703
FXUS61 KPHI 011120
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
720 AM EDT Tue Sep 1 2026

.WHAT HAS CHANGED...
A Heat Advisory was issued for the Philadelphia urban corridor,
which is in effect from noon through 8 PM today.

There have been no significant changes to the forecast regarding
the severe thunderstorm potential through Thursday.

Aviation section updated for 12Z TAFs.

&&

.KEY MESSAGES...
1. Scattered severe thunderstorms are forecast to develop late
this afternoon and evening across portions of New Jersey and
eastern Pennsylvania. Damaging wind gusts, large hail, and a
tornado or two will be possible.

2. Hot and humid conditions are forecast across the region today
and again on Thursday. A Heat Advisory is in effect for the
Philadelphia urban corridor for today. The temperature forecast
for Wednesday continues to trend cooler, however it will remain
humid.

3. Unsettled conditions will continue Wednesday through
Thursday, with potential for severe thunderstorms both days. A
few thunderstorms are possible on Friday, mainly across
Delmarva.

4. Labor Day Weekend is forecast to feature mostly dry weather
with below normal temperatures and lower humidity.

&&

.DISCUSSION...
KEY MESSAGE 1...Scattered severe thunderstorms are forecast to
develop late this afternoon and evening across portions of New
Jersey and eastern Pennsylvania. Damaging wind gusts, large
hail, and a tornado or two will be possible.

Upper ridging will strengthen to our southwest today. A frontal
boundary will lift just north of the area by this afternoon as a
shortwave trough approaches from the Great Lakes region by this
evening. This is expected to result in scattered thunderstorms
developing in the vicinity of the frontal boundary by late
afternoon and especially into the evening hours.

The relatively slow moving shortwave trough and frontal
boundary sinking back into the area from the northeast into the
late evening and overnight hours may support isolated to
scattered showers and thunderstorms persisting well into the
overnight hours before diminishing toward dawn. However, the
greatest severe thunderstorm risk will be with the evening
activity, which will occur with the greatest diurnally driven
instability coinciding with the greatest upper support and
shear for convective development as the shortwave trough
arrives.

Timing for severe thunderstorms appears to be mainly between
about 5 PM and midnight tonight, though cannot totally rule out
an isolated storm popping up before that time frame. As
previously mentioned, convection will likely linger into the
overnight period, though the severe risk will have largely ended
by then.

Storms will initially be focused near where ever the frontal
boundary stalls out this afternoon, which guidance generally
has placed across northern New Jersey and into northeastern
Pennsylvania. As storms develop, cold pools will eventually
start forcing a modified frontal boundary toward the southwest
in the direction of the Lehigh Valley, Philly metro, and coastal
New Jersey. As this occurs, new storms will likely develop along
the boundary into the aforementioned areas farther southwest.

Instability will be quite high by early September standards,
upwards of 2000-3000+ J/kg MLCAPE. This combined with deep layer
shear on the order of 35-45 kts will support well organized
convective development, with discrete supercells and multicell
clusters favored. A few localized bowing line segments could
also materialize. Mid level lapse rates are not overly
impressive, mainly around 6.5 C/km. Low level shear is also not
overly impressive (~10-15 kts), especially owing to relatively
weak boundary layer winds, though the shear profile is such that
low level hodograph curvature and SRH (100-200 m2/s2) will be
supportive of a tornado risk. The tornado risk will be maximized
with any discrete cells that can latch onto the frontal boundary
or any antecedent outflow boundaries.

Damaging winds will be the primary threat with this setup,
though large hail and a couple tornadoes are also notable
threats. Storms should be moving fast enough to preclude any
significant threat of flash flooding, however any brief training
of cells could quickly drop 1-3" of rainfall. This may pose a
localized flash flood threat for urbanized areas, and any areas
that have received excessive rainfall in recent days.

SPC has maintained a Slight Risk (level 2 out of 5) for much of
our NJ and PA areas, as well as northern DE. The probability of
any storms developing at all is lower farther south into
Delmarva, though cannot totally rule out an isolated storm
there.


KEY MESSAGE 2...Hot and humid conditions are forecast across
the region today and again on Thursday. A Heat Advisory is in
effect for the Philadelphia urban corridor for today. The
temperature forecast for Wednesday continues to trend cooler,
however it will remain humid.

The broad upper level ridging building to our southwest this
week will bring hotter and humid conditions reaching into
portions of our area starting today. Afternoon highs are
forecast to range from the mid 80s north of I-78 to the upper
80s and low to even some mid 90s for most areas south of I-78,
about 5-10 degrees above average for early September. Dewpoints
will also remain mainly in the low 70s. This will lead to heat
indices climbing into the upper 90s to low 100s for many areas
south of I-78. Forecast maximum heat indices are right near 100
degrees for the Philadelphia urban corridor and south into far
southern NJ and Delmarva. Since this is right at Heat Advisory
criteria for the Philadelphia urban corridor, we`ve opted to go
ahead with a targeted advisory for this area. The Heat Advisory
is in effect from noon through 8 PM today.

A frontal boundary is anticipated to sink into the area by
Wednesday, where it will likely remain much of the day. This
will keep temperatures in check, at least for our NJ and PA
counties. Forecast high temperatures are mainly in the mid 70s
to mid 80s, though portions of Delmarva could make a run at the
low 90s. Dewpoints will still remain in the upper 60s to low
70s, so this won`t exactly feel like a huge improvement and
conditions will remain very humid despite cooler temperatures.

