textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Extreme Heat Warning issued for all of southeastern Pennsylvania, northern Delaware, and portions of New Jersey.

Confidence continues to increase in a prolonged period of dangerously hot conditions beginning Wednesday and into the holiday weekend.

KEY MESSAGES

1. A warming trend in temperatures is expected to continue through today. There is some potential for a few thunderstorms mainly along and north of I-78 this afternoon and evening. There is also potential for isolated thunderstorms across much of the area on Wednesday. 2. Dangerous heat and humidity will build over the Northeast and Mid Atlantic Wednesday through Saturday. All time record high temperatures could be challenged Thursday and Friday.

3. There is the potential for afternoon and evening thunderstorms starting on Friday and lasting into the holiday weekend.

DISCUSSION

KEY MESSAGE 1...A warming trend in temperatures is expected to continue through today. There is some potential for a few thunderstorms mainly along and north of I-78 this afternoon and evening. There is also potential for isolated thunderstorms across much of the area on Wednesday.

A strengthening ridge of high pressure to our west will result in a warming trend in temperatures locally through today. Highs will be mainly in the upper 80s to low 90s this afternoon. With a lack of low level moisture return locally until later tonight, dewpoints are anticipated to mix out in the afternoon. So although it'll be quite warm, humidity won't be too oppressive until the heat ramps up on Wednesday. Lows in the mid 60s to low 70s tonight.

For this afternoon and evening, we could get some MCS activity coming into our region out of the eastern Great Lakes and southern Ontario region amid the northwesterly flow aloft. Latest CAMs suggest this MCS's remnants will stay mainly north of our area across Upstate New York and the Hudson Valley, but CAMs sometimes tend to place MCSs too far north. Will have to monitor this system as it progresses south today. Greatest chances of any thunderstorms will be focused mainly along and north of I-78 after 2 PM this afternoon through the evening. Some additional convection could develop later tonight as well. SPC's MARGINAL risk area is currently just to our north, so we'll continue to monitor the potential for any storms to drift south into our area. Damaging winds and frequent lightning would be the main threat with any thunderstorms today through tonight.

For Wednesday, instability is forecast to be high (potentially 3000 J/kg or greater) across the area, especially along and northwest of I-95 by the afternoon hours. Due to the strong subsidence and ridging building in from the west, there will also be a lot of mid level dry air contributing to high DCAPE (1000-1500 J/kg). Latest CAMs are hinting at potential for some convective initiation within this environment, despite the strong subsidence. While coverage of any thunderstorms would be very isolated, any storms that do form could be capable of producing strong microbursts with damaging winds. SPC has highlighted our northern CWA in a MARGINAL (level 1 out of 5) risk for severe thunderstorms.

KEY MESSAGE 2...Dangerous heat and humidity will build over the Northeast and Mid Atlantic Wednesday through Saturday. All time record high temperatures could be challenged Thursday and Friday.

Strong mid-level ridging is developing across the Mississippi Valley, Great Lakes, and Ohio Valleys early this week. Ridging will then build eastward into the Northeast and Mid Atlantic by Wednesday and persist into the holiday weekend. A west to southwest flow on Wednesday will shift more west to northwest by Friday. The ridge will finally start to break and retreat some starting Saturday into Sunday.

High temperatures will climb into the mid and upper 90s on Wednesday, and then from 100 to 105 degrees on Thursday and Friday. With a south to southwest flow on Wednesday, dewpoints are less likely to mix out significantly in the afternoon. Resulting heat indices are forecast to range mainly from around 100-105 degrees. By Thursday and especially Friday, the shift to a more west or northwest flow will allow for better mixing out of dewpoints during the afternoon, and also a component of downsloping (adiabatic warming) from the Appalachians. This will support temperatures climbing above 100 degrees as dewpoints mix out into the mid 60s during the afternoon hours. Resulting forecast heat indices are mostly in the 105-110 degree range. This is indeed the type of synoptic pattern that could challenge all time record high temperatures in our region. See the Climate section below for more information on record temperatures. Temperatures will abate somewhat on Saturday as the ridge begins to break and retreat to the southwest. However, high temperatures will still be in the mid 90s to near 100 degrees. The prevailing flow direction will dictate how much mixing out of dewpoints occur on Saturday. Currently, we have upper 60s to near 70 degree dewpoints, which results in 100-105 degree heat indices. Highs will remain in the low to mid 90s Sunday and Monday as well, with heat indices still in the mid 90s to low 100s. So we certainly could see heat related impacts continuing beyond Saturday despite a slight improvement in temperatures.

Given the increasing confidence in significant impacts from the 3 to 4 day excessive heat and humidity with heat indices reaching into the 100-110 degree range each day, the Extreme Heat Watch was upgraded to and Extreme Heat Warning for all of southeastern Pennsylvania, northern Delaware, and portions of New Jersey. This includes all areas where the previous watch was effective beginning Wednesday afternoon through Saturday. While heat indices may fall shy of warning criteria by Saturday, impacts will likely be worsened by the multiple days of heat and busy outdoor holiday festivities.

