Delaware Beaches.

For specific beach forecasts, visit weather.gov/beach/phi

CLIMATE

Here is a list of the high temperature records for July 2 (Thursday) and July 3 (Friday):

Thursday July 2 Allentown...102/1968 Atlantic City Airport...100/1966 Atlantic City Marina...97/1968 Georgetown...99/1953 Mount Pocono...93/1911 Philadelphia...103/1901 Reading...102/1966 Trenton...100/1901 Wilmington...98/1966

Friday July 3 Allentown...105/1966 Atlantic City Airport...104/1966 Atlantic City Marina...99/1919 Georgetown...101/1954 Mount Pocono...103/1911 Philadelphia...104/1966 Reading...103/1966 Trenton...102/1966 Wilmington...102/1966

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Heat Watch from Wednesday afternoon through Saturday evening for PAZ054-055-060>062-070-071-101>106. NJ...Extreme Heat Watch from Wednesday afternoon through Saturday evening for NJZ001-007>010-012>027. DE...Extreme Heat Watch from Wednesday afternoon through Saturday evening for DEZ001. MD...None. MARINE...None.