263
FXUS61 KPHI 210954
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
554 AM EDT Fri Jun 21 2024

.SYNOPSIS...
Bermuda high pressure remains in control through much of the
weekend. A back door cold front will move in from the north on
Friday and will stall over the area into Saturday before
returning north as a warm front. A stronger cold front will
approach the region Sunday looking to pass through Monday. High
pressure returns for Tuesday, followed by another front for the
middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The beat goes on with the heat continuing for the end of the
week. Temperatures will yet again be a touch higher with ambient
temperatures in the low to mid 90s and heat indices pushing
triple digits. The Heat Advisory that has been in effect for the
past several days remains in effect for counties north of a
line from Wilmington to LBI. However, not expecting any records
to fall as we have some impressive daily records for June 21st
(see climate section below for more info). The only climate site
with a remote chance is Trenton as it is the only location with
a forecasted high within 3 degrees of the record.

In addition to the heat, there is a small threat for severe
weather, mainly north of I-78 as a cold front sags down from the
north. Overall, the severe threat looks rather limited due to a
lack of shear. Most of the CAM guidance shows 0-3km shear
values less than 30 kt. The HREF ensemble shows 60-70% chance of
SBCAPE values exceeding 1000 J/kg though north of I-78, so
there is ample instability in place even with the lack of the
shear. What this all means is that thunderstorms will be
scattered in nature, with a few storms capable of producing some
stronger downbursts which could lead to damaging wind gusts.
However, given the lack of shear, any severe thunderstorms will
be relatively isolated, and thinking the bigger threat will be
north and west of the area where the better instability and
shear are. In terms of timing, the window for any severe storms
looks to be between 3-10 PM, though some showers/sub- severe
thunderstorms could linger into the night in northern NJ. With
weak flow aloft, any thunderstorms that do develop will be
relatively slow moving, so cannot rule out a quick inch or two
in some spots leading to poor drainage flooding. SPC has a
MARGINAL risk (1/5), which is about right. South of the I-78
corridor, it should stay dry and hot.

We will not get much relief tonight as temperatures will only
get down into the low to mid 70s. Though, compared to the next
couple nights, this will seem cool. Showers in northern NJ
slowly dissipate as the night goes on. Some patchy fog is
possible, especially in areas that see rain this afternoon and
evening as well as in more rural/sheltered areas such as the NJ
Pinelands.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
The heat and humidity continue to build into the region this
weekend with the most oppressive conditions of this heat wave so
far.

Back door cold front will be just over the Fall Line Saturday
morning and returns back north as a warm front on Saturday.
South to southwest flow will prevail, and an even warmer and
more humid air mass spreads into the Northeast and Mid-Atlantic.
Highs on Saturday rise into the mid and upper 90s for most of
the region, and in the upper 80s to low 90s in the southern
Poconos and northern New Jersey. Meanwhile, surface dew points
rise into the lower 70s in the morning, but look to mix down
into the upper 60s Saturday afternoon. This will yield max heat
index values in the upper 90s to low 100s. Current forecast has
max heat index values just under 105 for interior portions of
New Jersey. Plans for heat headlines is to convert the Excessive
Heat Watch to a Heat Advisory for portions of southeast
Pennsylvania and New Jersey. Will keep the Excessive Heat Watch
in effect for the urban corridor which includes eastern Chester
and New Castle counties north to Middlesex and Somerset
counties. This also includes Salem county, which, although not
technically part of the urban corridor counties, may touch a max
heat index of 105. It is close enough that those counties may
be converted to Excessive Heat Warnings, especially considering
the cumulative effects of the heat over the last week even if
max heat index values remain below 105. For the coastal strip of
New Jersey, southeast New Jersey, the eastern shores of
Maryland, and Kent and Sussex counties in Delaware, max heat
index values look to remain below 105, so will not hoist a Heat
Advisory in those counties for Saturday. There are currently no
Excessive Heat Watches for those counties for Saturday.

