993
FXUS61 KPHI 031037
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
637 AM EDT Wed Jul 3 2024

.SYNOPSIS...
High pressure moves offshore today. A more active period begins
Thursday with the arrival of a surface trough, bringing hot and
humid conditions along with chances for showers and storms each
day into the weekend. A frontal boundary may linger across the
region into early next week, with generally unsettled conditions
persisting.

&&

.NEAR TERM /THROUGH TONIGHT/...
Areas of dense fog have developed along the NJ and DE coasts,
mainly from Ocean County southward. Have issued a dense fog
advisory for these areas until mid morning. Otherwise no changes
to earlier thinking below.

Surface high pressure is currently centered just to our
northeast over southern New England, and it will slowly slide
eastward through today and tonight as a cold front, currently
over the western Great Lakes, presses southeastward toward us.
Despite this shift in surface pattern, the flow will not bring
the big surge of humidity until tomorrow, so we have one more
day of relatively low humidity, with afternoon dew points likely
to drop back into the 50s again. Aloft, the ridge axis currently
over western PA will shift over us by this afternoon, then off
shore tonight. Some mainly high clouds are passing over the
ridge and will keep today from being perfectly sunny, but
greater cloud cover will not invade until tonight. Today and
most of tonight should also be dry, but as the warm front with
the humidity push moves in late tonight and some dying
convection approaches from the northwest, our northwestern
zones, especially the Poconos, could see a late night shower or
thunderstorm. Highs today will be in the 80s for most, similar
to yesterday, with lows tonight staying a touch milder with the
clouds and humidity starting to build, with 60s common.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY/...
High pressure will continue to move offshore into Independence
Day as a surface low and associated upper level trough move
eastward through eastern Canada. This will push a warm front
through the region early in the day Thursday allowing the area
to get into the warm sector. The warm front could bring a few
isolated showers or storms early Thursday morning, mainly over
NE PA into NW NJ, with the greater chances for showers and
storms arriving later in the day Thursday. Prior to this, the
other story for the day will be the return of the heat and
humidity as highs 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 into the 90s for most areas along and south of
I-78. Cloud cover is expected to increase as the day progresses
and winds will be from the south to southwest near 5-10 mph
(15-20 mph near the coasts).

As for the storm threat later in the day, expect storms to fire
in the vicinity of a weakening frontal boundary draped over PA
by the afternoon hours with these storms moving 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. 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.
With ML CAPE values progged to max out around 1000-1500 J/kg
along with deep layer shear values around 30 knots, there is
the potential some storms could become severe. The SPC has
maintained the MARGINAL risk (1/5) for severe thunderstorms
across SE PA (including the Philadelphia area), Delmarva, and
parts of southern NJ for Thursday. 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 at this time. 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). Thus, storms will also be capable of producing very
heavy rain, which could lead to localized flash flooding. WPC
has also maintained the MARGINAL risk of excessive rainfall
across much of our area, outside of the immediate coast, the
Poconos, and northern NJ.

Storms may linger into a good portion of the evening Thursday
before diminishing overnight. It will be a very warm and muggy
night as the frontal boundary gets hung up over the area. Most
areas won`t see lows getting below the low/mid 70s (except 60s
over the Poconos). Some fog development is possible overnight
where ever rainfall occurs.

