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301
FXUS61 KPHI 291637
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1237 PM EDT Sat Jun 29 2024

.SYNOPSIS...
A warm front will lift across the region later today into tonight. A
cold front will then move across the area later Sunday. High
pressure builds in later Monday and Tuesday before moving offshore
by Wednesday. A surface trough arrives on Thursday as a cold front
may approach from the northwest Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
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Radar showing some spotty showers popping up across portions of
the Delaware Valley, Delmarva, and southern New Jersey. Will add
some slight chance PoPs, but not expecting much out of these
showers over the next couple of hours in these locations.

Warm and increasingly humid conditions will be the theme during
the daytime hours today as a surface low pressure system slowly
progresses eastward out of the Great Lakes and into eastern
Canada. The warm front associated with this system will lift
across the region today and turn winds more southerly, drawing
more warm and moist air into the region. The result will be
highs generally in the mid to upper 80s, though in the 70s along
the coasts and in the southern Poconos. With surface dew points
increasing into the upper 60s to around 70, max heat index
values will be in the upper 80s to low 90s. The passage of the
front may help spark a few afternoon showers, most likely in the
northwestern portion of the area, but the vast majority of the
area should stay dry today.

However, convection will develop to the west of the region late this
afternoon across the eastern Ohio River Valley and progress eastward
this evening, riding along a secondary wave within the warm sector
of the low pressure system. The result will be showers and scattered
thunderstorms that move through the region, mainly during the
overnight hours, with the approach and passage of this wave. With 0-
6 km Bulk Shear increasing to 35 to 40 kt through the overnight
hours, and MUCAPE increasing to 750-1000 J/kg, this convection could
potentially remain organized as it progresses eastward. However,
with some CAM soundings depicting an inversion above the rather low-
LCLs, as well as the unfavorable timing during the overnight hours,
it remains to be seen how much of this convection is surface-based.
Strong to potentially damaging wind gusts would be the primary
threat if the storms are surface based. SPC has most of the area
near and northwest of the I-95 corridor in a MARGINAL risk as a
result with a SLIGHT risk for the most western portion of the area
where storms will arrive earliest.

Additionally, with abundant low level moisture over the area, PWATs
will be around 2.5 inches and thunderstorms will be capable of heavy
rain and localized flooding. With the aforementioned shear values,
flow should be strong enough for storms to move quickly and mitigate
widespread flash flooding. This correlates with a continued MARGINAL
risk for excessive rainfall from WPC.

Outside of any convection, a warm and humid night is on tap with
lows in the low to mid 70s for most of New Jersey and southeast
Pennsylvania, but in the mid to upper 70s along the I-95 corridor
from Philadelphia to Wilmington, and across Delmarva.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Very warm/hot and humid Sunday with the potential for some severe
thunderstorms, then cooling and less humid for Monday.

A few showers and thunder may be ongoing to start Sunday morning,
especially north and west of I-95, within an area of increasing
instability and local convergence. This activity should diminish
with cloud cover giving way to at least some sunshine and therefore
an increasingly hot and humid day. High temperatures are expected to
be in the upper 80s to low 90s for much of the region. Surface dew
points are forecast to be in the 70s, however we will have to see if
these can mix out some during peak heating prior to additional
convective developed. The combination of the very warm to hot
temperatures and dew points in the 70s will yield heat indices
Sunday afternoon well into the 90s with some areas getting to around
100 degrees. A Heat Advisory remains in effect for the urban I-95
corridor where the heat index criteria for an advisory through June
30th starts at 96 degrees, however given the latest forecast
expanded this into western Monmouth, Middlesex and Somerset counties
where the heat index criteria starts at 100 degrees. Elsewhere,
forecast heat indices do not reach advisory criteria. Local relief
from the heat will occur especially Sunday afternoon as showers and
thunderstorms develop.

While low pressure tracks well to our north, an incoming upper-level
trough will push a cold front across our region later Sunday. Some
additional showers and thunderstorms are forecast to develop,
however a pre-frontal trough may be far enough east to focus most 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 and low/mid level shear of 30-40 knots should
result in some discrete storms with even some clustering or line
segments occurring. Some of the convection could become severe 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 as at least some model forecast
soundings show an inverted-V low-level profile. This would enhance
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. The low-level flow is also
forecast to veer some with time and therefore the 0-1km SRH
decreases with time. The precipitable water values may start to
lower during the day although this will depend on the progression of
the trough and cold front, however locally heavy rain will occur
with any stronger convection. It will dry out Sunday night in the
wake of the cold front, and dew points will also be on the decrease
by daybreak Monday.

As we go through Monday, an upper-level trough departs and a ridge
starts to arrive by Tuesday. A cooler and less humid air mass will
be ushered in under a north to northeast wind as surface high
pressure builds in. High temperatures are forecast to be in the low
80s for many places. A comfortable air mass with dew points in the
50s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
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.

