textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation section regarding smoke visibility restrictions through Thursday.

KEY MESSAGES

1. Above normal temperatures will continue through Thursday.

2. Smoke from Canadian wildfires will infiltrate the area beginning tonight through Thursday, and potentially lingering through Friday into Saturday.

3. Isolated thunderstorms possible Thursday afternoon and evening for Philly Metro and south.

4. The first half of the weekend is shaping up to be active and unsettled with showers and thunderstorms expected for Saturday into Saturday night. Some of these could be severe and also produce heavy rainfall resulting in flash flooding.

DISCUSSION

KEY MESSAGE 1...Above normal temperatures will continue through Thursday.

A weak cold front will pass through the area tonight, which will knock down both temperatures and dew points a bit for Thursday. While it will still be quite hot, with highs in the 90s, heat indicies will only be a few degrees higher from the actual air temperature. Also, have to consider how thick the smoke will be in the area (see more on that below), which may limit daytime heating on Thursday. This could result in temperatures being several degrees cooler than what is currently forecast. Considering this and upon discussion with neighboring offices, have opted to hold off on any Heat Advisories for Thursday and see how much of an impact smoke aloft will have on temperatures.

KEY MESSAGE 2...Smoke from Canadian wildfires will infiltrate the area beginning tonight through Thursday, and potentially lingering through Friday into Saturday.

The latest hi-res guidance continues to indicate near surface smoke the wildfires in western Ontario will spread into the region following tonight's cold frontal passage. The smoke appears likely to persist in the area through Thursday night, and potentially lingering through Friday and Saturday as well. The smoke is anticipated to result in visibility restrictions and potential for poor air quality, but exactly to what degree this may be is uncertain. This smoke event is not currently anticipated to be as severe as the June 2023 event, but this will be difficult to predict ahead of time.

For more information about air quality in your area, visit your state Department of Environmental Protection agency. We will also relay any Air Quality Alerts they may issue on our website.

KEY MESSAGE 3... Isolated thunderstorms possible Thursday afternoon and evening for Philly Metro and south.

A lingering cold front bisecting the region Thursday may provide a sufficient area of convergence to allow for another round of isolated afternoon showers and thunderstorms to develop. Uncertainty remains high, however, with hi res guidance in disagreement on timing and coverage as well as the positioning of the front. Furthermore, the presence of wildfire smoke in the region will add another uncertain factor to the mix. The upshot here is there is a limited conditional potential for thunderstorm development Thursday afternoon and evening, but should they develop, a few storms could pack some gusty winds, similar to what occurred Wednesday afternoon. Along and south of the front, some hi res ensemble members have SBCAPE values in the 2000-2500 J/kg range once again along with 30 kts of deep layer shear. Further more, forecast soundings also show a setup for an inverted-V profile, which will be favorable for downbursts in thunderstorms that can develop. At this time, the best placement for the frontal boundary will be around the Philly Metro/PA Turnpike/I-195 corridor with the best chance for convective development along and south of the front.

KEY MESSAGE 4...The first half of the weekend is shaping up to be active and unsettled with showers and thunderstorms expected for Saturday into Saturday night. Some of these could be severe and also produce heavy rainfall resulting in flash flooding.

We are still a few days out, but are monitoring the potential flash flooding and severe weather setup as we get into the weekend.

As we get towards Friday night, low pressure will be approaching as it moves into the Great Lakes region and pushes a warm front ahead of it northwards towards the area. This could bring some showers and storms into the area as early as the overnight period Friday night with more widespread showers and storms expected for Saturday into Saturday night as low pressure moves from the lower Great Lakes eastward towards New England. As is often the case with these summer patterns, there is some potential for both severe weather and heavy rain/flash flooding. At this vantage point the shear looks to be moderate to potentially strong with the main question being how unstable it will get. This will determine the extent of any severe weather threat but at this point we see at least some potential for severe weather Saturday...especially during the afternoon and evening. Precipitable water values will also be increasing to around 2+ inches. That is well above the 90th percentile for this time of year, increasing the risk for heavy rain/flash flooding. It appears storms may come through in multiple rounds with one or more rounds of storms coming in later Friday night into the day Saturday with the warm front and then another coming in late day into Saturday night ahead of the cold front. So the bottom line is that Saturday could be quite unsettled and most of the region has POPs in the 70- 90 percent range reflecting this.

