628
FXUS61 KPHI 041428
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1028 AM EDT Thu Jul 4 2024

.SYNOPSIS...
High pressure will remain east of the area through the weekend.
Low pressure moves well north of the area today and sends a few
weak fronts towards eastern Pennsylvania and northern New
Jersey. These fronts remain in the area through Saturday before
diminishing. Another system arrives for Tuesday of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
1015 AM...For the mid morning update, we reduced POPs some for
the first part of the afternoon based on the latest trends in
obs and high resolution models. It`s looking more and more like
showers/storms across the area will hold off until at least
around 2 PM if not even a bit later until around 4-5 PM. Until
then it will be increasingly hot and muggy under variable but
generally increasing cloud cover. High temperatures for most
areas will reach the upper 80s to low 90s (cooler near the
coast and over the Poconos) with dewpoints reaching the upper
60s to low 70s. This will result in max heat indicies well into
the mid to upper 90s for most areas along and south of I-78. Winds
will be from the south to southwest near 5-10 mph (15-20 mph
near the coasts).

By later this afternoon and evening, expect storms to fire in
the vicinity of a weakening frontal boundary draped over PA and
lee side thermal trough by the mid afternoon hours. The
convection should move generally eastward into our eastern PA
and northern NJ zones by later in the afternoon, and then
potentially affecting Delmarva and southern and coastal NJ by
the evening. So it won`t be an all day type rain event, but
unfortunately the timing won`t be great as it could interfere
with Independence Day festivities. Lack of synoptic forcing is
the main limiting factor, although the unstable air and subtle
surface forcing mechanisms should overcome this to some degree.
CAMs will likely struggle with today`s setup. Our POPs generally
max out around 40% to 50% over much of the area, though are a
bit lower near 30% to 40% near the coast and across Delmarva and
a bit higher (around 60%) around Chester and Berks Counties in
SE PA where the greatest instability is anticipated.

With ML CAPE values progged to max out around 1000-1500 J/kg
along with deep layer shear values around 25 knots, there is the
potential some storms could develop some organization and
become severe. The SPC has a MARGINAL risk (1/5) for severe
thunderstorms extending east into SE PA and Delmarva. Water
loaded downdrafts could lead to a few strong to locally damaging
wind gusts. Given the shear, convection could exhibit some
organization, though widespread severe convection is not
expected. Most guidance has PWats over 2.0" across the entire
area, and largely within the 2.25" to 2.5" range. These values
are near record PWat values for the month of July at IAD and OKX
(read: highly anomalous). WPC keeps most of our area in a
MARGINAL risk for excessive rain. Storms will be capable of
producing very heavy rain which could lead to localized flash
flooding, especially if storms set up over urbanized areas.

Storms may linger into a good portion of the evening before
diminishing overnight. It will be a very warm and muggy night as
weak warm advection persists. Most areas won`t see lows getting
below the low/mid 70s (except 60s over the higher terrain NW).
Some areas of fog development are possible overnight, especially
where appreciable rainfall occurs. Lingering smoke from evening
fireworks will make for ideal cloud droplet nuclei in the very
warm and humid environment with light winds, despite the broken
cloud cover overnight.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Extremely moist and rather warm air mass will dominate Friday
and Saturday, with PW`s over 2 inches, dew points well into the
70s, and high temperatures approaching or exceeding 90 in many
spots. This will bring two main issues... 1, the excessive heat
risk, with heat advisory now in effect Friday for I-95 and
Delmarva and possibly needed Saturday... 2, the thunderstorm
risk due to increasing instability. We likely start out fairly
dry, perhaps with patchy fog/low clouds, early Friday before
some clearing brings quick warming to the low 90s for much of
the area, mid 90s possible in Delmarva. With high dews, expect
heat indices around I-95 to pass 100 and Delmarva to pass 105.
There isn`t a strong source of forcing for convection, but with
all the heat and instability, along with the weakened front
which drifts into the area tonight, plus potential lee side
trough and a weak shortwave passing aloft, its quite possibly
enough to get a round of storms going, especially northwest of
I-95 where we`ve placed the highest POPs. With plenty of
moisture available, locally heavy rain is likely biggest
concern, but with the available instability some isolated severe
certainly looks possible.

Shower and thunderstorm chances likely linger Friday night as a
leading shortwave crosses the area, increasing forcing even as
instability decreases. While severe/flood risks likely diminish
a bit with loss of heating, will still need to be on guard after
sunset for isolated problems. Otherwise, a steamy night with
lows mostly in the 70s.

