textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Smoke from Canadian wildfires appears increasingly likely to impact the region with smoky conditions beginning tonight through Thursday, and potentially lingering through Friday into Saturday as well. Otherwise, no significant changes to the forecast.

KEY MESSAGES

1. Above normal temperatures will continue through the end of the week. Today will be the hottest day of the stretch, with dangerously hot and humid conditions expected. Extreme Heat Warnings and Heat Advisories are in effect for our entire forecast area for today.

2. A cold front will pass through the region late this afternoon into tonight. Isolated to scattered thunderstorms will be possible in the afternoon and early evening hours for parts of the area, some of which could be severe. The front will also usher in smoke from Canadian wildfires, which looks to permeate the region beginning tonight through Thursday, and potentially lingering through Friday into Saturday.

3. Tonight's cold front passage will usher in smoke from Canadian wildfires, which looks to permeate the region beginning tonight through Thursday, and potentially lingering through Friday into Saturday.

4. The weekend is shaping up to be active and unsettled with chances for showers and thunderstorms each day. Some storms could be severe and also may produce heavy rainfall leading to flash flooding.

DISCUSSION

KEY MESSAGE 1...Above normal temperatures will continue through the end of the week. Today will be the hottest day of the stretch, with dangerously hot and humid conditions expected. Extreme Heat Warnings and Heat Advisories are in effect for our entire forecast area for today.

Heat and humidity will peak today as strong upper ridging remains to our southwest. A low pressure system moving through eastern Canada will result in warm advection amid westerly flow.

High temperatures are forecast to range from the mid 90s to near 100 degrees in most areas, hottest along and southeast of I-95, with increasing humidity. While some areas will likely reach the century mark again, it will not be quite to the magnitude that we experienced to start July. Humidity will be a little less as well, and the hottest conditions will only last for one day. However, heat index values are still forecast to range from 100-109 degrees for most of the area, again hottest from along and southeast of I-95. A modest westerly, offshore wind near 10-15 mph should help keep the sea breeze from making much inland progress during the afternoon hours. Thus, we expect high temperatures in the mid to upper 90s and heat indices near 100-105 degrees even for the New Jersey Shore and Delaware Beaches.

The Canadian wildfire smoke does not look to play much of a factor in today's temperatures and heating, as guidance indicates we'll be in a clear spot between smoke plumes aloft to our north and southwest.

A weak cold front which pass into the area tonight, which will knock down temperatures and dewpoints a bit. However, Thursday will still be quite hot, with maximum heat indices in the upper 90s to low 100s along and southeast of I-95. It is possible some additional Heat Advisories could be needed on Thursday for our southern zones, however the wildfire smoke couple put a damper on daytime heating. Thus, we do not currently have any heat related headlines up for Thursday still, as this remains uncertain. Friday looks to bring more relief from the humidity at least, as drier dewpoints advect in from the north despite highs in the low to mid 90s remaining for some areas.

KEY MESSAGE 2...A cold front will pass through the region late this afternoon into tonight. Isolated to scattered thunderstorms will be possible in the afternoon and early evening hours for parts of the area, some of which could be severe.

Most of today is still expected to be dry, with increasingly hot and humid conditions ahead of an approaching cold front. However, concern has increased somewhat that isolated to scattered storms could form across the northern half of the area during the late afternoon hours and continue into the evening.

While mid-level ridging will be in place to start the day, it will slowly become suppressed to the south of the region through the afternoon hours. A shortwave looks to dig southeastward north of the region across portions of New England. While the shortwave appears likely to remain mostly north of the region, it will still cause height falls through the afternoon and into the evening, with H5 flow on the order of 50-55 kt overspreading the area. The environment during the afternoon and early evening hours ahead of the cold front will become strongly unstable, with SBCAPE values of 2000-2500 J/kg. With some mixing taking place, surface dewpoints will likely fall into the upper 60s F. Low-level lapse rates will be quite steep, with 0-3 km lapse rates near dry adiabatic. Mid-level dry air from subsidence will contribute to very high DCAPE values around 1200-1500 J/kg. With the enhanced mid-level flow overspreading the region, effective shear will range from 40-50 kt, greatest across northern portions of the area. All of this being said, the environment in place will be conditionally supportive of severe winds, and perhaps some marginally severe hail as well, for any stronger storms that are able to develop.

