685
FXUS61 KPHI 202307
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
707 PM EDT Mon Jul 20 2026

.WHAT HAS CHANGED...
Updated aviation section for 0z TAFs

&&

.KEY MESSAGES...
1. Several rounds of showers and thunderstorms return Tuesday into
Wednesday with an increasing potential for severe weather and flash
flooding.

&&

.DISCUSSION...
KEY MESSAGE 1...Several rounds of showers and thunderstorms return
Tuesday into Wednesday with an increasing potential for severe
weather and flash flooding.

Beginning early Tuesday morning, a warm front will lifts northward
throughout the day. As this warm front lifts northward, winds turn
more southerly with moisture being transported northward. PWAT
values are forecast to surge upwards of 2.00-2.25 behind the front.
As this front lifts north, hi res guidance continues to show
isolated to scattered showers and thunderstorms to develop along the
front, bringing periods of heavy downpours during the morning rush
hour. Areas of greatest concern will be mainly around the I-78 and I-
80 corridors across northeast PA and northern NJ, some areas of
which were hit hard with flash flooding just this past weekend. At
some point, the warm front will stall nearby the NJ/NY/PA border,
but it is uncertain where. The area of greatest flash flood risk
will likely be where the front stalls. A general 1-2 inches of rain
can be expected from this first of many rounds of precipitation with
locally higher amounts. While this may not sound significant, many
locations, especially in northern NJ, remain vulnerable to excessive
rainfall after this past weekend. As such, rainfall rates of 1-2
inches per hour at times will be sufficient to result in scattered
incidents of flash flooding during the critical morning rush hour
Tuesday. Furthermore, there is a nonzero severe threat with these
storms given the deep layer shear present along with the presence of
the frontal boundary. All this said, high uncertainty remains with
the timing and overall coverage of the morning convection.

As the day progresses into the late morning and the afternoon, some
guidance continues to develop additional isolated showers and
thunderstorms across the area. The second round of thunderstorms
will be developing across PA during the afternoon hours, which carry
the greatest severe potential with widespread coverage as an upper
level trough moves through. With PWAT values near or over 2 inches
along with instability building to around 2000-3000 J/kg or greater
and deep layer shear in the 30-40 kt range, the environment will be
favorable for severe thunderstorms with the primary hazard being
damaging wind gusts, though a tornado or two cannot be ruled out.
With that potential, the Storm Prediction Center has upgraded the
region to an an ENHANCED (3 out of 5) risk for the entire region.
The fly in the ointment for the severe potential will be how robust
the morning convection ends up being. More thunderstorm coverage in
the morning could mean less severe potential for the I-78 corridor
and north should the atmosphere not recover in time. Regardless, the
risk for severe weather remains with uncertainty remaining elevated
as well. What is more certain, however, is the potential for
torrential downpours from these thunderstorms. Rainfall rates around
2 inches per hour will be possible with this round of storms as
well, bringing the risk of flash flooding to the rest of the region.

As this second round of storms move through the region, hi res
guidance continues to suggest additional rounds of storms thereafter
as the cold front pushes in from the west, in which a disorganized
and chaotic storm mode could begin to take hold by the evening into
overnight hours. The severe risk will begin to gradually diminish
through the evening as the environment gets worked over, but
flooding concerns will persist through at least 2AM with the
continue risk for heavy downpours and the possibility of training
storms given the messy and widespread coverage of thunderstorms
expected over the urban corridor. Thunderstorm coverage could
continue into the early morning hours of Wednesday, but hi res
guidance suggests most storms will be concentrated along the coastal
plains by that point. By the time we reach the overnight hours, an
additional 1-2 inches could have fallen across the region with
locally higher amounts.

Given the potential for flash flooding across the urban corridor, a
Flash Flood Watch was issued from 6AM Tuesday through 2AM Wednesday
morning.

On Wednesday, the SPC and WPC continue to have portions of
Delmarva and southern NJ outlooked for severe weather and a risk
for excessive rainfall, respectively. Part of this set-up will
depend on the placement of the front and timing but latest CAM
guidance continues to emphasize re-development of convection
along the coastal plains of Delmarva and New Jersey through much
of the day Wednesday. Primary threat with any severe storms
is likely to be damaging wind gusts in addition to heavy
rainfall leading to flash flooding, particularly in the more
urbanized areas along the immediate coast.

Once the front moves away by late Wednesday, we are drier for
Thursday and beyond into the weekend.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...Mainly VFR through around 09Z. MVFR after 09Z with
VCSH/VCTS. Southeast winds around 5 kt or less. High confidence.

Tuesday...Mainly MVFR cigs to start with iso/sct SHRA/TSRA in the
morning around PHL and north. More widespread TSRA in the afternoon.
Periods of sub-VFR in TSRA. Winds generally SSE early in the day
veering to SSW by mid to late afternoon and increasing to around
10 gusting up to 20 knots. Moderate confidence in storm
coverage, low confidence in storm timing.

Outlook...

Wednesday...Sub-VFR conditions likely at times in SHRA/TSRA.

Thursday through Saturday...VFR. No significant weather.

&&

.MARINE...
Sub-advisory conditions will prevail tonight. Winds around 10 to 15
kts or less with seas of 2-3 feet.

Winds and seas reach SCA criteria by Tuesday afternoon and linger
into the overnight. Winds will gust to 25-30 kts at times with seas
building to 5-7 feet.

Outlook...

Wednesday...SCA conditions continue with seas in excess of 5
feet and could linger into Wednesday night. SCA remains in
effect thru Wednesday afternoon for the ocean zones.

Thursday through Saturday...Conditions are expected to remain
below SCA criteria.

Rip Currents...

For Tuesday, winds increase out of the south to around 15-20
MPH, with either a shore parallel or slight onshore flow
expected depending on location. Breaking waves around 2 to 4
feet as well will result in more energy within the surf zone. As
a result, elected to go with a MODERATE risk at all beaches.

For Wednesday, winds turn a bit more southwesterly but still
around 10-20 MPH. However, breaking wave heights are expected to
increase, with breaking waves as high as 5 feet possible. Still
a 7 second period though which will mitigate the rip current
threat some. However, thinking a MODERATE risk is sufficient for
Wednesday`s threat.



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

&&


.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch from 6 AM EDT Tuesday through late Tuesday night
     for PAZ070-071-101>106.
NJ...Flood Watch from 6 AM EDT Tuesday through late Tuesday night
     for NJZ008>010-012>015-017>019.
DE...Flood Watch from 6 AM EDT Tuesday through late Tuesday night
     for DEZ001.
MD...None.
MARINE...Small Craft Advisory from noon Tuesday to midnight EDT Tuesday
     night for ANZ430-431.
     Small Craft Advisory from noon Tuesday to 6 PM EDT Wednesday
     for ANZ450>455.

&&

$$

DISCUSSION...AKL/MJL
AVIATION...AKL/Fitzsimmons/MJL
MARINE...AKL/Hoeflich/MJL