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660
FXUS61 KPHI 251927
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
327 PM EDT Sat May 25 2024

.SYNOPSIS...
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A weak cold front approaches from the west tonight and washes out
before it crosses the region. Surface high pressure tries to build
in, but several upper level troughs will impact the area Sunday
night through Monday. A stronger cold front will pass through the
region Monday night. Weak high pressure returns Tuesday, but weak
low pressure passes through on Wednesday. High pressure closes out
the new work week.
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&&

.NEAR TERM /THROUGH SUNDAY/...
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A relatively nice start to the weekend is ongoing this afternoon. A
warm front is currently located just north of Philadelphia and
lifting northward. Behind it within the warm sector, some
scattered/broken cumulus clouds are developing. Starting to see the
makings of a sea and bay breeze as well. Relatively tranquil
conditions should continue for the next few hours.

Some convection is starting to sprout up off to the west as a
surface trough pushes through Central PA. Orographic effects and
incoming shortwaves are also contributing to the forcing as well. As
the surface trough and shortwaves push east, the developing
convection will make it into our area. Instability will be
sufficient for thunderstorms to make it into the Lehigh
Valley/southern Poconos. However, shear is quite meager, resulting
in storms generally being sub-severe. Scattered
showers/thunderstorms will gradually weaken as they push east, as
instability wanes with the loss of daytime heating. Some scattered
showers are still possible through the night around the I-95
corridor, but only have around a 15-20% chance of showers. Some
patchy fog is expected as well, especially in areas that see heavy
rain later this evening and near the coast, where some marine fog
will move ashore. Temperatures will drop into the low to mid 60s.

For tomorrow, any fog/stratus will mix out by mid-morning. It will
be mainly dry through the early afternoon, though another shortwave
approaches. This will result in some scattered showers/isolated
thunderstorms developing, especially near a stalled boundary north
and west of Philadelphia. PoPs are only around 15-30% from the I-95
corridor on west. Not expecting any severe weather either (that
potentially comes Monday). Should be dry near the coast, and another
nice beach day down the shore. Temperatures will climb into the mid-
80s with low to mid 70s at the coast.
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&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
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No major changes with the forecast in this period. Low pressure will
continue to organize and develop Sunday night and will lift into the
Great Lakes and eastern Canada Monday and Monday night. A warm front
extending out from that low will lift north through the region
Sunday night into Sunday morning, and as it does so, brings in what
is expected to be round 1 of showers and storms on Sunday. With the
first round, there is some elevated instability, so lightning is a
concern, but risks for other hazards appear low.

There should be a lull before we then have round 2 of showers and
storms moving into the region. This round will be in response to a
cold front (may be occluding by the time it reaches our region)
approaching from the west. There are a few concerns of hazards with
this round.

Severe (hail, wind): the threat for severe storms will likely be
limited to areas south of the warm front by the time the cold front
reaches our region, which at this point looks to be along and south
of an ABE to TTN to ACY line (but as with most warm fronts 48+ hours
out, there is considerable uncertainty in exactly where this will
be). Even for areas south of the warm front, model soundings depict
a low level cap staying in place, thus limiting the risk for surface
based instability. If this holds true, this should limit the risk
for the tornado risk despite some veering on model hodographs. The
bigger concern is wind and hail. Even with limited mixing (thanks to
the inversion), if the storms can organize into a QLCS, there could
be enough to mix stronger winds down to the surface. Deep layer
shear is a bit on the lower side (20 - 30 kt) for a well organized
QLCS, and the best upper level forcing will be to our north and
west, so there remains some uncertainty about that. Even with
elevated instability, model CAPE values are well above 1000 J/kg
over Delmarva. This increases the risk for hail, although a
relatively warm boundary layer will work against that. None the
less, there appears to be at least some potential for severe hail,
probably on the lower end (around 1 inch).

Heavy rain/flooding: There is a risk for heavy rain and flooding,
especially further inland, however, there remains some uncertainty
with this threat. Here are the factors favoring a flooding threat:
modeled precipitable water values between 1.75 and 2 inches (which
is near or slightly above the 90th percentile for this time of
year), the earlier (round 1) storms priming the soils ahead of the
main round in the afternoon and evening, and at least one
deterministic model (GFS) depicts model soundings very favorable for
the heavy rain threat (high average RH, deep warm cloud layer).
However, on the other end of the argument, this set up isn`t a
classic Maddox set up for heavy rain, other model soundings aren`t
as favorable for heavy rain, and the cold front (and thus storm
motions) are relatively fast moving.