The front will remain draped over the area Friday as it extends
to a new low developing over the midwestern States that will
lift northward towards the Great Lakes. This will result in
another hot, and even more humid day with chances for afternoon
and early evening storms once again. Storms should be a little
less widespread though compared to Thursday and this is
reflected in our POPs which are more in the 30% to 40% range.
The higher dewpoints Friday, possibly getting into the mid 70s,
will result in max heat indices in the mid 90s to near 100
degrees in PA and NJ along and south of I-78 and away from the
coasts, and near 100 to 105 degrees in Delmarva. A Heat
Advisory may be needed to highlight the hot and humid
conditions, mainly within the Philly metro and perhaps the
eastern shore of Maryland.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Low pressure lifts northward into Ontario Friday night into
Saturday with a strengthening southerly flow developing in the
warm sector over the area ahead of an approaching cold front.
This will ratchet up the mugginess even further with dewpoints
potentially reaching over 75 degrees in some areas by Saturday
afternoon. Combined with forecast highs for Saturday once again
in the upper 80s to low 90s for most areas, this could bring
peak heat indices in the range of 102 to 107 degrees, meaning
heat headlines would be needed if this forecast holds. The
synoptic scale lift provided by the cold front along with
abundant moisture looks to result in widespread showers and
storms by Saturday afternoon into the evening. And at this
point, both severe weather and flash flooding look to be a
threat. We remain particularly concerned seeing progged PWat
values by both deterministic models and ensembles in the range
of 2.25 to 2.5 inches which is again is close to climatological
maxes.

Looking beyond Saturday into early next week, the cold front
will move through into Sunday but also tend to stall and "wash
out" as it moves across. Thus, while Sunday into early next week
should be a little more comfortable compared to Saturday in
terms of heat and humidity, it will still be quite warm and
humid. Each day will also see continuing chances for scattered
showers/storms, mainly during the afternoon and early evening
hours, with POPs generally around 30 percent.

&&

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

Through 0Z this evening...Mainly VFR. Areas of fog and low
clouds at KMIV and KACY should dissipate near or just after 12Z
but may need to include tempo in 12Z TAFs. Otherwise, just some
ci clouds with some diurnal Cu possible too. Light S winds
becoming 5-10 kts. High confid.

Tonight...VFR with no significant weather. Winds generally
southerly around 5 kts. High confidence.

Outlook...

Thursday through Sunday...Sub-VFR conditions possible with
chances (30-60%) for SHRA and TSRA, especially in the afternoon
and evenings each day. The best chances for impacts from most
widespread showers and thunderstorms will be Saturday.

&&

.MARINE...
Marine dense fog advisory in effect this morning from Ocean
County NJ waters southward based on observations on land
showing dense fog in Toms River, Atlantic City, Millville,
Wildwood and possibly Georgetown (low ceilings). Otherwise, sub-
SCA with mostly fair weather today and tonight. Light S or SE
winds increasing to 10-15 kts by this afternoon and tonight.

Outlook...

Thursday through Sunday...Conditions are forecast to be below
Small Craft Advisory criteria through the period, though
southerly winds may gust to around 20-25 knots at times. There
will also be daily chances for showers and storms through the
period, especially in the late afternoon and evening periods.
Best chance for a Small Craft Advisory will be Saturday
afternoon and evening as a cold front approaches, yielding the
best chances for wind gusts near 25 knots and seas near 4-5
feet, albeit marginal.

Rip Currents...

Today...Southeast winds around 5-10 mph. Breaking waves remain
around 1-2 feet. Despite more of an onshore flow, with light
conditions expected, have continued with a LOW risk of dangerous
rip currents for both the Jersey Shore and Delaware Beaches.

Independence Day (Thursday)...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 2 to
3 feet, though some 4 foot waves are possible within the
MODERATE risk area.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Dense Fog Advisory until 10 AM EDT this morning for NJZ020>027.
DE...Dense Fog Advisory until 10 AM EDT this morning for DEZ003-004.
MD...None.
MARINE...Dense Fog Advisory until noon EDT today for ANZ430-431-451>455.

&&

$$

SYNOPSIS...Fitzsimmons/RCM/Staarmann
NEAR TERM...RCM
SHORT TERM...Fitzsimmons/Staarmann
LONG TERM...Fitzsimmons/Staarmann
AVIATION...Fitzsimmons/OHara/RCM/Staarmann/Wunderlin
MARINE...Fitzsimmons/Hoeflich/OHara/RCM/Wunderlin