Forcing is still expected to be somewhat weak, so storm coverage is expected to be limited. We have maintained PoPs from the Philly metro northward to around 20%. Any isolated storms that develop should track southeastward into the evening hours before running into increasing inhibition and decreasing instability, therefore, it currently appears unlikely that any convection would make it much farther south than the Philly metro area.

KEY MESSAGE 3...Tonight's cold front passage will usher in smoke from Canadian wildfires, which looks to permeate the region 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 and Thursday night, and potentially lingering through Friday into 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 4...The weekend is shaping up to be active and unsettled with chances for showers and thunderstorms each day. Some storms could be severe and also may produce heavy rainfall leading to flash flooding.

We are still several days out, but are monitoring the severe weather setup as we get into the weekend.

Another cold front will approach on Friday into Friday night. The daytime Friday looks dry at this point, but showers and storms may increase in coverage into the overnight period.

Guidance still focuses storm chances when the front finally sinks into the region Saturday, when upper level support for forcing will also be maximized. The front will stay over or at least near the region into Sunday. As is often the case with these patterns following a building heat pattern mid week, there is some potential for both severe threats and heavy rain/flash flooding threat. At this early 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 threats. Precipitable water values will also be increasing to around 2+ inches. That is well about the 90th percentile for this time of year, increasing the risk for heavy rain/flash flooding.

There's uncertainty though regarding how fast this front pushes through and also whether it may get hung up near the area into early next week. If it moves through faster as the GFS is indicating, things could start to clear out by later Sunday afternoon with fair weather to follow for Monday. However the ECMWF hangs up the front over the area into next Monday with continuing chances for showers and storms. Regardless, chances for showers and storms will remain into early next week as well, but details are very unclear at the moment.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

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

Today...Primarily VFR. Smoke should remain north of our area through at least 21Z. Isolated storms are possible after around 20Z for RDG/ABE/TTN/PNE/PHL, which could lead to brief restrictions, but confidence is not currently high enough to include in the TAFs. West winds increasing to around 10 kts with gusts 15-20 kts. High confidence in prevailing VFR and winds.

Tonight...MVFR visibility from smoke appears increasingly likely to impact at least the I-95 terminals and north, beginning between 00-05Z from north to south. Westerly winds near 10 kts early will shift northwest or north around 5-10 kts following a frontal passage, which will also signal the beginning of visibility restrictions. Moderate confidence overall.

Outlook...

Thursday through Thursday night...MVFR visibility is likely to be prevalent during this period due to Canadian wildfire smoke, though could improve to VFR at times especially early in the day. Visibility could drop near IFR between 18Z and 00Z. Westerly winds 10-15 kts, with gusts up to 20 kts. High confidence in smoke resulting in visibility restrictions at times, but low confidence in details.

Friday through Sunday...Restrictions will be possible in isolated to scattered showers and storms, especially Saturday and Sunday. Smoke may also linger during this period as well, especially Friday.

MARINE

West to southwest winds 10-15 kts and seas 1-3 feet. An isolated thunderstorm with gusty winds possible north of Atlantic City this evening.

Smoke from Canadian wildfires will filter into the region overnight Wednesday night, potentially causing visibility restrictions as low as 3 NM.

Outlook...

Thursday through 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 during this period.

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

Rip Currents...

Today, west 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 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.

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

CLIMATE

A brief 1-day spike in temperatures is expected today, with highs likely to approach records. Here are the records for Wednesday, July 15:

Allentown/ABE 98 in 1995 AC Airport/ACY 100 in 1995 AC Marina/55N 99 in 1995 Georgetown/GED 96 in 2024 and 1993 Mount Pocono/MPO 92 in 1954 Philadelphia/PHL 103 in 1995 Reading/RDG 100 in 1995 Trenton/TTN 101 in 1995 Wilmington/ILG 99 in 1997 and 1995

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Heat Warning from 10 AM this morning to 8 PM EDT this evening for PAZ070-071-104-106. Heat Advisory from 10 AM this morning to 8 PM EDT this evening for PAZ054-055-060>062-101>103-105. NJ...Extreme Heat Warning from 10 AM this morning to 8 PM EDT this evening for NJZ010-012>015-017>020-027. Heat Advisory from 10 AM this morning to 8 PM EDT this evening for NJZ001-007>009-016-021>026. DE...Extreme Heat Warning from 10 AM this morning to 8 PM EDT this evening for DEZ001. Heat Advisory from 10 AM this morning to 8 PM EDT this evening for DEZ002>004. MD...Heat Advisory from 10 AM this morning to 8 PM EDT this evening for MDZ012-015-019-020. MARINE...None.