Elsewhere across far southern New Jersey, the New Jersey barrier islands, and Delmarva, the Extreme Heat Watch remains in effect beginning Thursday afternoon through Saturday. Within the remaining watch area, heat indices up to 100 degrees are possible on Wednesday, but the more notable heat won't begin until Thursday.

The mid-level ridge will break and retrograde back towards the south and west beginning 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 and Monday, as well as the potential for showers and thunderstorms for the holiday weekend (see Key Message 3 below).

KEY MESSAGE 3...There is the potential for afternoon and evening thunderstorms starting on Friday and lasting into the holiday weekend.

The mid-level ridge looks to break down and retrograde back to the south and west starting on Friday, and this will allow mid- level troughiness to develop over the Northeast. With several shortwaves riding over the top of the ridge and into the local area, this may support afternoon and evening showers and thunderstorms starting on Friday, but the better chances for convection arrive on Saturday and Sunday. Currently, PoPs are around 20-30% on Friday and 30-50% on Saturday and Sunday.

While details of this activity won't become clear until later this week, it is important to point out that any holiday weekend festivities could be impacted by thunderstorms (in addition to the extreme heat). The environmental setup would be favorable for strong to severe thunderstorms, if any storms do develop. We'll continue to monitor this potential.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

Today...VFR. Winds south to southwest and increasing to near 10 kts with gusts up to 18 kts after 18Z. High confidence.

Tonight...VFR. South to southwest winds remaining near 5-10 kts overnight. High confidence.

Outlook...

Wednesday through Friday...VFR. Low chance (20%) of a thunderstorm Wednesday and Friday, otherwise no significant weather.

MARINE

No marine hazards are forecast through most of today. However, southerly winds will increasing to 15-20 kts. Winds as high as 25 kts north of Barnegat Light. Seas increasing to 2-3 feet by evening. Winds and seas diminishing overnight.

A Small Craft Advisory was issued between 2 PM and midnight to highlight the potential for winds up to 25 kts later this afternoon and evening.

Outlook...

Wednesday through Friday...There is potential for marginal advisory criteria to be met with winds increasing to near 20-25 kts with seas nearing 5 feet both Wednesday and Thursday afternoon and evening, mainly north of Atlantic City.

Rip Currents...

For today, southerly winds increase to 15 to 25 mph with breaking waves in the surf zone around 1-3 feet. There will also be multiple swell groups and we'll only be one day removed from today's Full Moon. For these reasons, we'll go with a MODERATE risk of dangerous rip currents for coastal Ocean County south through Cape May County along the NJ shore. For coastal Monmouth County and the Delaware Beaches we'll continue with a LOW risk.

For Wednesday, similar conditions to today can be expected, but with slightly larger breaking waves. So we'll continue to go with a MODERATE risk of dangerous rip currents for coastal Ocean County south through Cape May County along the NJ shore. And for coastal Monmouth County and the Delaware Beaches we'll continue with a LOW risk.

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

CLIMATE

Record breaking heat is forecast from Wednesday through Saturday. Temperature records for our climate sites are listed below:

All Time Record High Temperatures

Site Record / Date Allentown (ABE) 105 / July 3, 1966 AC Airport (ACY) 106 / June 28, 1969 AC Marina (55N) 104 / August 7, 1918 Georgetown (GED) 104 / July 22 & 23, 2011 Mount Pocono (MPO) 103 / July 3, 1911 Philadelphia (PHL) 106 / August 7, 1918 Reading (RDG) 106 / July 22, 2011 Trenton (TTN) 106 / July 9, 1936 & July 22, 2011 Wilmington (ILG) 107 / August 7, 1918

All Time Record Warmest Low Temperatures

Site Record / Date Allentown (ABE) 79 / July 15, 1995 AC Airport (ACY) 84 / July 23, 2011 AC Marina (55N) 82 / July 21, 2019 Georgetown (GED) 84 / July 23, 2011 Mount Pocono (MPO) 76 / July 12, 1952 Philadelphia (PHL) 83 / July 24, 2010 & July 23, 2011 Reading (RDG) 81 / August 7, 1918 Trenton (TTN) 84 / August 11, 1895 Wilmington (ILG) 84 / August 7, 1918

Record Number of Consecutive 100+ Degree Days

Site Record / Date Allentown (ABE) 3 days / July 9-11, 1936 & July 2-4, 1966 AC Airport (ACY) 3 days / July 2-4, 1966 & July 22-24, 2011 AC Marina (55N) 1 day / Occurred multiple times Georgetown (GED) 2 days / Most recent June 24-25, 2025 Mount Pocono (MPO) 1 day / July 3, 1911 Philadelphia (PHL) 3 days / July 2-4, 1966 & July 8-10, 1993 Reading (RDG) 3 days / Most recent July 21-23, 2011 Trenton (TTN) 3 days / Most recent July 21-23, 2011 Wilmington (ILG) 3 days / Most recent July 9-11, 1936

