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837
FXUS61 KPHI 301954
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
354 PM EDT Tue Apr 30 2024

.SYNOPSIS...
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A cold front settles south and east of our area tonight before
moving back north as a warm front Wednesday night into Thursday.
A weak cold front then settles to our south Thursday night into
Friday. Another cold front slowly approaches Saturday and
Sunday, then it gradually settles to our south and east early
next week.
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&&

.NEAR TERM /THROUGH WEDNESDAY/...
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Following the passage of a backdoor cold front coming in from
the northeast last night, actually reaching down past
Philadelphia with a push of cool air on northeast winds, we had
low clouds across the northeastern half of the area this
morning. Those clouds have since cleared the vast majority of
our area, except right along the shore from Barnegat Light
northward, where the chilly marine air is fending off the push
of milder air from the south and west. Temperatures there are
actually stuck in the mid to upper 50s this afternoon.
Meanwhile, in areas where the cool air did not push in, highs
temperatures are into the 80s this afternoon, even up to around
Philadelphia. In between that, temperatures vary from the 60s
across much of northern NJ to the 70s closer to the Delaware
and Lehigh River Valleys down to around Trenton.

As a result of that push of cooler air, the atmosphere is
relatively stable across much of northern and eastern NJ.
Meanwhile, RAP analysis suggest around 1000 j/kg of CAPE from
near the Poconos down to near Wilmington Delaware. That will be
a key factor for showers and thunderstorms trying to push into
our region from the west and northwest as we head into this
evening. A shortwave trough and a weakening cold front are
helping to trigger showers and thunderstorms this afternoon
across central PA and upstate NY, but due in part to less
instability and moisture, there is very limited cumulus
development in our area, and when this activity encounters the
more stable air toward NJ, they will be weakening.

That said, along with the CAPE mentioned above, there is
800-1000 j/kg of DCAPE analyzed by the RAP across eastern PA as
well, and effective bulk shear of around 40 kt. SPC has added
our area into a `marginal` risk of severe thunderstorms, and
that seems on point given these conditions. CAMs continue to
suggest activity will start to move into our area toward 5 PM,
and particularly between 6-8 PM, possibly reaching the I-95
corridor around 8-9 PM, but likely weakening as it does so.
Expect the main threat with any of these storms to be gusty
winds, capable of bringing some tree limbs down, but that threat
will diminish toward sunset and as the storms reach the more
stable air closer to the Delaware River Valley.

Showers will continue to weaken and diminish in coverage heading
toward midnight, but with the trough still yet to move through,
some light showers will linger, mainly east of I-95, possibly
through dawn on Wednesday. Some vorticity may continue to advect
in on WNW flow aloft in the wake of the main trough on
Wednesday. However, moisture will be very limited, and the
atmosphere will be fairly stable, so while a pop up shower,
perhaps along the sea-breeze, cannot be completely ruled out,
do not have anything mentioned in the forecast. Otherwise,
Wednesday looks like it will turn out to be a fairly pleasant
day as clouds tend to give way to sunshine, especially away from
the coast. Look for highs mainly in the 70s, but much cooler
toward the shore thanks to a pronounced onshore breeze.
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&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
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As a shortwave trough slides across New England Wednesday night and
Thursday, an upper-level ridge is forecast to build eastward with
its axis cresting our region Friday or Friday night. At the surface,
weak high pressure shifts east of our area into Thursday, however
there may be a weak low pressure sliding across northern New England
Thursday. An associated weak cold front may settle south across our
area Thursday afternoon and Thursday. As the ridge aloft arrives
Friday, surface high pressure centered well to or north-northeast is
forecast to extend southward into our area keeping a front to our
southwest.

Our sensible weather through this time frame may be limited to some
times of more cloudiness as the system moving through is on the weak
side and moisture looks limited. A warm air mass will be in place
Thursday, however light enough flow should keep it much cooler
closer to the coast with a sea breeze circulation. In the wake of
the weak system, some cooling is forecast for Friday although
daytime temperatures are still above average. Even on Friday,
temperatures will be much cooler closer to the coast once again with
more of a marine influence.
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&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
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Summary...Much cooler over the weekend with increasing chances for
showers, then turning warmer early next week.