Another front approaches on Saturday, reading to tap the very
steamy air mass, though upper forcing is a bit weaker than
during the night Friday night. However, with the soupy air mass
and potential slowing of the front/storms running parallel to
the boundary for a time, have kept high chance/low likely pops
with at least isolated flooding and severe weather remaining
risks. It is notable that guidance has trended down a bit in
temps, more into the 80s to near 90, likely due to greater cloud
cover/precip coverage, and this may reduce instability a little,
limiting hazardous weather risks, as well as keeping us slightly
below heat advisory levels.

Front slowly clears us out Saturday night, with drier air
trickling southeastward and showers/storms ending northwest to
southeast. Cooler spots manage to drop into the 60s.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Dew points drop into the 60s as high pressure briefly dominates
Sunday and Monday, with little chance of precip and decent
sunshine. Highs should manage high 80s to low 90s however, with
lows in the 60s to low 70s. Lower dew points keep us just below
heat advisory levels, with Monday being the slightly hotter and
more humid day of the pair.

Uncertainty grows toward mid-week, but looks likely additional
shortwave energy with surface frontal boundaries heading
eastward into the area, combined with greater flow of moisture
heading northward ahead of them, will lead to some showers and
storms Tuesday and Wednesday. Still a lot to be worked out on
the details. Dews will return to the 70s but temps may struggle
to reach 90 with greater cloud cover and precip coverage.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Prevailing VFR conditions with increasing clouds.
Scattered showers and thunderstorms are expected to develop
mainly after 18z, and these could bring restrictions if any of
these storms move over a TAF site. The best chances (50-60%
chance) for this look to be around RDG/ABE with lower chances
(30-50% chance) farther south and east toward I-95 (PHL), MIV,
and ACY. Winds mainly south to southwest around 5-10 kts, except
10-15 kts around ACY. Moderate confidence overall, low
confidence exactly where storms will impact terminals.

Tonight...Initially VFR outside of any lingering showers or
thunderstorms, however some areas of fog/haze and/or stratus
development are possible with MVFR restrictions. Winds diminish
to around 5 kts or less, favoring a southerly direction. High
confidence in initially prevailing VFR conditions, but low
confidence on the occurrence and timing of any restrictions in
fog/haze/stratus overnight.

Outlook...

Friday through Saturday night...MVFR/IFR conditions possible due
to chance TSRA and potential low clouds/fog during late
night/early morning periods when precip tapers and higher clouds
break.

Sunday through Monday...Mainly VFR with NO SIG WX currently
anticipated.

&&

.MARINE...
No marine headlines anticipated through tonight. Southerly
winds increasing into the afternoon to around 15-20 kts, then
decreasing overnight to near 10 kts. Seas building to 3-4 feet.
Scattered showers and thunderstorms possible during the evening
hours, but otherwise fair weather is expected.

Outlook...
Friday...Sub-SCA. Wind gusts to 15 kt. Wave heights up to 4
feet. Slight chance TSRA.

Friday night through Saturday night...Areas of SCA likely, with
highest probability on Saturday. Wind gusts to 25 kt. Wave
heights up to 6 feet. Chance TSRA.

Sunday through Monday...Sub-SCA likely. Winds less than 15 kts.
Wave heights less than 5 feet. Slight chance of TSRA Sunday, NO
SIG WX thereafter.

Rip Currents...

Today...Southerly winds strengthen with breaking wave heights
increasing. With an onshore component to the wind for Cape May,
Atlantic, and Ocean County, there is a MODERATE risk for rip
currents. With wind direction more shore parallel/offshore at
the Delaware Beaches and northern Jersey Shore, have elected to
go with a LOW risk. Breaking wave of 1 to 3 feet, though some 4
foot waves are possible within the MODERATE risk area.

Friday...A New Moon will occur. Winds weaken a bit from
Thursday but become more onshore for much of the NJ shore. There
is a MODERATE risk for rip currents for Cape May, Atlantic,
Ocean counties in NJ, and a LOW risk for Monmouth NJ and the
Delaware Beaches. Breaking waves of 2 to 3 feet.

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

&&

.TIDES/COASTAL FLOODING...
With the New Moon to occur Friday, some locations may see some
spotty minor coastal flooding during the next few high tide cycles.
Any advisory minor coastal flooding is not expected at this time.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 10 AM to 8 PM EDT Friday for PAZ070-071-104-
     106.
NJ...Heat Advisory from 10 AM to 8 PM EDT Friday for NJZ015>019.
DE...Heat Advisory from 10 AM to 8 PM EDT Friday for DEZ001>003.
MD...Heat Advisory from 10 AM to 8 PM EDT Friday for MDZ012-015-019-
     020.
MARINE...None.

&&

$$

SYNOPSIS...OHara/RCM
NEAR TERM...Fitzsimmons/Staarmann
SHORT TERM...RCM
LONG TERM...RCM
AVIATION...Fitzsimmons/OHara/RCM/Staarmann
MARINE...OHara/RCM/Staarmann
TIDES/COASTAL FLOODING...