-- Discussion containing changed information from previous version are highlighted. --
090
FXUS61 KPHI 262128
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
528 PM EDT Sun May 26 2024

.SYNOPSIS...
A warm front lifts north through the region late tonight, followed
by a strong cold front Monday night. Weak high pressure builds
through the region Tuesday with an area of low pressure passing
through on Wednesday. High pressure returns to close out the
week.

&&

.NEAR TERM /THROUGH MONDAY/...
Main changes with this forecast update was to refine the timing
of the two rounds of showers and storms tonight and tomorrow.
Also, increasing confidence of fog along a narrow corridor right
along the coast continuing tonight into early tomorrow morning.
Additionally, the severe risk for tomorrow has been expanded to
include more of the region.

Through this evening, still looking at isolated to scattered
shower and storm coverage in areas that have not seen the bay or
sea breeze move through. These will be very much diurnally
driven, so expect these showers to dissipate by sunset. Expect
pulse storms due to very weak shear. However, with some elevated
DCAPE values, could see isolated downbursts and small hail with
the storms today.

The other concern is fog along the coast. Still looks like this
is staying limited to a narrow corridor right along the coast,
and expect that trend to continue until closer to midnight, when
winds should settle out of the southeast, allowing the fog, at
least temporarily to progress inland.

As the warm front lifts northward tonight, expect scattered
showers and storms to move into the region. This should help to
dissipate the fog. With this first round of showers and storms,
some lightning will be possible, and localized downpours, but
risk of other thunderstorm hazards is low at this time.

There should be a lull in the showers and storms starting
mid to late morning (starting first in SE PA). Although the
warm front will be north of the area, and southerly flow
develops, clouds and rainfall should keep temperatures down
compared to Sunday. Highs will top off in the mid to upper 70s
to around 80 for most of southeast Pennsylvania and New Jersey,
and in the low to mid 80s for Delmarva. However, the bigger
change will be in surface dew points, as dew points will rise
well into the 60s and even the low 70s in Delmarva. These dew
points are about 5 degrees higher than Sunday, but once dew
points are in the 60s, any change higher can be quite
noticeable.

Models are indicating that the highest instability levels will
be over Delmarva, which is where the warmest temperatures and
highest dew points will be. Due to a low level inversion/cap, we
may not fully realize surface based CAPE values, but even with
mixed layer CAPE values above 1000 J/kg. DCAPE across Delmarva
may be near 1000 J/kg as well, but the cap may limit this a bit.
0-6 km Bulk Shear values will get up around 35 to 40 kt over
Delmarva, and will generally be 25 to 30 kt across areas north
and west of the Fall Line. PWATs throughout will be 1.5 to 2
inches.

The Storm Prediction Center has most of the region in a Slight
Risk (2 out of 5) for severe weather. The strongest storms will
be capable of damaging wind gusts and even large hail. Although
given the relatively warm boundary layer, the risk for hail is
not as high as the risk for damaging wind gusts. Can`t rule out
the tornado threat, especially in the vicinity of the cold
front, but if the instability is elevated, that should help to
limit the tornado threat.

Although heavy rain is a threat throughout the region, given
the lower shear values across northern areas, there is a higher
potential for bands of heavy rain to develop across areas north
and west of the Fall Line. In this areas, the Weather Prediction
Center has a Slight Risk (2 out of 4) for Excessive Rainfall.

This round of showers and storms should move out by about
midnight tomorrow night.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
Tuesday will be mostly dry as the primary cold front moves through
and high pressure tries to nudge in. While the primary front will
not knock down temperatures much, if at all, there will be some
relief from the humid air as dewpoints fall. As an upper trough
moves in from the Great Lakes, a shortwave will round the base of
said trough and move towards our region. This could spark off a few
showers/thunderstorms in the afternoon, mainly west of the I-95
corridor, though PoPs are only around 20% in these spots. For the
rest, it will be a dry day with temperatures in upper 70s/low 80s.
Any showers/thunderstorms will collapse after sunset with the loss
of daytime heating, and it will be a tranquil Tuesday night with
lows in the upper 50s/low 60s.

