textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Extreme Heat Warning issued for the urban corridor and adjacent areas of central and southern New Jersey. Heat Advisory issued for portions of the Eastern Shore of Maryland.

Small Craft Advisory issued for Atlantic coastal waters of New Jersey north of Great Egg Inlet.

Aviation section updated for 12Z TAFs.

KEY MESSAGES

1. Above normal temperatures beginning today and continuing 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. 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 beginning today and continuing 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 will begin to build 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. This will occur as a low pressure system moves through eastern Canada, resulting in warm advection strengthening into Wednesday.

Highs today will generally range from the upper 80s to the mid 90s, but dewpoints and humidity will only be in the moderate range so conditions are expected to stay below heat advisory or warning levels. Maximum heat indices today will be in the mid to upper 90s from around the Philly metro and north. However, Wednesday will be a different story as the heat continues 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.

A mostly dry cold front looks to pass sometime late Wednesday which will knock down temperatures and dewpoints a bit, but medium range guidance and ensembles indicate above normal temperatures with highs ranging from the upper 80s into the low/mid 90s could remain through the end of the week and potentially into the weekend. The front should be mostly dry, but would not be surprised to see a few isolated storms developing along it late Wednesday afternoon and evening, mainly across the Lehigh Valley, Poconos, and northern New Jersey north of I-195. The latest HRRR and RAP guidance support near surface smoke advecting in from the wildfires in western Ontario behind the front as well, unfortunately. This will be preceded by smoke and haze aloft beginning today, but not reaching the surface until Wednesday from north to south as the day progresses. The smoke could impact temperature forecasts.

Thursday will still be quite hot, with maximum heat indices in the upper 90s to low 100s along and southeast of I-95. While we aren't explicitly forecasting advisory criteria to be reached, it's 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...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. At this point, the cold front that comes through the area late Wednesday into early Thursday looks to be mainly dry with just some spotty storms possible over the southern Poconos into northern NJ late Wednesday.

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 /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. Southwest winds increasing to around 10-12 kts by 16Z. Some gusts 18-20 kt possible. High confidence.

Tonight...VFR. Southwest winds 5-10 kts. High confidence.

Outlook...

Wednesday 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 Saturday...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 was issued for the New Jersey Atlantic coastal waters north of Great Egg Inlet valid 2 PM to 2 AM. Winds and seas diminishing overnight.

Outlook...

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

Saturday...Winds could get near advisory levels by the afternoon.

Rip Currents...

For today, flow largely turns offshore with a 7 to 9 second period swell. Wave heights remain around 2 feet or less as well, resulting in a LOW risk for rip currents for most of the Jersey Shore and Delaware Beaches. The exception is along the southern New Jersey coast in Atlantic and Cape May counties, where south facing beaches look to have some onshore flow in the afternoon as winds turn back a bit more southerly with a sea breeze. Waves in the surf zone look to increase up to 3 feet, and as a result there is a MODERATE risk for the development of dangerous rip currents for this area.

For Wednesday, flow looks to remain offshore all day across the entire coast, with a 6 to 8 second period swell. Wave heights will be around 2 feet or less, resulting in a LOW risk for rip currents for all 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 from 2 PM this afternoon to 2 AM EDT Wednesday for ANZ450>452.