Cold front works its way through the region Monday night. Showers
and thunderstorms taper off after midnight from west to east and
will end by daybreak Tuesday.

During the day Tuesday, may see another round of storms Tuesday
afternoon in the southern Poconos, but in general, should have
mostly dry conditions. Winds shift to more southwesterly, which may
mean that temps, especially closer to the coast are near or even
slightly above Monday temps.
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&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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In the wake of the early week cold front, the airmass will not be
appreciably cooler, but it will be much less humid as surface dew
points drop from the mid and upper 60s to the mid and upper 50s. An
upper level trough with a sharp shortwave trough will move from the
Great Lakes into the Ohio Valley and western New York and western
Pennsylvania, bringing some storms to our region Tuesday night.
Conditions will otherwise be dry and mild.

Low pressure develops over the Mid-Atlantic on Wednesday as that
upper trough builds east, touching off scattered showers and
thunderstorms during the day.

Thereafter, surface high pressure builds in from the north and west
as an upper trough remains over the Northeast. Going into the end of
the week, conditions will be mainly dry, but cannot rule out an
isolated shower or thunderstorm. It will also be cooler with below
normal temperatures.
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&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas...

Rest of Today...VFR. Southerly winds 5-10 kt. High confidence.

Tonight...A period of sub-VFR conditions with some showers and a
thunderstorm possible (25-35% chance) for the Lehigh Valley
terminals. Have included VCTS for KABE/KRDG with the 18z TAF
issuance. Some patchy fog possible there late. Winds light and
variable. Moderate confidence overall.

For the I-95 terminals, currently have VFR tonight as
showers/thunderstorms should dissipate before reaching these
terminals. Winds generally light/variable but will favor a southerly
direction around 5 kt or less at times. Some patchy fog is possible,
but only put visibility restrictions at around 20% so have not
included any MVFR conditions in the 18z TAF.

For KMIV/KACY, some marine stratus/fog will make its way inland.
Have rather high confidence in some sort of restrictions at KACY,
with a lower confidence that the marine layer makes it to KMIV.
Confidence in timing and extent of restrictions is also low. Winds
will also go light/variable, though favor a southerly direction at
times at 5 kt or less.

Sunday...Any fog/stratus will dissipate shortly after daybreak.
After, VFR expected. Winds out of the south/southeast around 5 kt.
High confidence.

Outlook...

Monday...periods of MVFR and IFR with TSRA.

Tuesday...VFR conditions expected.

Wednesday...predominantly VFR, but brief impacts with TSRA
possible.

Thursday...VFR conditions expected.
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&&

.MARINE...
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No marine headlines in effect through Sunday. Winds out of the
south/southeast around 10 kt with 1 to 2 foot seas. Some patchy
dense fog possible (40-60% chance) on the waters tonight. A Marine
Dense Fog Advisory may be necessary.

Outlook...
Sunday through Thursday...Sub-SCA conditions overall, but a
brief period of SCA conditions with 25 kt wind gusts and seas
around 5 feet may occur Monday night through Tuesday morning.
Thunderstorms Monday afternoon and night could result in strong
wind gusts and VSBY restrictions in heavy rain. Fog possible
Monday morning.

Rip Currents...

Both Saturday and Sunday will feature a LOW risk for the
development of rip currents. While there will be a light onshore
flow with south/southeast winds, breaking waves of only 1 to 2
feet and a short period of less than 8 seconds will mitigate the
rip current potential.

As always, rip currents can still develop, and often occur in
the vicinities of jetties and piers. Utilize any guarded beaches
this holiday weekend if venturing out into the water.
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&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Air Quality Alert until midnight EDT tonight for PAZ070-071-
     101>106.
NJ...Air Quality Alert until midnight EDT tonight for NJZ013-014-
     016>027.
DE...None.
MD...Air Quality Alert until midnight EDT tonight for MDZ012-015.
MARINE...None.

&&

$$

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