210
FXUS61 KPHI 292001
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
401 PM EDT Mon Apr 29 2024

.SYNOPSIS...
An upper ridge builds along the East Coast through the remainder of
today. A backdoor cold front approaches this evening and stalls
across the region on Tuesday as an upper trough approaches. Showers
and thunderstorms return later Tuesday and possibly into Wednesday,
before weak high pressure builds in for Thursday and Friday. Another
cold front approaches by the weekend which may bring some unsettled
weather through early next week.

&&

.NEAR TERM /THROUGH TUESDAY/...
An upper-level ridge axis is passing over the East Coast today, with
500 mb heights approaching 580 dm and 850 mb temperatures near 15C.
With some decent mixing, this is translating into high temperatures
well into the 80s across almost our entire region away from the
immediate coast this afternoon. Even a few spots have neared 90F,
and a few of our `climate sites` will be in jeopardy of tying or
even breaking their daily record highs for April 29th. A sea breeze
has been gradually marching inland and west-northwestward across New
Jersey, and should make it to the urban corridor around 8 PM this
evening. That will tend to cool temperatures back into the 70s, with
a light southeast breeze.

Meanwhile, we will need to keep an eye on the radar through about
sunset for pop-up showers and even a stray thunderstorm or two
possible, mainly north of an Allentown to Tom`s River line.
We had one shower pop-up in Morris County, but otherwise with the
upper-level ridge and associated subsidence, and very limited
forcing, convection will struggle to ignite and maintain itself for
very long. CAPE near 1500 j/kg including DCAPE values around 700
j/kg, at least according to RAP analysis, and effective shear of 20-
30 kt over northeast PA and northern NJ will mean that any
thunderstorm might be capable of becoming briefly strong, but should
not push severe limits.

Otherwise, heading through tonight it will be unseasonably mild once
again, with lows in the low 60s for much of our area.
However, a back door front will press in from the northeast as
surface high pressure noses southward across New England. That will
bring a push of cooler air on northeast to east winds, as well as
some low clouds and perhaps even patchy fog from midnight through
early Tuesday morning. Model guidance varies on how far south that
will reach, but it could come close to Philly by morning. That will
have an effect on high temperatures on Tuesday, even while the
clouds lift back north and scatter.
For now am still forecasting low 80s around Philly, but only near
70F at Trenton, and much of north NJ to the Poconos and down the
shore will be stuck in the 60s with some clouds hanging around on
Tuesday...a stark contrast from Monday.

By later Tuesday afternoon, we will once again need to watch out for
showers and thunderstorms, this time approaching from eastern
Pennsylvania. Outside of the influence of the cooler airmass from
the backdoor front and onshore flow, CAPE may reach 1000-2000 j/kg
along with shear of 30-35 kt of bulk shear. With some larger scale
synoptic forcing from an approaching shortwave trough, and an
associated weakening cold front, that should help support some
stronger thunderstorms. However, it is questionable how far east
across PA the better instability will extend, especially as we start
to lose daytime heating toward sunset. As a result, showers and
storms will likely weaken as they approach the I-95 corridor toward
or shortly after sunset.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
Any thunderstorm and shower activity that does enter our
northwestern areas will begin to dwindle and lose their punch as we
move deeper into Tuesday night. Not to say that embedded rumbles of
thunder won`t continue, but the firepower of the storms compared to
during the day will be much less due to the loss of diurnal heating.
Regardless, best chances for showers/thundershowers on Tuesday night
continue to be north and west of the I-95 corridor where up to a
half inch of rain is possible. For areas south and east, most places
are expected to remain quite dry aside for a passing shower under
mostly cloudy skies. Lows will be in the 50s for most, with temps
remaining in the 60s across the Delmarva.

By Wednesday the low pressure moves offshore, however while temps
drop, humidities levels increase as dew points remain in the upper
50s. With upper trough slowing down as it passes by, there likely
will be a few showers lingering across South Jersey and the Delmarva
into Wednesday afternoon with rainfall amounts up to a tenth of an
inch in these areas. Most places north of Philadelphia should dry
out despite stubborn cloud cover hanging around most of the day.
Temps will be in the 70s for much of the area except along the shore
which will be stuck in the low 60s with continued onshore flow.

