881
FXUS61 KPHI 050031
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
831 PM EDT Thu Jul 4 2024

.SYNOPSIS...
High pressure will remain located offshore through Friday.
Several waves of low pressure and weak fronts approach the area
through Saturday, before a cold front crosses through the area on
Saturday night. High pressure returns for Sunday and Monday before
another low pressure system approaches the area around the middle of
next week.

&&

.NEAR TERM /THROUGH FRIDAY NIGHT/...
An area of showers and thunderstorms have picked up and
intensified over Berks county and western portions of the Lehigh
Valley. These storms are moving southeast towards the Fall Line
and may get as far south as the Philadelphia metro area.
Guidance suggest these storms should lose their intensity over
the next hour or two. However, there has been some backbuilding
in the Reading area, suggesting localized flash flooding is
possible. Although there is a loss of diurnal heating owed to
sunset, it remains quite unstable with with 1500 to 2000 J/kg of
SB CAPE and up to 1500 J/kg of MU CAPE. 0-6 km Bulk Shear up to
30 to 40 kt and PWATs are just over 2 inches. PoPs range from
high chance over the western zones to low chance/slight chance
over the far eastern and southern zones. 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. Also, WPC keeps most of our area in
a MARGINAL risk for excessive rain as storms will be capable of
producing very heavy rain which could lead to localized flash
flooding, especially if storms set up over urbanized areas.

Once storms taper off, a weak frontal boundary will remain
draped across the region west to east over eastern PA into NJ
and this will extend back to a new surface low developing over
the midwestern states. Meanwhile weak impulses aloft will
continue to move through in the upper level flow. For this
reason there could be some additional showers and a few storms
overnight...mainly over eastern PA into adjacent portions of NJ.
It will otherwise 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.

A very moist and warm to hot air mass will be in place for
Friday with 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, with heat advisory
remaining in effect starting Friday morning for I-95 corridor
and Delmarva (now extended through Saturday) and 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 the
high dew points, expect heat indices around I-95 to pass 100 and
Delmarva to pass 105 in some spots. There isn`t a strong source
of forcing for convection, but with all the heat and
instability, along with the lingering weak front there will
likely be some more scattered showers and storms developing for
the afternoon into the early evening period, especially around
and 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 linger Friday night as a 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.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY/...
A similar weather story is expected on Saturday as the
extremely moist airmass continues. Although temps may be a
degree or two cooler than Friday mostly due to cloud cover
expected in the afternoon, dew points are anticipated to be at
least 2-3 degrees higher. In addition, PWATs are expected to
remain anomalously high in excess of the 95 percentile, which
essentially tells the story itself. With this in mind, the two
concerns for Saturday remain being the heat and the other being
the thunderstorm potential with approaching cold front.

In terms of the heat, it is forecast that highs will reach into
the upper 80s to low 90s across much of the area. With dew
points in the 74-77 degree range, this equates to heat indicies
around or in excess of 100 degrees for most of the immediate
I-95 corridor, with heat indicies around 105 degrees across the
Delmarva. Heat indicies in the 90s are expected up across the
Lehigh Valley and north Jersey. As a result, this warrants Heat
Advisory criteria for the same areas as Friday in addition to
Middlesex and Somerset Counties, which have been issued with
this forecast update.

In terms of thunderstorm potential, there isn`t too much
forcing present as the front does appear to rather weak. Despite
the moisture rich atmosphere, it doesn`t appear that there will
be too much of a severe threat this go around which supports
the thinking from the SPC of just a general thunderstorm risk
for the region. The main concern should be in terms of heavy
rainfall thanks to the the moisture-loaded atmosphere where any
storm will be capable of heavy rain. As a result, the WPC has
the eastern half of the forecast area in a Marginal Risk (Level
1/4) for excessive rainfall for Saturday and Saturday night.

