textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

The chance for isolated to scattered showers and storms, some of which could be severe, has increased slightly for Wednesday afternoon and evening.

Aviation section updated for the 00Z TAF issuance.

KEY MESSAGES

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

2. A cold front will pass through the region Wednesday afternoon into Wednesday night. Isolated to scattered showers and storms will be possible in the afternoon and early evening hours for parts of the area, some of which could be severe.

3. 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. Wednesday will be the hottest day of the stretch, when dangerously hot and humid conditions are expected. Extreme Heat Warnings and Heat Advisories are in effect for our entire forecast area.

Heat and humidity has begun building over the area today as an upper level area of high pressure over the central part of the country expands eastward towards the East Coast. A low pressure system moving through eastern Canada will result in warm advection strengthening into Wednesday.

On Wednesday, the heat will continue to build over the area. High temperatures should 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 limiting factor in how much we heat will be smoke from Canadian wildfires. We are already seeing the first plume of wildfire smoke dropping south into the the Mid-Atlantic this afternoon and that will insulate the region from the highest heat. The smoke should again help mitigate how much heating we receive tomorrow so while temps are going to be hot and there are no changes to any heat headlines, we are not fully maximizing the potential heat that the thermal profile suggests we could warm to.

A cold front looks to pass sometime late Wednesday 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. 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 Wednesday afternoon into Wednesday night. Isolated to scattered showers and storms will be possible in the afternoon and early evening hours for parts of the area, some of which could be severe.

Most of the day Wednesday 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 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. 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.

Forcing is still expected to be somewhat weak, so storm coverage is expected to be limited. We have increased PoPs from the Philly metro northward to around 20%, and to around 30% near and just north of I-78. 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...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. Some models are trending stormier with this period. For now though, the consensus blend of guidance is still mostly dry through this period, but will be watching trends within this period closely.

Regardless, most models are still focusing storm chances when the front finally sinks into the region Saturday. 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.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

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

Tonight...VFR. Southwest winds trending more west with time at 5-10 kt. LLWS out of the northwest at RDG and ABE at around 35 kt. High confidence.

Wednesday...Primarily VFR. Smoke could briefly bring down visibility below 6 miles. 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 kt with gusts 15-20 kt. Moderate confidence overall.

Outlook...

Wednesday night through Thursday night...Mainly VFR, however some smoke moving in from the wildfires in western Ontario could result in some MVFR visibility at times. Otherwise, no significant weather.

Friday through Sunday...Restrictions will be possible in isolated to scattered showers and storms. The chance for impacts to terminals currently appears greatest Saturday afternoon and evening.

MARINE

South to southwest winds increasing as the day progresses to near 15-25 kts, strongest north of Atlantic City. Seas building 3-5 feet. A Small Craft Advisory is in effect from 2pm through 2am issued for the New Jersey Atlantic coastal waters north of Great Egg Inlet. Winds and seas diminishing overnight.

Outlook...

Wednesday through Friday...No marine hazards expected. A few thunderstorms possible Wednesday evening.

Saturday through Saturday night...South to southwesterly winds could approach SCA criteria, with 10-15 kt winds and gusts 20-25 kt. Seas generally under 5 feet.

Sunday...No marine headlines currently anticipated with winds under 25 kt and seas under 5 feet.

Rip Currents...

On Wednesday, 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, 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 to 8 PM EDT Wednesday for PAZ070-071-104-106. Heat Advisory from 10 AM to 8 PM EDT Wednesday for PAZ054-055- 060>062-101>103-105. NJ...Extreme Heat Warning from 10 AM to 8 PM EDT Wednesday for NJZ010-012>015-017>020-027. Heat Advisory from 10 AM to 8 PM EDT Wednesday for NJZ001- 007>009-016-021>026. DE...Extreme Heat Warning from 10 AM to 8 PM EDT Wednesday for DEZ001. Heat Advisory from 10 AM to 8 PM EDT Wednesday for DEZ002>004. MD...Heat Advisory from 10 AM to 8 PM EDT Wednesday for MDZ012-015- 019-020. MARINE...Small Craft Advisory until 2 AM EDT Wednesday for ANZ450>452.