On Thursday, the frontal boundary is anticipated to lift north
once again, which will allow for high temperatures reaching into
the upper 80s to mid 90s again for many areas. These
temperatures combined with dewpoints in the low to mid 70s will
again result in heat indices well into the 90s to low 100s south
of I-78, perhaps even as high as 105 degrees for Delmarva.
Another Heat Advisory may need to be considered for Thursday if
the trend for hotter temperatures persists.


KEY MESSAGE 3...Unsettled conditions will continue Wednesday
through Thursday, with potential for severe thunderstorms both
days. A few thunderstorms are possible on Friday, mainly across
Delmarva.

We continue to monitor the potential for additional rounds of
severe thunderstorms Wednesday through Thursday. Friday could
also experience a few severe storms across Delmarva, depending
on how quickly the frontal boundary pushes south of the area
toward the end of the week.

SPC maintains a Slight Risk (level 2 out of 5) for severe
thunderstorms on Wednesday for much of the area, and a Slight
Risk has been introduced for Thursday across Delmarva.
Wednesday`s severe potential still looks to be conditional, as
a frontal boundary looks to be draped across our area with
potential for easterly winds and marine influence at the
surface across at least the northeastern half of our area. This
doesn`t mean there would be zero severe potential, but adds a
further degree of uncertainty to Wednesday`s setup.

The northwest flow pattern that will remain in place will
continue to be supportive of all severe threats, including
damaging winds, large hail, and a few tornadoes each day,
depending on how all the mesoscale details pan out. The threat
for flash flooding is typically lower with these patterns,
however given multiple days with the potential for storms and
recent heavy rainfalls, at least some risk for flash flooding
will exist each day as well.

Friday`s 15% probabilities for severe thunderstorms have shifted
just south of our Delmarva zones, though there is still some
uncertainty with this. Our latest forecast has about a 30%
chance of storms south of Dover in Delmarva.

The details regarding any severe thunderstorms beyond today
still remain unclear. Stay tuned for updates on these threats
over the coming days.


KEY MESSAGE 4...Labor Day Weekend is forecast to feature mostly
dry weather with below normal temperatures and lower humidity.

By Saturday, current model consensus indicates the frontal
boundary will be pushing south of our region with Canadian high
pressure building across the Northeast. This will favor an end
to any shower and thunderstorm potential. Thus, Labor Day
Weekend is still shaping up to feature nice weather, with
temperatures trending slightly below normal and lower humidity.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Any MVFR to IFR ceilings/visibility lift to VFR by 14Z.
Scattered thunderstorms developing after 21Z from northeast to
southwest. Have opted to carry VCTS everywhere, with PROB30 for
TSRA visibility restrictions. Any TSRA may be capable of produce
brief strong wind gusts and hail. West to southwest winds 5-10
kts. Low confidence on timing of TSRA, but probability of TSRA
at all terminals at some point after 22Z is at least 50-60%.
High confidence on prevailing VFR through the daytime hours.

Tonight...Scattered SHRA/TSRA prior to 06Z. MVFR/IFR ceilings
developing after 06Z. Variable winds early settling out of the
northeast around 5 kts after 06Z. Low confidence on timing of
restrictions.

Outlook...

Wednesday through Thursday...Restrictions possible in showers
and thunderstorms each day, mainly during the afternoon to
evening hours.

Friday through Saturday...Primarily VFR.

&&

.MARINE...
No prevailing marine hazards expected. South to southwest winds
around 5-15 kts. Seas 1-2 feet. Scattered thunderstorms
developing by evening from north to south along the coast into
the overnight hours. Winds and seas higher in and near any
thunderstorms. There is also potential for hail and waterspouts.

Outlook...

Wednesday through Thursday...No prevailing marine hazards
expected. Chances for thunderstorms each day, mainly in the
afternoon and evening hours.

Friday through Saturday...No marine hazards expected. Aside from
a low chance of a thunderstorm near the Delaware coast on
Friday, fair weather is expected.

Rip Currents...

For today, south-southwest winds will average around 10 mph.
Breaking wave heights around 1 to 2 feet with a 1 to 2 foot
southeast swell around 7 to 8 seconds. Given these conditions,
have opted to continue with a LOW risk for the development of
dangerous rip currents for both the Jersey Shore and Delaware
Beaches.

For Wednesday, east-northeast winds will average around 10 to
15 mph. Breaking wave heights will vary between 2 to 4 feet for
northern beaches, with 1 to 3 feet for southern beaches. A
south-southeast swell is expected with multiple periods. Given
the strengthening onshore flow and higher breaking wave heights,
have opted to go with a MODERATE risk for dangerous rip
currents for Monmouth, Ocean and Atlantic County Beaches in New
Jersey; while opting for a LOW risk for dangerous rip currents
for Cape May County in New Jersey and Delaware Beaches.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from noon today to 8 PM EDT this evening for
     PAZ070-071-102-104-106.
NJ...Heat Advisory from noon today to 8 PM EDT this evening for
     NJZ015-017>019.
DE...Heat Advisory from noon today to 8 PM EDT this evening for
     DEZ001.
MD...None.
MARINE...None.

&&

$$

DISCUSSION...AS
AVIATION...AS
MARINE...JD/AS