226
FXUS61 KPHI 300744
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
344 AM EDT Tue Apr 30 2024

.SYNOPSIS...
Backdoor front will remain stalled over the region today. A
cold front crosses through tonight and may stall over the region
through Wednesday. Weak high pressure looks to influence the
region Wednesday night through Friday. A cold front approaches
this weekend, which may bring some unsettled weather into early
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Back door cold front has made quick progress into our region,
already into the Philly metro area. This is faster and further west
than most guidance depicted. Consequently, I went below the mean
blend of guidance for high temperatures today. I expect the front
should slow or even stall soon, so I don`t expect it to make much
further progress east, but will be watching trends closely to see if
further adjustments will be needed.

In addition to temperatures, the other forecast concern today is the
potential for showers and thunderstorms. As mentioned by the
previous shift, the main round of showers and storms is expected
with a weak cold front approaching from the west. The timing for
this looks to have trended slightly later, likely 5 PM or later.
That being said, can`t rule out some isolated storms developing
along the first (backdoor) cold front if we are able to reach the
convective temperature. This is a bit of a question as there is
likely to be a stout inversion in place around 800 mb for much of
the day.

All that being said, once showers and storms develop from either of
the fronts mentioned above, there is a potential for strong storms
with gusty winds. Model soundings show CAPE values near 1000 J/kg in
areas well west of the backdoor cold front. That being said, the
larger scale forcing will be coming in after peak heating, when
instability will be waning. The main hazard with the stronger storms
is likely to be gusty winds. In addition to bulk shear values near
30 kt, model soundings show a mid level dry layer that could promote
a downburst or two.
Instability within the hail growth zone is too low to suggest much
of a concern with that.

Showers should weaken and decrease in coverage through the overnight
hours as the front progresses further east. Depending on how much
rain we get, low level stratus could develop over portions of the
region late tonight.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The weak cold front that comes through the region Tuesday night is
forecast to stall over the region for Wednesday. This is will likely
cause things to remain unsettled Wednesday before weak surface high
pressure builds in for the Wednesday night time frame. Weak high
pressure is then forecast to hold influence over the region into
Friday. However, weak surface high pressure influence may not mean
freedom from unsettled conditions. A small upper-level trough just
north of the Great Lakes region Wednesday night looks to slide
southeastwards with time, remaining just to our north into Thursday
night. Some upper-level shortwave energy may continue to plague the
region through the short term.

Slight chance and chance of PoPs for Wednesday thanks to the stalled
front and lingering shortwave energy; PoPs will decrease with time
Wednesday. NBM and most model guidance suggests a dry forecast for
Wednesday night through Thursday night; no PoP included in the
forecast. However, with the upper-level trough movement to our north
and possible shortwave energy, a few stray and isolated pop-up
showers should not be ruled out. If any showers do manifest
Wednesday night onwards, they will not be all that impactful.

Temperatures will remain rather warm for this time of year. Looking
at mostly 70s across the region for Wednesday with colder
temperatures for the coastal areas. More interior and urban
areas will likely get into the mid 70s. Temperatures are
expected to be warmer Thursday than Wednesday. More interior and
urban areas could see high temperatures around 80F Thursday.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Ensemble guidance suggests an upper-level ridge axis will be placed
over the eastern CONUS Friday. This ridge axis is forecast to move
eastward and breakdown with time rather slowly. Eventually, a more
zonal pattern looks to take hold around the time frame of Sunday
through the rest of the long term; this zonal upper-level pattern
being quite slow and stagnant to change. Really the whole upper-
level pattern evolution looks rather slow/stagnant and suggests a
slow-to-change situation will be in place for most of the long term.

At the surface-level, a cold front well to our west Friday looks to
march eastward while slowing down, and decaying some, with time.
Cold front could look to approach our region for several
periods, keeping things unsettled while slowly drawing near.
Cold front may not even cross through the region until Sunday or
later. Given the upper-level situation, the front may even
stall over our region beyond its arrival.

Given the descriptions above, expect an unsettled and showery
long term. PoPs increasing from the west Friday night into
Saturday with slowing frontal approach. Chance PoPS included
Saturday through Sunday night. Slight chance PoPs remaining
Monday with the front possibly stalling.

Temperatures around average look to return for Friday onwards.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through 12Z...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...Any low clouds may linger towards 15Z. The TAF site most
at risk for this is KTTN. Otherwise, VFR conditions for the bulk of
the day. 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.

Tonight...Showers and storms are expected to progress from west to
east. The storms could start as early as 21Z, but more likely near
or after 00Z for most of our region. Because of how close this is
occuring to sunset, expect the coverage of TSRA to be very limited.
Consequently, have only mentioned SHRA in the TAFs for now. Once
showers and storms decrease in coverage between 03 and 06Z, we could
then see a return of low level stratus with MVFR restrictions. Winds
light and variable. Moderate confidence.

Outlook...

Wednesday...Mainly VFR, though some sub-VFR conditions could linger
in the morning.

Wednesday night thru Friday...VFR.

Friday night/Saturday...Sub-VFR conditions possible.

&&

.MARINE...
Winds and seas are mostly expected to stay below Small Craft
Advisory criteria through tonight. However, In the Atlantic Coastal
waters of South Jersey and Delaware, seas could briefly build near 5
ft late this afternoon into this evening. Low confidence of this
occuring, and even if it does, it should be just barely to criteria.
Therefore, decided not to issue an SCA at this time.

Outlook...

Wednesday through Saturday...No marine headlines anticipated.
Showers possible (20-30%) for Wednesday. Showers possible
starting Friday night into the weekend.

&&

.CLIMATE...
Allentown broke the previous daily record high temperature
yesterday. Philadelphia and Trenton tied with previous records.
Check out the RERs.

&&

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

&&

$$

SYNOPSIS...Wunderlin
NEAR TERM...Johnson
SHORT TERM...Wunderlin
LONG TERM...Wunderlin
AVIATION...Johnson/Wunderlin
MARINE...Johnson/Wunderlin
CLIMATE...PHI