510
FXUS61 KPHI 290427
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1227 AM EDT Mon Apr 29 2024

.SYNOPSIS...
An upper-level high pressure ridge will build along the East
coast, with unseasonably warm temperatures expected Monday.
A backdoor cold front will then arrive Monday night and
hang across our region on Tuesday, while an upper-level trough
approaches. Showers and thunderstorms may threaten 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/...
As of 12:15AM...Conditions are relatively quiet across the
region. Some shortwave energy remains and some rain showers can
still be noted in the more northern parts of our forecast area.

Tonight...A mostly quiet and warm night will be tap for the region.
Any remaining scattered showers should dissipated by 1 or 2AM. Lows
right around 60F can be expected with winds around 5 mph or less.
Some areas could see winds go light and variable at times. Added
some patchy fog to the forecast with this update; many of the model
soudings suggest some patchy fog could develop across areas during
the early morning hours. Warm air advection will continue to occur
and RHs should rise with time.

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 be near 60F, so the Heat
Index won`t be much different than the air temperature. No heat
headlines are anticipated. 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.

Monday night...Overall, another warm night anticipated. A backdoor
front will sag over region from the north/northeast and become
stationary over the region. 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. 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 some patchy fog development once again
for some areas.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
Beginning on Monday evening, we may see a few lingering shower
mainly across northern New Jersey, perhaps extending down to
northern portions of the shore, but those will likely dissipate
quickly after sunset with very weak synoptic-scale forcing and
a mid/upper-level ridge axis aloft. Otherwise expect a mainly
dry and quiet night. Low temperatures will be quite mild across
much of our region, with many areas south and west of about
Trenton forecast to stay above 60F, which would be the mildest
night since last October. The exception, with lows into the
lower 50s, will be toward the northern NJ shore. Surface high
pressure presses southward across New England and sends a
shallow backdoor cold front into the northeast corner of our
area. That can be seen in the wind fields of the latest hi-
resolution models.

While that ridge axis will start to give way to an approaching
upper-level trough Monday night into Tuesday, shifting off the
East Coast, mid-level temperatures will only lower slightly.
That warm air aloft will not manifest at the surface for
northeast portions of our region, though, thanks to that back-
door front. Models disagree on how far south that cooler airmass
will seep, but expect most of the shore to stay in the 60s with
east to southeast winds off the still chilly Atlantic waters.
Water temperatures are still mainly in the low to mid 50s. As
the warmer air aloft overrides that cooler airmass, low clouds
will develop late Monday night and may reach as far south as
near Philly to Brigantine NJ early Tuesday morning. Some of
those clouds should lift out heading toward the afternoon,
especially away from the shore, but will still keep temperatures
much cooler than Monday. For now we are still forecasting low
80s in Philly, but it will be a tricky forecast. Expect most
areas northeast of there to struggle to reach higher than the
low to mid 70s, with some model guidance suggesting parts of
northern NJ to the Poconos will stay mostly cloudy and in the
60s on Tuesday.

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 that while the absence of a distinct
surface cold front and weaker forcing aloft may limit
thunderstorm potential, anything that does develop across
eastern PA could be rather strong, even locally severe, before
it weakens. Do not expect widespread storms though, with fairly
limited rainfall area-wide Tuesday afternoon and evening.

Even on Wednesday, as low pressure pushes offshore, a fairly
moist atmosphere will remain in place, while some model
guidance slows the upper-level trough passage enough so that it
will not pass by portions of our area until the afternoon. That
may allow a few spotty showers to linger, 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 will take control heading into the last couple
days of the work-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 `back-door` shot of cooler air in, keeping at least much
of the shore and perhaps much of northern NJ in the 60s to low
70s. Dry weather is expected 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 /01Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR with some scattered clouds possible. Winds mainly
5 kts or less favoring a southwest direction initially, then
shifting northwest overnight. Some isolated SHRA/TSRA possible
through 02Z at ABE. High confidence in prevailing conditions.

Monday...VFR conditions with scattered clouds. Winds initially from
the northwest near 5-10 kts will shift to to southwest or even south
after 18Z. A 10-20% chance of an isolated shower. High confidence in
prevailing conditions.

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...

South to southeast winds may pick up to near 15 kt off the
southern NJ and Delaware coast enough to raise seas above 4 ft.
Some scattered showers and even a stray thunderstorm will
threaten by Tuesday evening and may linger into Wednesday.
Winds will tend more E to NE on Wednesday but still just around
10 kt or so, with seas near 3 ft. Expect very similar conditions
for Thursday as high pressure builds southward from New England
for the end of the work-week, with winds tending more southerly
by Friday. Small Craft Advisory conditions are not anticipated
through the week ahead.

&&

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

Record High Temperatures
                           Jan 25
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...Wunderlin
NEAR TERM...Johnson/Staarmann/Wunderlin
SHORT TERM...Dodd
LONG TERM...Dodd/Staarmann/Wunderlin
AVIATION...Dodd/Johnson/Staarmann/Wunderlin
MARINE...Dodd/Staarmann/Wunderlin
CLIMATE...