Synoptic Overview...A mid/upper level ridge arrives Tuesday into
Wednesday, then some influence from a Canadian upper-level trough
will push the ridge southward some Thursday and Friday. At the
surface, high pressure overhead Tuesday before shifting to our south
and east into Wednesday. A warm front lifts northward Wednesday then
a trough arrives Thursday with a cold front approaching from the
northwest Friday.

For Tuesday...A ridge arrives with a less humid air mass remaining
in place. High temperatures are forecast to be a few degrees warmer
than Monday given some warm air advection starting. The surface flow
however turns more east and southeast resulting in some cooling
along the coast. A comfortable air mass with dew points still in the
50s.

For Wednesday through Friday...The ridge aloft is forecast to
gradually weaken and shift southward and this will take surface high
pressure with it. A more south to southwest flow will become
established as a result, and this will also boost the warm air
advection. A warm front should also be lifting on by Wednesday with
a hotter air mass overspreading 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. Given the uncertainty at this time range,
kept PoPs on the lower side (mainly 20-30 percent Wednesday night
onward). The heat and humidity is forecast to build with this
spiking on Thursday as high temperatures rise into the lower 90s for
many places, and with dew points into the lower 70s results in humid
conditions. Peak heat indices on Thursday are forecast to be around
100 degrees for many places, and therefore a Heat Advisory may be
needed at least for some areas. The air temperatures may lower some
Friday, however the dew points are forecast to remain in the lower
70s for much of the area. Some convection cannot be ruled out on
Friday as a cold front lurks to our northwest. This front may be
slow to even get into our area with the main upper-level trough
still well to our west and a wave potentially developing along
it.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...MVFR, or IFR in a few spots, to start the day, improving to
VFR later in the morning into the afternoon. Southeast winds 5-10
knots, become southerly through the morning into the afternoon, with
gusts 15-20 knots. High confidence overall, moderate confidence on
timing of improving conditions.

Tonight...Sub-VFR conditions expected. Prevailing MVFR conditions
likely develop with ceilings again around 1500-2500 feet. IFR
conditions are possible, particularly with showers or thunderstorms
that are expected to move through the region. South-southwest winds
around 10 knots. Low confidence on extent of IFR conditions.

Outlook...

Sunday...Areas of MVFR/IFR conditions possible in the morning due
mostly to some low clouds, then some areas of sub-VFR conditions in
the afternoon and evening associated with showers and thunderstorms.
Some storms will be capable of producing locally strong winds. The
conditions improve from west to east by later at night.

Monday through Wednesday...VFR.

&&

.MARINE...
A Small Craft Advisory is in effect beginning at 6 PM this evening.
Sub-SCA conditions are expected through this afternoon with
southeast winds around 10-15 knots and gusts up to 20 knots. Seas of
2-4 through this afternoon as well. Some fog may develop offshore
during the day and reduce visibilities.

Heading into this evening, winds turn more southerly and increase to
15-20 knots with gusts up to 25 knots. Seas also build to 4-5 feet.
These conditions are expected to continue through the overnight
hours tonight.

Sub-SCA conditions are expected on the Delaware Bay.

Outlook...

Sunday...A Small Craft Advisory is in effect for the New Jersey and
Delaware Atlantic coastal waters. Some morning fog possible, otherwise
showers and thunderstorms mainly in the afternoon and evening with
potentially strong winds and frequent cloud to surface lightning.
The conditions improve at night.

Monday through Wednesday...The conditions are anticipated to be
below Small Craft Advisory criteria.

Rip Currents...

Today...Southeast winds 10-20 mph with breaking waves of 2 to 4
feet at the beaches and an 8-9 second period are expected. With
the steady onshore flow (especially in coastal Cape May,
Atlantic and Ocean counties) and increasing wave heights, there
is a HIGH risk for the development of dangerous rip currents at
the Jersey Shore. At the Delaware beaches, wind is more shore
parallel and therefore a MODERATE risk for the development of
dangerous rip currents.

Sunday...Wind turns more south/southwest, though with a shore
parallel wind and 2 to 3 foot breakers with a remaining 8 second
swell, have gone with a MODERATE risk for the development of
dangerous rip currents for coastal Cape May, Atlantic and Ocean
counties with a LOW risk for coastal Monmouth County and the
Delaware Beaches.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 11 AM to 7 PM EDT Sunday for PAZ070-071-102-
     104-106.
NJ...Heat Advisory from 11 AM to 7 PM EDT Sunday for NJZ010-012-013-
     015-017>020-027.
     High Risk for Rip Currents until 8 PM EDT this evening for NJZ014-
     024>026.
DE...Heat Advisory from 11 AM to 7 PM EDT Sunday for DEZ001.
MD...None.
MARINE...Small Craft Advisory from 6 PM this evening to 7 PM EDT Sunday
     for ANZ450>455.

&&

$$

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