As we head towards the latter part of the weekend, the latest indications are for the cold front to move through a little faster Saturday night into early Sunday. This will keep some chances for showers/storms around into the first part of Sunday but but based on this faster timing of the front, it looks like we will start to clear out from NW to SE by the latter part of Sunday. As such, the threat for severe weather and/or flash flooding appears low at this time for Sunday.

High pressure builds in to start the new week before the next frontal system may affect the area next Tuesday into Wednesday with renewed chances for showers and storms.

AVIATION /01Z THURSDAY THROUGH MONDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Tonight...Any VFR conditions to start will lower to MVFR between 00- 05Z from north to south. Periods of MVFR visibilities from wildfire smoke will be possible through the remainder of the overnight, especially for areas north of KPHL. West winds around 5- 10 kt early in the evening will become north- northwest overnight around 5 kt or less. Moderate confidence overall.

Thursday...MVFR visibilities likely through much of Thursday from wildfire smoke. There are indications that some denser smoke will arrive later in the day (which may cause IFR visibilities at times), but this primarily looks to hold off until Thursday night. Cannot rule out an isolated thunderstorm around KPHL south around 21-01Z, but uncertainty remains high at this time. West- southwest winds around 10-15 kt with gusts up to 20 kt in the afternoon. Moderate confidence overall.

Outlook...

Friday...Some restrictions possible due to lingering smoke from the Canadian wildfires

Saturday through Saturday night...At least some restrictions likely at times due to showers and storms.

Sunday...Lingering showers/storms possible in the morning then becoming mainly VFR by the afternoon.

Monday...VFR expected.

MARINE

No marine headlines are in effect through Thursday. Southwest winds around 10-15 kt through this evening will become north-northest overnight around 5-10 kt following a frontal passage. Winds will then become south-southwest again on Thursday around 10-15 kt. Seas around 2-3 feet through Thursday.

Generally fair weather is expected, however wildfire smoke will filter into the region tonight into Thursday. This may cause some localized areas of visibility restrictions, however no marine dense smoke advisories for this are warranted at this time.

Outlook...

Friday...Canadian wildfire smoke may cause visibility restrictions at times. Otherwise, fair weather and no marine hazards anticipated.

Saturday through Saturday night...South to southwesterly winds could approach advisory criteria, with 10-15 kt winds and gusts 20-25 kt. Seas generally under 5 feet. Showers and thunderstorms likely. Smoke may also linger some during this period potentially still causing some visibility restrictions.

Sunday...No marine hazards currently anticipated with winds under 25 kts and seas under 5 feet. Scattered showers and thunderstorms possible.

Monday...Fair weather expected with winds under 25 knots and seas around 3 feet.

Rip Currents...

On Thursday, south to southwest winds will average 10 to 15 mph with a light southeast swell with a period of 6 to 8 seconds. Breaking waves will average 1 to 2 feet. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and at Delaware Beaches.

On Friday, north winds around 10 mph in the morning will veer to the southeast in the afternoon with a light southerly swell with a period around 6 seconds. Breaking waves will average 1 to 2 feet. As a result, there is a LOW risk for the development of dangerous and life threatening rip currents at the Jersey Shore and at Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...Air Quality Alert until midnight EDT Thursday night for PAZ054- 055-060>062-070-071-101>106. NJ...Air Quality Alert until midnight EDT tonight for NJZ001- 007>010. DE...None. MD...None. MARINE...None.