Synoptic Overview...An upper-level ridge axis should be shifting to
our east or weakening Saturday. An upper-level trough sliding across
the Midwest is forecast to shift east and especially northeast
through the weekend into early next week. A cold front attached to
the associated surface low should be slow to arrive into our area
over the weekend as the main system tracks well to our northwest and
north. The front itself may not settle east and south of our area
until sometime Monday. This may result in a period of more unsettled
weather.

For Saturday and Sunday...As an upper-level trough slides across
south-central Canada to the Midwest, it mostly shifts east-northeast
more into Canada. A surface low associated with it then tracks well
north of our region across Canada. An attached cold front however
will slowly be approaching our area during this time frame. Given
how far removed the main trough is from the surface cold front, the
front should be slow to get into our area. A lead shortwave trough
however may provide enough lift for some increase in showers later
Saturday through Sunday. The amount of shower activity that occurs
with this system is less certain at this time as it will depend on
the forcing strength that arrives. Surface high pressure centered to
our northeast Saturday before shifting farther east will keep an
onshore wind in place and therefore a much cooler air mass across
our entire region. The marine influence should also limit the amount
of instability.

For Monday and Tuesday...The aforementioned upper-level trough
across mostly eastern Canada shifts eastward with more of a zonal
flow aloft across our region. Weak high pressure is forecast to be
working its way into our area as a weakening cold front settles
south and east of our area. This time frame could be precipitation
free, however given some timing uncertainty with the front went with
the NBM PoPs (20-40 percent). An offshore wind to start should turn
more southerly with time and therefore allowing temperatures to warm
quite a bit. Some cooling may still occur however along the coast as
the wind becomes more off the ocean again.
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&&

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

Through tonight...Showers and a few thunderstorms are expected
to progress from west to east. The storms could start as early
as 21-22Z around ABE/RDG, but more likely near or after 00Z for
most of our region, possibly down to I-95. Have not carried
mention of TEMPO TSRA outside of ABE and RDG, as activity will
likely weaken toward sunset, with more stable air across NJ. Once
showers and storms decrease in coverage between 03 and 06Z,
CIGs may lower to MVFR toward morning, mainly north of PHL
toward TTN and ABE. Winds tending light NE, or variable at times.
Low confidence.

Wednesday...Some MVFR or at least low VFR CIGs possible in the
morning, particularly from PHL northeastward. Some light SHRA
possible around ACY early in the morning, otherwise mainly VFR
with partial clearing from west to east heading past 15-18Z.
Winds will become more WNW toward the afternoon from I-95
northwest, but remain E or SE toward the coast, around 4-8 kt.
Moderate confidence.

Outlook...

Wednesday night through Friday...Mainly VFR.

Saturday and Sunday...Sub-VFR conditions probable with increasing
chances for showers.
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&&

.MARINE...
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Winds and seas are mostly expected to stay below Small Craft
Advisory criteria through Wednesday. However, In the Atlantic
Coastal waters of South Jersey and Delaware, seas could briefly
build near 5 ft as southwest winds pick up to 15-20 kt with some
occasional gusts near 25 kt for a few hours through this
evening. This seems brief and localized enough so that a SCA
doesn`t seem warranted. Winds easing and tending more northerly
later tonight, then becoming onshore and generally SE around 10
kt on Wednesday, with seas around 3 ft.

Outlook...

Wednesday night through Sunday...The conditions are anticipated to
be below Small Craft Advisory criteria, although seas could build to
5 feet for a time Saturday night and Sunday on the ocean zones.
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&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$

SYNOPSIS...Gorse
NEAR TERM...Dodd
SHORT TERM...Gorse
LONG TERM...Gorse
AVIATION...Dodd/Gorse
MARINE...Dodd/Gorse