The cold front slowly crosses the area on Saturday night with
drier air gradually filtering into the region in its wake.
Overall, this should lead to a pretty nice day on Sunday under a
mix of sun and clouds as high pressure builds in from the north
and west. Although afternoon temps will reach into the upper
80s to around 90 degrees, it will feel a bit drier compared to
Saturday as dew points fall into the mid 60s. Mostly clear skies
are expected to continue into Sunday night as high pressure
persists. Lows for Saturday and Sunday night will both be in the
mid 60s to low 70s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
High pressure remains in control through Monday with above
normal temps continuing. A slight chance of a shower or
thunderstorm may approach our western counties late in the day
as the next weather system approaches toward the middle portion
of next week. An upper trough will be located over the Great
Lakes on Tuesday and Wednesday as some shortwave energy
spreading east into our neck of the woods. Due to uncertainty of
timing and spread amongst model guidance, heavily based the
forecast on NBM guidance. This generally yields a 30-50% chance
of showers and/or thunderstorms each day through Thursday. This
is not an indication that any one day will be a washout, instead
there will likely be some shower activity around each day as
variability exists amongst current forecast guidance. Temps for
the middle portion of the week are expected to remain seasonable
to a few degrees above normal.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas...

Through tonight...Scattered SHRA/TSRA may affect the terminals
into 04Z or so, and restrictions are likely should a storm pass
over a given terminal. Best chances for this look to be a RDG,
ILG, PHL, PNE. Showers/storms diminish by the late evening
however some areas of fog/haze and/or stratus development are
possible overnight with MVFR restrictions. Winds diminish to
around 5 kts or less, favoring a southerly direction. Overall
lower than average confidence given the uncertainty regarding
both the convection and timing/degree of overnight restrictions.

Friday...Lingering restrictions in the morning should give way
to period of VFR by around midday with further restrictions
possible by later in the day due to more showers storms. South
to southwest winds 5 to 10 knots. Moderate confidence.

Outlook...

Saturday through Saturday night...Sub-VFR probable with a chance of
showers and thunderstorms. Locally gusty winds possible in
thunderstorms.

Sunday through Monday...VFR. No significant weather expected.

Monday night through Tuesday...Sub-VFR conditions possible. A slight
chance of a shower or thunderstorm.

&&

.MARINE...
No marine headlines anticipated through tonight. Southerly winds 10
to 20 knots with seas 2 to 4 feet. Scattered showers and
thunderstorms possible during the evening hours, but otherwise fair
weather is expected.

Conditions should stay Sub SCA for Friday but by Friday night both
winds and seas could approach Small Craft Advisory levels.

Outlook...

Saturday through Saturday night...SCA conditions likely due to seas
around 4-6 feet. Winds are likely to remain below 25 kt during this
period, however. A chance of thunderstorms possible.

Sunday through Tuesday...No marine headlines expected. Winds are
expected to remain below 20 kt and seas of 2-4 feet. Fair weather
through Monday, with a slight chance of a thunderstorm on Tuesday.

Rip Currents...

Friday and Saturday...A New Moon will occur on Friday. Winds
will be around 10-15 MPH out of the south both days, with an
onshore component to the wind in some spots. 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 4 feet within the moderate risk and 1 to
3 feet within the low risk.

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.

With continuing S/SE flow and the approach of the New Moon, some
guidance has areas of Cumberland County NJ seeing advisory minor
coastal flooding with the upcoming evening high tide cycle. Though
guidance may be running a bit high, a short-fused coastal flood
advisory may be needed later on.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 10 AM Friday to 8 PM EDT Saturday for
     PAZ070-071-102-104-106.
NJ...Heat Advisory from 10 AM Friday to 8 PM EDT Saturday for
     NJZ010-012-015>019.
DE...Heat Advisory from 10 AM Friday to 8 PM EDT Saturday for
     DEZ001>003.
MD...Heat Advisory from 10 AM Friday to 8 PM EDT Saturday for
     MDZ012-015-019-020.
MARINE...None.

&&

$$

SYNOPSIS...DeSilva
NEAR TERM...Fitzsimmons/MPS/Staarmann
SHORT TERM...DeSilva
LONG TERM...DeSilva
AVIATION...DeSilva/Fitzsimmons
MARINE...Fitzsimmons/Hoeflich
TIDES/COASTAL FLOODING...