596
FXUS61 KPHI 301042
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
642 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/...
Very warm/hot and humid air mass today. Severe thunderstorms
possible this afternoon and evening, roughly from 4-10 PM. Drying
and cooling tonight as dew points lower especially overnight.

As of 640 AM, convection continues across far southeastern New
Jersey and parts of Delmarva. A subtle shortwave looks to be
maintaining these within an unstable air mass and also high
moisture content air. Locally very heavy rain has been occurring
and earlier some wind gusts to around 35 mph were measured.
Some additional development could take place northward some over
the next few hours as a warm front continues to work northward
in conjuction with some low-level warm air advection and
southerly flow up and over an outflow boundary. Adjusted the
PoPs based on radar trends.

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 /12Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...An area of showers and thunderstorms near KACY and KMIV
early this morning. Spotty low clouds have occurred, however
this looks to remain less widespread. Fog also has not really
developed. Otherwise, mainly VFR for a while. More organized
showers and thunderstorms are expected from about 19z onward.
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 from 11 AM this morning to 7 PM EDT this evening
     for PAZ070-071-102-104-106.
NJ...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NJZ010-012-013-015-017>020-027.
DE...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for DEZ001.
MD...Heat Advisory from 10 AM this morning to 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
SHORT TERM...AKL
LONG TERM...AKL/Gorse/Johnson
AVIATION...AKL/Gorse
MARINE...AKL/Gorse