Record High Temperatures July 1 Site Record / Year Allentown (ABE) 98 / 2018 AC Airport (ACY) 99 / 1968 AC Marina (55N) 98 / 1968 Georgetown (GED) 99 / 2012 Mount Pocono (MPO) 88 / 1963, 1964, 1968, & 2018 Philadelphia (PHL) 102 / 1901 Reading (RDG) 101 / 1901 Trenton (TTN) 98 / 1898, 1901, & 1945 Wilmington (ILG) 97 / 1931 & 1963

Record Warmest Low Temperatures July 1 Site Record / Year Allentown (ABE) 74 / 1930 AC Airport (ACY) 77 / 1945 AC Marina (55N) 79 / 1945 Georgetown (GED) 76 / 1945 Mount Pocono (MPO) 67 / 1946, 1971, 2013, 2017, 2025 Philadelphia (PHL) 82 / 1901 Reading (RDG) 76 / 1968 Trenton (TTN) 77 / 1945 Wilmington (ILG) 75 / 2013

Record High Temperatures July 2 Site Record / Year Allentown (ABE) 102 / 1966 AC Airport (ACY) 100 / 1966 AC Marina (55N) 97 / 1968 Georgetown (GED) 99 / 1953 Mount Pocono (MPO) 93 / 1911 Philadelphia (PHL) 103 / 1901 Reading (RDG) 102 / 1966 Trenton (TTN) 100 / 1901 Wilmington (ILG) 98 / 1941 & 1966

Record Warmest Low Temperatures July 2 Site Record / Year Allentown (ABE) 75 / 2013 AC Airport (ACY) 79 / 2002 AC Marina (55N) 76 / 2012 Georgetown (GED) 77 / 1959 Mount Pocono (MPO) 70 / 1903 Philadelphia (PHL) 82 / 1901 Reading (RDG) 79 / 1901 Trenton (TTN) 79 / 1901 Wilmington (ILG) 75 / 1941 & 2014

Record High Temperatures July 3 Site Record / Year Allentown (ABE) 105 / 1966 AC Airport (ACY) 104 / 1966 AC Marina (55N) 99 / 1919 Georgetown (GED) 101 / 1954 Mount Pocono (MPO) 103 / 1911 Philadelphia (PHL) 104 / 1966 Reading (RDG) 103 / 1898 & 1966 Trenton (TTN) 102 / 1966 Wilmington (ILG) 102 / 1966

Record Warmest Low Temperatures July 3 Site Record / Year Allentown (ABE) 75 / 2018 AC Airport (ACY) 78 / 2018 AC Marina (55N) 76 / 1983, 2002, & 2018 Georgetown (GED) 79 / 1953 Mount Pocono (MPO) 69 / 2013 Philadelphia (PHL) 77 / 1876, 1901, &2002 Reading (RDG) 76 / 2018 Trenton (TTN) 76 / 1901 & 1966 Wilmington (ILG) 76 / 2002

Record High Temperatures July 4 Site Record / Year Allentown (ABE) 102 / 1949 AC Airport (ACY) 102 / 1966 AC Marina (55N) 99 / 1966 Georgetown (GED) 100 / 1966 Mount Pocono (MPO) 99 / 1911 Philadelphia (PHL) 103 / 1966 Reading (RDG) 100 / 1911 & 1966 Trenton (TTN) 99 / 1919, 1999, & 2010 Wilmington (ILG) 102 / 1966

Record Warmest Low Temperatures July 4 Site Record / Year Allentown (ABE) 75 / 1999 AC Airport (ACY) 75 / 1974, 1983, 1999, & 2002 AC Marina (55N) 79 / 2002 Georgetown (GED) 75 / 1999 & 2012 Mount Pocono (MPO) 70 / 2002 & 2013 Philadelphia (PHL) 79 / 1919 Reading (RDG) 78 / 1966 Trenton (TTN) 78 / 1966 Wilmington (ILG) 76 / 1999

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Heat Warning from noon Wednesday to 8 PM EDT Saturday for PAZ054-055-060>062-070-071-101>106. NJ...Extreme Heat Warning from noon Wednesday to 8 PM EDT Saturday for NJZ001-007>010-012>020-027. Extreme Heat Watch from Thursday afternoon through Saturday evening for NJZ021>026. DE...Extreme Heat Warning from noon Wednesday to 8 PM EDT Saturday for DEZ001. Extreme Heat Watch from Thursday afternoon through Saturday evening for DEZ002>004. MD...Extreme Heat Watch from Thursday afternoon through Saturday evening for MDZ012-015-019-020. MARINE...Small Craft Advisory from 2 PM this afternoon to midnight EDT tonight for ANZ450-451.