249
FXUS61 KPHI 042011
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
411 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/...
330 PM...As of mid afternoon, we are starting to see some scattered
showers and thunderstorms developing towards central PA due to a
combination of daytime heating, terrain features, and warm front
that`s transitioning to becoming more of a lee side trough.
Meanwhile, still pretty quiet farther east over PHI forecast area
but becoming increasingly hot and humid with cu developing. As we go
through the next several hours (late afternoon through the first
half of this evening) expect the convection to our west to become at
least a bit more widespread and also move eastward into eastern PA
towards northern Delmarva and the I-95 corridor. So 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. Our POPs through
this evening range from around 50-60 percent over our western zones
such as Berks and Chester Counties in PA to around 40 to 50 percent
near the I-95 corridor, then decreasing farther to around 20 to 30
percent near the coast. ML CAPE values are increasing to around 1000-
1500 j/kg with deep layer shear around 25-35 knots and PWATS of 2.1+
inches. So for these reasons we are still thinking 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. 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.

We are generally expecting that storms will diminish through the mid
to late evening. However 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 /20Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas...

Through tonight...Scattered showers and storms may affect the
terminals through the first part of this evening bringing
restrictions if they do in fact move over any of the TAF sites. Best
chances for this look to be a RDG, ILG, PHL, PNE where we`ve
included TEMPO groups. 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.
Any advisory minor coastal flooding is not expected at this time.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 10 AM Friday to 8 PM EDT Saturday for
     PAZ070-071-104-106.
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/Staarmann
SHORT TERM...DeSilva
LONG TERM...DeSilva
AVIATION...DeSilva/Fitzsimmons
MARINE...Fitzsimmons/Hoeflich
TIDES/COASTAL FLOODING...NWS PHI