193
FXUS61 KPHI 291428
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1028 AM EDT Mon Apr 29 2024

.SYNOPSIS...
An upper-level high pressure ridge will build along the East
coast bringing unseasonably warm temperatures today. A backdoor
cold front will approach this evening and hang across our
region on Tuesday, while an upper-level trough approaches.
Showers and thunderstorms return later Tuesday and possibly
linger into Wednesday, before weak high pressure builds in for
Thursday. A cold front could approach from the west later Friday
and pass through during the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Through mid to late morning on Tuesday...weather conditions
remain quiet across the region, with a rather `muggy` feel to
the air for one of the first times this year. Dewpoints have
crept into the low 60s across the region, which will feel very
comfortable and refreshing in a few months, but most of us
haven`t acclimated yet. Refined POPs for this afternoon and this
evening given the latest CAMs. Generally no change with respect
to the expectation of some scattered showers and perhaps a
thunderstorm north of an Allentown to Trenton to Tom`s River
line, mainly between 4 PM and 8 PM.

Otherwise for today...500 mb heights look to be near 580 dam,
values more typical of summertime. At the surface, high pressure
will remain centered offshore of the Carolinas, helping to
advect warmer and more moist air into the region. The result of
this will be very warm day for the region with much above normal
temperatures.

High temperatures are expected to soar into the mid to upper 80s in
most interior locations under partly cloudy skies. Highs in the more
urbanized areas could top out closer to 90 degrees, though readings
of 90 degrees or higher still appear unlikely. Northwest flow will
add a downsloping component to the winds and thus also help to boost
temperatures locally near the urban corridor. The offshore wind will
also allow for a notable warmup even along the immediate coast, with
highs getting to the low to mid 70s before a sea breeze kicks in.

As the gradient flow weakens and shifts eventually to the southwest
then south, a sea breeze will likely make significant inland
progress into the I-95 corridor and perhaps even farther inland from
there during the evening. This will cool temperatures off into the
70s and 60s by late evening. Dewpoints will remain near or above
60F for most of our region; humid for late April, but nothing
that will bring the Heat Index much higher than the actual air
temperatures. With some additional weak shortwave energy
rounding the top of the upper- level ridge, some isolated
showers or storm are possible north of Philly, however the
subsidence will be quite strong and any convection will likely
struggle to materialize.

Tonight...Overall, another warm night anticipated. A backdoor
front will sag over the region from the north/northeast and
become stationary. Where exactly the front ends up stalling will
be a forecast challenge. Low temperatures expected to be in the
low to mid 60s southwest of the front and in the mid to upper
50s northeast of the front for most locations. Mostly cloudy
skies anticipated for areas northeast of the front; mostly clear
to partly cloud skies anticipated for areas southwest of the
front. With some lingering shortwave energy, some isolated
showers remain possible, mainly for our northern areas. Model
soundings support fog development once again during the early
morning hours; cannot rule out patchy fog development once
again.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
High pressure starts to give way on Tuesday as an upper level
trough digs into the Carolinas region. This starts to being a
return of some moisture to the area however the moisture looks
to ride along the top of the departing ridge. The net effect is
that Tuesday remains abnormally warm across DelMarVa and
southern PA/NJ with some near normal temps working in over the
Poconos and northern NJ. Cloud cover will likely be the
determining factor in how warm or cold temps end up as guidance
suggests a stalled front should hang slightly to the north
Trenton.

Aside from the temperatures Tuesday, we will have to keep an
eye out for showers and thunderstorms developing across central
and eastern PA ahead of the approaching upper-level trough. The
southern stream portion of the trough will split and hang back
across the Tennessee Valley, with the northern stream shortwave
tending to `pull-up` and weaken as it approaches. Additionally,
a new surface low will develop around SE PA into northern
Delaware on Tuesday, before tracking ENE offshore Tuesday night.
All of this will tend to limit instability and forcing from the
Delaware Valley eastward across NJ, so that storms will
eventually weaken as they head into that airmass. That said,
guidance does indicate 1000-2000 j/kg of CAPE and perhaps 30-35
kt of deep layer shear, so there is potential for some strong to
possible severe thunderstorms to develop. However currently the
missing ingredient is a trigger to fire the storms. Do not
expect widespread storms though, with fairly limited rainfall
area-wide Tuesday afternoon and evening.

By Wednesday the low pressure pushes offshore, however while
temps drop, humidities levels increase. Thus with a slowing of
the upper- level trough passage there may be a few spotty
showers lingering along with a fair amount of cloud- cover.
Lowered high temperatures a bit from NBM guidance otherwise
area-wide, as much of our region will be more exposed to cooler
east- northeast flow, keeping highs mainly in the low to mid 70s
from the Delaware Valley west, and in the 60s toward the shore.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
High pressure returns for the middle of the week and should
persist through the end of the week. While upper- level ridging
will start to shift eastward across the Appalachians toward
Friday, another surface high pressing southward from New England
may bring another backdoor front with associated cold air
advection, keeping at least much of the shore and perhaps much
of northern NJ in the 60s to low 70s. Sensible weather is
expected to be dry with fairly plentiful sunshine, and slightly
above normal temperatures farther southwest from the marine
influence. Some showers may threaten as early as late in the day
Friday, with increasing clouds toward eastern PA, as the next
storm system approaches. That looks to be another surface low
lifting across the Great Lakes, with a weakening surface cold
front heading our way over the weekend. Stayed with NBM POPs.
Not looking like a washout, but some scattered showers and
possibly some thunderstorms possible. Temperatures should remain
pretty close to typical values for early May, if not slightly
above normal.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR conditions with few to scattered clouds. Winds
initially from the W/NW near 5-10 kts will back with time. Sea
breeze will develop and penetrate further inland with time
during the afternoon/evening, causing winds to shift out of the
S/SSE. Slight chance to chance (20-40%) of isolated/scattered
showers in the afternoon into the evening, mainly for sites
north of KTTN. High confidence overall, lower confidence in
details (e.g., exact sea breeze timing).

Tonight...Mainly VFR for most sites. However, guidance indicates
a stratus layer could move in from the northeast. How far this
layer gets will be determined by where the front ends up
stalling. CIGS could lower to 10k ft or less for sites north of
KPNE in the later half of end of the period (i.e., KABE/KTTN).
Some patchy fog may once again occur at some sites. Mainly
variable or easterly winds 5 kts or less. Low confidence.

Outlook...

Tuesday and Wednesday...Mainly VFR but occasional MVFR and even
briefly/locally IFR conditions possible in scattered showers and
thunderstorms later Tuesday afternoon and evening. Some of those
showers may linger into Wednesday.

Thursday and Friday...VFR conditions expected.

&&

.MARINE...
No marine headlines anticipated through Monday night. WSW winds 5-10
kts will shift briefly W then NW this early morning due to a
land breeze. Winds shift to the SSE/S by late morning then hold
S through Monday night. Seas 2-3 feet.

Outlook...

Winds and seas expected to be sub SCA through the period.
Generally south to southwest winds through Wednesday veering to
the northeast then onshore late Wednesday. Fairly persistent
onshore flow anticipated through the end of the week. Medium
chance (40-50%) of scattered showers with isolated thunderstorms
Tuesday night into Wednesday morning.

&&

.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
NEAR TERM...Dodd/Johnson/Staarmann/Wunderlin
SHORT TERM...Dodd/Deal
LONG TERM...Dodd/Deal/Staarmann
AVIATION...Dodd/Deal/Wunderlin
MARINE...Dodd/Deal/Wunderlin
CLIMATE...Staarmann