691
FXUS61 KPHI 301517
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1117 AM EDT Sun Jun 30 2024

.SYNOPSIS...
Showers and thunderstorms early this morning and then again this
afternoon and evening prior to a cold front moving across the
area. High pressure builds in Monday through Tuesday before
moving offshore Wednesday. A surface trough arrives on Thursday
as a cold front may approach from the northwest Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Severe Thunderstorm Watch 489 is now in effect for most of the
area.

The Storm Prediction Center has New Jersey and portions of
southeast Pennsylvania, including Philadelphia, in an Enhanced
Risk (3 out of 5) for severe weather this afternoon and evening.

Otherwise, an active day is anticipated especially this
afternoon and early this evening. Our region becomes more
situated within a warm sector and this will boost high
temperatures into the lower 90s for many places. This occurs
once areas of lower clouds lift, although at least some clouds
are anticipated to persist. A cold front then works its way
through the region tonight. The high dew points advecting over
the cooler ocean waters should result in some marine fog along
the coast this morning.

Deep southerly flow will be maintained today ahead of a cold
front. This will strengthen some and as the boundary layer
warms, a gusty southerly breeze should commence especially
across the coastal plain this afternoon. The surface dew points
will be in the 70s this morning for much of the area. There may
be enough mixing in the afternoon for the dew points to drop a
little. Heat index values will be in the upper 90s to low 100s.
No changes to the Heat Advisory as peak heat indices this
afternoon should be around 100 degrees within the urban I-95
corridor (lower advisory criteria through today) and around 105
for our eastern shore of Maryland counties. If more cloud cover
prevails and/or convection gets going sooner, than high
temperatures and therefore heat indices will be impacted
(lower).

While low pressure tracks well to our north, an incoming upper-
level trough will push a cold front across our region late today
and this evening. Some showers and thunderstorms are forecast
to develop, however a pre-frontal trough may focus much of the
convective development along and south/east of the I-95
corridor. Given the heat and high humidity in place coupled with
ample instability (MLCAPE up to 2000 J/kg) and effective shear
of 40 knots will support some discrete storms although these
should organize into more of a linear mode. Severe thunderstorms
are certainly possible, especially the more organized they
become, with damaging straight- line winds and some hail the
main hazards. The downburst potential could become enhanced
especially given the high precipitable water environment and
with some model forecast soundings showing an inverted-V low-
level profile. This would increase the downdraft instability or
DCAPE. The environment looks conducive for thunderstorms that
have a taller and stronger core to produce frequent cloud to
ground lightning and stronger outflow winds. The low-level flow
is also forecast to veer some and therefore the 0-1km SRH
decreases with time. The precipitable water values may start to
lower later in the afternoon although this will depend on the
eastward progression of the trough/cold front, however locally
very heavy rain will occur with any stronger convection.
Convection will be moving and therefore the flash flooding
threat looks more limited. It will then dry out by later tonight
in the wake of the cold front, and dew points will also be on
the decrease especially overnight.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
High pressure will be the name of the game during the short term
forecast period. This will bring a dry and pleasant start to
the new month across the region. Ushered in by breezy north-
northwest winds, temperatures on Monday will top out in just
the upper 70s to low 80s across the region. This is 5 degrees
or so below normal for the beginning of July. Dew points will be
lower as well, in the 50s, making conditions quite comfortable.
Lows Monday night look to be in the upper 50s to low 60s.

Highs on Tuesday will be closer to normal, mainly in the low-
mid 80s. However, with dew points forecast to remain in the 50s,
another pleasant day looks to be on tap. Lows Tuesday night will
be a touch warmer as well, mainly in the low-mid 60s.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Summary...Less humid to start then the heat and humidity spikes
Thursday, followed by a little cooling but remaining humid. Some
convection possible late in the week. Few changes from the
previous forecast.

