textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Aviation section updated for 06Z TAFs.

The Small Craft Advisory in effect for Tuesday evening was allowed to expire for the coastal waters from Sandy Hook to Little Egg Inlet. A new Small Craft Advisory has been issued for all of the nearshore Atlantic coastal waters from 12 PM Wednesday to 12 AM Thursday.

KEY MESSAGES

1. There is another chance 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 additional and more widespread afternoon and evening thunderstorms starting on Friday and lasting into the holiday weekend.

DISCUSSION

KEY MESSAGE 1...There is another chance for isolated thunderstorms across much of the area on Wednesday.

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 continue hinting at potential for some convective initiation within this environment, despite the strong subsidence. While coverage of any thunderstorms would be relatively isolated, any storms that do form could be capable of producing strong microbursts with damaging winds. SPC has highlighted our northern half of our CWA in a MARGINAL (level 1 out of 5) risk for severe thunderstorms, with a SLIGHT (level 2 out of 5) in the far north.

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 previously upgraded to an 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, and the highest heat indices are more probable on Friday.

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 additional and more widespread 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 more widespread afternoon and evening showers and thunderstorms starting on Friday, but especially on Saturday and Sunday. Currently, PoPs are around 10-20% 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 /06Z WEDNESDAY THROUGH SUNDAY/

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

Through 12Z...VFR. South to southwest winds remaining near 5-10 kts overnight. High confidence.

Wednesday...VFR. Very low risk of a shower or t-storm in the afternoon. South to southwest winds remaining near 5-10 kts with gusts upwards of 18 kt for KACY after 18Z. High confidence.

Wednesday night...VFR. Winds settling out of the southwest around 5-10 kts. High confidence.

Outlook...

Thursday through Saturday...VFR. Low chance (20%) of a thunderstorm Friday, better chance (30-40%) Saturday, otherwise no significant weather.

MARINE

The Small Craft Advisory has been allowed to expire for the coastal waters from Sandy Hook to Little Egg Inlet. A new Small Craft Advisory has been issued for all of the nearshore Atlantic coastal waters from 12 PM Wednesday to 12 AM Thursday.

Winds tonight are out of the south-southwest at 10-15 kt with gusts to 20 kt. Seas are 3-4 feet. Tomorrow, winds increase out of the south-southwest at 15-20 kt with gusts of 25 kt. Seas build to 4-6 feet. Through Wednesday night, winds and seas gradually lessen.

Outlook...

Thursday 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 Thursday afternoon and evening, mainly north of Atlantic City.

Rip Currents...

For Wednesday, breaking wave heights in the surf zone will be increasing to around 3 to 4 feet with S/SW winds around 25 mph. There will also be multiple swell groups and we'll still only be 2 days removed from the Full Moon. Given these factors, the latest indications are that shore areas in Cape May and Atlantic Counties in NJ will have a HIGH risk for the development of dangerous rip currents. For our remaining NJ shore zones as well as the Delaware Beaches, we'll continue with a MODERATE risk.

For Thursday, conditions look similar except both wind and wave heights look to come down some and we'll also be another day removed from Monday's Full Moon. For these reasons we're forecasting a MODERATE risk for the development of dangerous rip currents for our NJ shore zones and a LOW risk for the Delaware beaches.

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

Annual temperature extremes For Trenton (TTN), this may be the first year with temperatures both below zero and 100+ since 1988. The low so far this year was -1 on 1/21.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Heat Warning from noon today to 8 PM EDT Saturday for PAZ054-055-060>062-070-071-101>106. NJ...Extreme Heat Warning from noon today to 8 PM EDT Saturday for NJZ001-007>010-012>020-027. Extreme Heat Watch from Thursday afternoon through Saturday evening for NJZ021>026. High Risk for Rip Currents from 8 AM EDT this morning through this evening for NJZ024-025. DE...Extreme Heat Warning from noon today 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 noon today to midnight EDT tonight for ANZ450>455.