By Wednesday night and Thursday, upper trough moves far enough
offshore that the area is no longer impacted as upper ridge begins
to approach from the west. Coinciding high pressure builds nearby
as well leading to the return of sunny skies by Thursday. Lows will
be in the mid 40s to mid 50s on Wednesday night before getting back
up into the mid 70s to around 80 degrees on Thursday.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
High pressure remains nearby Thursday night and Friday before
beginning to vacate the area into Friday night. As it does so,
guidance is keen on having another backdoor cold front sinking
southwestward across the area with another push of some cooler air.
On the synoptic scale back to the west, another upper trough will
begin to approach the area over the weekend which will lead to
periods of showers and possible thunderstorms throughout the
weekend. Not looking like a washout by any means as PoPs are only in
the 30-50% range, but definitely think there will be some shower
activity around with abundance of low-level moisture and approaching
cold front from the west. Depending on timing of the front, shower
activity may linger into the start of next week as well, but the
exact details on this is yet to be seen considering it is a week
away.

Temperatures throughout the long term period are progged to be quite
seasonable for early May with a mixed bag of 60-70s across the area.

&&

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

Tonight...Mainly VFR for most sites. However, guidance indicates a
stratus layer will push in from the northeast, associated with a
backdoor cold front. As a result, ceilings could lower to IFR or
even LIFR from TTN to ABE northward, possibly reaching as far south
as RDG, PHL, and ACY for a few hours early Tuesday. Some patchy fog
may once again occur at some sites. Mainly variable or easterly
winds 5 kts or less. Low confidence.

Tuesday...Some IFR/LIFR may persist until 15Z or so but should lift
north of TTN and ABE before too long into the morning.
Otherwise, VFR conditions for the bulk of the day. Showers and even
a few thunderstorms will try to push in from the NW in the
afternoon, reaching ABE/RDG after 20-21Z, and possibly surviving to
near I-95 around 00Z or so. Winds will be mainly easterly at the TAF
sites, but may vary around PHL and ACY as some SW winds try to push
in for the afternoon. Winds will be 10 kt or less, though. Moderate
confidence.

Outlook...

Tuesday night...VFR early, likely becoming sub-VFR at KABE/KRDG with
showers and thunderstorms, possible sub-VFR along the I-95 corridor
terminals with shower activity as well. Winds light and variable.
Moderate confidence.

Wednesday through Friday...Any lingering sub-VFR CIGs early
Wednesday will become VFR by the afternoon and persist through
Friday. High confidence.

Friday night through Saturday...Sub-VFR possible with rain showers.
Low confidence.

&&

.MARINE...
No marine headlines anticipated through Tuesday. S to SE winds
around 10 kt will become more SW later tonight into Tuesday near
and south of Atlantic City, while ENE winds will persist on
Tuesday farther north. Those southerly winds could gust 20-25 kt
Tuesday afternoon, with seas building above 4 ft, which could
briefly flirt with SCA criteria.

Outlook...

Tuesday night through Saturday...Winds and seas on both the ocean
waters and the Delaware Bay are expected to remain below SCA
criteria. Onshore flow varying between NE to SE winds are expected
through the period. Shower activity is possible Tuesday night into
Wednesday with another period possible on Saturday.

&&

.CLIMATE...
Near record breaking high temperatures are forecast today.
Records for our climate sites are listed below:

Daily Record High Temperatures for April 29th

Site                  Record/Year
Allentown (ABE)           86/1974
AC Airport (ACY)          92/1974
AC Marina (55N)           88/2017
Georgetown (GED)          91/2017
Mount Pocono (MPO)        82/1974
Philadelphia (PHL)        90/1974
Reading (RDG)             91/1888
Trenton (TTN)             88/1974
Wilmington (ILG)          91/1974|

&&

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

&&

$$

SYNOPSIS...Deal/DeSilva
NEAR TERM...Dodd
SHORT TERM...Deal/DeSilva
LONG TERM...Deal/DeSilva/Staarmann
AVIATION...Dodd/DeSilva/Wunderlin
MARINE...Dodd/DeSilva/Wunderlin
CLIMATE...WFO PHI