Synoptic Overview...A mid/upper level ridge remains in place
Wednesday, then some influence from a Canadian upper-level
trough will push the ridge southward some Thursday and Friday.
At the surface, a warm front lifts northward Wednesday then a
trough arrives Thursday with a cold front approaching from the
northwest Friday.

Wednesday...Surface high should remain the dominant feature for
one more day, leading to one more day of tranquil conditions. As
the high shifts further off shore though, southwesterly flow
will develop, leading to increasing dewpoints, although,
guidance still shows the dew points in the lower 60s even late
on Wednesday, which is humid, but not oppressively humid.

Thursday (Independence Day) through Saturday...Southerly flow
becomes more established leading to hot and humid conditions all
three days (though Saturday is dependent on if a cold front will
stay northwest of our region. The influence of a Canadian
upper-level trough will start to approach during Thursday,
however the timing and amplitude of this trough is less certain.
As of now, the main trough is more toward the Midwest and
therefore a cold front may be slow to approach our area late in
the week. There may be a pre-frontal trough in our vicinity
Thursday into Friday and this may provide enough focus amid a
destabilizing air mass to initiate some convection.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...SHRA/TSRA continue over KMIV and KACY. A small cluster
of TSRA is approaching KILG/KPHL as well. ONce this ends, there
should be a break in the convection until around 19Z or so when
more organized SHRA/TSRA develop. Strong west to northwest
winds can accompany the more intense thunderstorms later this
afternoon. Southwesterly winds increasing to 10-15 knots with
some gusts to around 20 knots, then winds should start to become
westerly toward early evening. Low confidence on the details.

Tonight...Areas of sub-VFR conditions with showers and
thunderstorms for a portion of the evening, then VFR. Strong
west to northwest winds can accompany the more intense
thunderstorms. West to northwest winds 5-10 knots, becoming
north-northwest 10-15 knots. Low confidence on the details.

Outlook...

Monday through Wednesday...VFR with no significant weather.

Thursday...Sub-VFR conditions possible with chances (25-35%)
for SHRA and TSRA, especially in the afternoon.

&&

.MARINE...
Southerly flow will be maintained today ahead of a cold front
with some increase as the low-level flow also increases. Mixing
over the cooler waters will be limited some. Will maintain the
Small Craft Advisory for the New Jersey and Delaware Atlantic
waters into early this evening for now as some gusts up to 25
knots and seas up to 5 feet are still possible. Some showers and
a thunderstorm early this morning, then additional showers and
thunderstorms later this afternoon and tonight with locally
strong winds. Some fog may occur through this morning especially
nearshore with visibility restrictions. The conditions improve
overnight as a cold front shifts out to sea.

Outlook...

Monday through Thursday...Conditions are forecast to be below
Small Craft Advisory criteria.

Rip Currents...

Today...South-southwest winds 15-20 mph with breaking waves of
2-3 feet are expected. Swell periods are expected to be up to
6-7 seconds in length. Due to winds and a longer swell period,
have gone with a MODERATE risk for the development of dangerous
rip currents for both the Jersey Shore and Delaware Beaches.

Monday...North-northeast winds 10-15 mph with breaking waves
around 2 feet. Given the elevated winds and waves still around
2 feet, opted to go with a MODERATE risk for the development of
dangerous rip currents for both the Jersey Shore and the
Delaware Beaches.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory until 7 PM EDT this evening for PAZ070-071-102-
     104-106.
NJ...Heat Advisory until 7 PM EDT this evening for NJZ010-012-013-
     015-017>020-027.
DE...Heat Advisory until 7 PM EDT this evening for DEZ001.
MD...Heat Advisory until 7 PM EDT this evening for MDZ012-015-019-
     020.
MARINE...Small Craft Advisory until 7 PM EDT this evening for
     ANZ450>455.

&&

$$

SYNOPSIS...AKL/Gorse/Johnson
NEAR TERM...Gorse/MPS
SHORT TERM...AKL
LONG TERM...AKL/Gorse/Johnson
AVIATION...AKL/Gorse/MPS
MARINE...AKL/Gorse