A stronger shortwave approaches for Wednesday with an associated
surface cold front that will actually bring some cooler air. Showers
could begin as early as midday, though the afternoon brings the
highest chances for showers/thunderstorms. The strongest forcing and
highest instability will be along and west of the I-95 corridor,
with rain chances around 50-70%. The strongest shear will be
displaced further south, with severe chances looking much lower
compared to Memorial Day. Some areas south and east of the I-95
corridor could stay dry (PoPs only 25-50%). Temperatures will be
in the mid to upper 70s.

The cold front pushes through on Wednesday night though some
showers/isolated thunderstorms could linger through the first half
of the night. By the early morning hours of Wednesday, we should be
dry region-wide. Overnight lows will be in the low to mid 50s.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Long term period looks rather quiet overall. A cold front moves
offshore by Thursday morning with high pressure moving in from
Canada. A trough will hang over the northeastern US, though it
should remain dry through the end of the week. With lower heights
and Canadian high pressure moving in, temperatures for
Thursday/Friday will be a few degrees below normal. Looking at upper
40s/low 50s for nighttime temperatures and upper 60s/low 70s for
daytime highs.

High pressure looks to dive southeast and offshore, resulting in
return flow setting up over our region. This should bring
temperatures back to near normal levels for next weekend (upper
70s). Deterministic guidance and ensembles currently show a dry
first half of the weekend, though another system could move in for
the back half.

&&

.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas...

Today...Mostly VFR. As of 5 PM, still some storms near PHL and
PNE so these will pose a risk of lightning until about 23z
before dissipating. Winds mainly southeast 5 to 10 knots.

Tonight...The next round of showers is now looking like it will
be arriving later. Consequently, there is an increasing risk for
fog or low stratus to develop ahead of the showers moving in.
Expect fog/low stratus to develop after 00Z, likely affecting
KACY first, before expanding over much of the rest of the
region. Currently have IFR in the TAFs, but there is
considerable variability between guidance on the flight
category, between LIFR and MVFR. Conditions should improve
slightly, maybe as high as MVFR, as the showers move in
generally 08Z or later. Winds settle out of the SE around 10 kt.
Moderate confidence on the overall pattern, but low confidence
on details.

Monday...Starting IFR or MVFR with SHRA and some TSRA in the
morning. Then expect a lull and brief return to VFR starting
around 15Z. Another round of TSRA is expected Monday afternoon,
generally near or after 21Z. With this second round of TSRA,
could see very tempo IFR conditions, variable and gusty winds,
and possibly small hail. Winds out of the SE and S around 10
KT. Moderate confidence on the overall pattern, but low
confidence on details.


Outlook...

Monday night...Once storms pass, returning to mostly VFR
conditions.

Tuesday through Tuesday Night...Primarily VFR. Isolated
shower/thunderstorm possible at KRDG/KABE (20%).

Wednesday through Wednesday Night...Restrictions likely (50-70%)
especially for the I-95 and Lehigh Valley terminals with scattered
showers/thunderstorms moving through in the afternoon/evening.
Lesser chances (20-40%) for showers/storms at KACY/KMIV.

Thursday through Friday...VFR. No significant weather expected.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Winds and seas may get close to Small Craft Advisory criteria
Monday afternoon and evening, but it is marginal, so confidence
is too low to issue an SCA at this time. There are also a few
other hazards to watch for. First, fog has developed in a narrow
corridor along the NJ and DE coast. While there may be some
transient clearing, the continued onshore flow should keep the
fog mostly in place through the overnight hours. The other
concern is thunderstorms, primarily tomorrow afternoon and
evening which may be capable of localized gusty winds and large
hail.

Outlook...

Tuesday through Friday...No marine headlines expected. Fair weather
outside of some showers and thunderstorms on the waters Wednesday
afternoon and evening.

Rip Currents...

A LOW risk for the development of rip currents will continue through
today.

On Monday, the onshore flow will strengthen as southeast winds
increase to around 15 to 20 knots. This will lead to larger breaking
waves increasing the rip current risk to MODERATE for the NJ
beaches. For the Delaware Beaches, the rip current risk will remain
LOW for Monday as the winds will have somewhat less of an onshore
component.

For Tuesday, winds will shift to more of an offshore direction as
they become west to southwest. This will lead to a LOW risk for rip
currents for both the NJ and Delaware beaches.

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.
-- End Changed Discussion --

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until 8 AM EDT Monday for ANZ450>455.
-- End Changed Discussion --

&&

$$

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