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000
FXUS61 KPHI 141928
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
328 PM EDT Sun Apr 14 2024

.SYNOPSIS...
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A slow moving cold front will drop south through our region tonight
into Monday. High pressure builds back in by late Monday into
Tuesday, before a series of frontal boundaries and low pressure
systems affect the east coast Wednesday through Friday.
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&&

.NEAR TERM /THROUGH MONDAY/...
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The forecast for our severe weather potential this evening into
tonight remains largely unchanged and on track with a high shear,
low CAPE severe risk setup expected.

Latest observations show a weak warm front pushing through central
PA this afternoon and headed towards eastern PA. Behind it, clear
skies allow for abundant diurnal heating and gradual destabilization
ahead of an advancing surface low and trailing cold front across the
southern Great Lakes. Surface dew points are evidently rather
unimpressive at the moment, with most locations struggling to reach
even 50 degree dew points. With that said, high res guidance is
showing a gradual increase in surface moisture through the afternoon
with plentiful low-level moisture arriving via the low-level jet.
Mesoanalysis and hi res guidance also show favorable veering shear
profiles with this afternoon with 0-6km bulk shear in the ballpark
of 30-50 kts with 0-1km bulk shear in the low levels expected to be
around 30-40 kts. These shear profiles, in turn, are rather
supportive of a dominant linear storm mode in the form of an QLCS
with the potential for strong, gusty winds and an isolated quick
spin-up tornado embedded within the line. Some small hail will be
possible too, mostly with any storms that form ahead of the main
line. The only ingredient really missing this evening will be
instability with overall CAPE values expected to only reach 500-750
J/kg. This lack of instability will limit the widespread extent and
longevity of severe weather tonight. That said, favorable shear
profiles and modest lapse rates in the low and mid levels ahead of
the main line of the advancing cold front should help create
sufficient lift to take advantage of whatever instability we
ultimately end up with tonight and, thus, we could still see some
isolated severe weather as a result. A SLIGHT RISK for severe
weather across northeastern PA and northwestern NJ with a Marginal
Risk for the I-95 corridor and northwest is very fitting for our
current setup.

One potential fly in the ointment to this forecast, however, would
be from the showers that passed through the Poconos earlier this
afternoon. It will be interesting to see how these showers may
influence the mesoscale environment ahead of the line of storms
expected this evening. One one hand, these showers may have created
a more favorable environment with additional low-level moisture the
models may have not accounted for. On the other hand, increased
cloud coverage and any lingering outflow boundary may limit
convective development later or even relocate the area of convective
initiation. Unfortunately, it is difficult to tell which factor will
win out over the other at this time. Regardless, we will be keeping
an eye to the sky across the region.

The cold front passes overnight and any showers should dissipate
shortly after midnight or so. Temperatures will drop into the 50s
with some locations across the Delmarva only cooling to 60.

The main forecast challenge for Monday will be a cold front that
will still be our vicinity. There is some uncertainty regarding how
quickly it will move through with a variety of guidance in
disagreement on how far south the front will actually reach by
Monday afternoon. Consensus from guidance suggest the front will
slow to a crawl by the time it reaches southern NJ and the Delmarva
on Monday. Then, afternoon heating will allow for some isolated
convection to form along the front as it continues to inch
southward. Given this, added a 20-30 percent chance for showers and
an isolated thunderstorms for southern NJ and the southern Delmarva
for Monday afternoon. It will be another warm day with most areas
seeing highs in the 70s except cooler near the coast and over the
Poconos.
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&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
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Tranquil weather is expected Monday night into Tuesday as high
pressure builds in from the northwest. In the mid an upper levels, a
short wave ridge is expected to slide eastward toward our region by
Tuesday night. This should limit coverage of showers on Tuesday
night despite a warm front approaching from the south.

By Wednesday however, the shortwave ridge will weakens the next
closed low approaches from the west, leading to increasing chances
for showers though the day. Thanks in large part to the warm front
in the region, there will likely be a robust temperature inversion
through the day, resulting in limited instability. Therefore, not
expecting any thunderstorms at this time.
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&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
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The closed low and a trailing cold front continues to approach the
region Wednesday night into Thursday. Consequently, there are
chances for widespread showers and thunderstorms in the Wednesday
night/Thursday period. As far as thunderstorms, instability,
especially surface based instability may remain limited through this
period, so it is uncertain how widespread the coverage of storms
will be. At this point, it appears as if we are likely to have
widespread showers with some embedded thunderstorms, primarily late
Wednesday night into Thursday morning, though there remains quite a
bit of uncertainty with the timing.

Tranquil weather should return again, albeit briefly, as the low
departs our region Thursday night or Friday, before the next cold
front approaches to start the weekend. At this point, the forecast
has rain chances almost throughout this period, but it is unlikely
to be a washout through the period, rather there remains uncertainty
in the exact timing of the cold front. Once the cold front passes,
we should have drying conditions.
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&&

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

Rest of Sunday (through 00Z)...VFR expected at all terminals.  SW
winds 10-15 kt with gusts reaching up to 25-30 kt. Showers and
scattered thunderstorms will approach the north and west terminals
(ABE/RDG), but likely stay away through sunset.

Sunday night...Some showers and scattered thunderstorms are
anticipated to move in during the late afternoon/early evening,
mainly for KRDG/KABE. Sub-VFR conditions are possible within any
thunderstorm that does occur. Any lingering sub-VFR conditions
Sunday night will lift to VFR once the cold front comes through.
LLWS concerns for a few hours ahead and along the frontal boundary,
diminishing after 05-06Z. Moderate confidence overall.

Monday...Prevailing VFR with fair weather for most terminals. An
afternoon shower possible for MIV/ACY which could briefly bring sub-
VFR conditions. Moderate confidence.

Outlook...

Monday night and Tuesday...VFR expected.

Wednesday through Thursday...Sub-VFR conditions possible (30-40%)
with showers.

Thursday night and Friday...VFR likely, but a brief period of sub-
VFR is possible (20%) especially Friday.
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&&

.MARINE...

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Robust SSW flow this Sunday afternoon has warranted an SCA to be
issued for the Delaware Bay. Winds will gust to 25-30 kts this
afternoon before gradually diminishing behind the cold front
overnight. Seas will remain elevated around 4 to 7 feet through
tonight before gradually diminishing early Monday morning. Sub-SCA
conditions will persist through the rest of the day on Monday.

Outlook...

Monday night through Wednesday...winds and seas should stay below
SCA criteria.

Wednesday night through Friday...SCA conditions are likely (60%) for
at least a portion of this period as winds and seas build.
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&&

.PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ430-
     431.
     Small Craft Advisory until 8 AM EDT Monday for ANZ450>455.
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&&

$$

SYNOPSIS...Johnson
NEAR TERM...MJL
SHORT TERM...Johnson
LONG TERM...Johnson
AVIATION...Johnson/MJL
MARINE...Johnson/MJL