With the front north of the area, a thermal trough develops
over central Pennsylvania. Some shortwave energy approaches from
the west and passes through the region Saturday afternoon. This
will interact with the front just north of the area and will
spark off scattered afternoon and evening showers and
thunderstorms, mainly for areas north and west of the I-95
corridor. For areas north of I-80, showers and thunderstorms may
become likely. SB CAPE values will be 2000 to 2500 J/kg, and
PWATs will be around 2 inches. DCAPE values will even be around
1000 J/kg. The limiting factor may be shear, as 0-6 km Bulk
Shear could get up to 25 kt or so late in the day. Heavy rain,
gusty winds, and frequent lightning can be expected in the
stronger storms. The Storm Prediction Center has a Marginal Risk
(1 out of 5) for the southern Poconos, northern New Jersey, and
Lehigh Valley, mainly areas north of I-78.

Disgustingly warm and humid Saturday night. Lows generally in
the 70s, though not much below 80 in/around the I-95 corridor,
including Philadelphia and Delmarva.

Sunday looks to be the hottest and most humid day of this
stretch, however, there are some factors that may limit the
worst conditions from developing. Bermuda high pressure remains
anchored offshore and a cold front approaches from the west.
Southwest flow increases over the area, ushering hot and humid
air into the region. Forecast high temperatures will get into
the upper 90s for urbanized areas of southeast Pennsylvania,
most of southern New Jersey, and Delmarva, and in the mid 90s or
so for the areas north of the Fall Line. The southern Poconos
and northern New Jersey areas will be in the upper 80s to low
90s. Surface dew points will be in the lower 70s. Should the
highest temps be realized with these dew points, then widespread
max heat index values will range from 100 to 110. Will keep the
Excessive Heat Watch in effect for the entire region for
Sunday.

Should clouds associated with that cold front move into the
region in the afternoon, any increased cloud cover would
preclude the highest temperatures from occurring. In addition,
timing of convection will also have a major impact on if those
highest temperatures can be realized. For now, will keep the
Excessive Heat Watch in effect for the entire forecast area for
Sunday.

Back to that cold front. There are concerns about the timing of
the front. 00Z/21 NAM has convection firing up ahead of the
front on a pre-frontal trough late Sunday afternoon and moves
through Sunday night. The front itself does not move through
until late Sunday night. This signal is also showing up on the
00Z/21 GFS, however, the development of convection on that pre-
frontal trough develops earlier in the day compared to the NAM.
00Z/21 RGEM and ECMWF also showing something similar, though QPF
not as intense as NAM and GFS.

Should storms fire up in the afternoon, that would keep the
highest temperatures from developing, and and that would keep
instability levels down, potentially keeping the strongest
storms from developing.

Overall, will stick with the high temperature forecast in the
upper 90s. Will carry mostly chance PoPs for Sunday afternoon,
increasing to likely PoPs Sunday night. Will also mention
potential for gusty winds, heavy rain, and frequent lightning.
SB CAPE values will be up around 2500 J/kg, along with 1000 to
1500 J/kg DCAPE. 0-6 km Bulk Shear will be 20 to 25 kt through
the afternoon, but rising to 30 to 35 kt Sunday evening as that
front edges closer. Main concern for timing of the strongest
storms will be from 18Z Sunday to 06Z Monday. Showers and
thunderstorms continue through the night as the cold front
slowly passes through the region.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Cold front continues to work its way through the region on
Monday, but the relatively cooler and drier are does not make
its way through the region until Monday afternoon. Surface dew
points will be in the low 70s in the morning, lowering to the
60s late in the day. Highs will be in the upper 80s to low 90s
with max heat index values in the mid 90s in southern New Jersey
and Delmarva. Lingering showers and thunderstorms will pass
through the region with the passage of that front.

High pressure builds into the region Tuesday with dry
conditions and highs in the upper 80s to low 90s with, compared
to this weekend, low humidity as surface dew points will be in
the low 60s.

A series of frontal boundaries are possible Wednesday and
Thursday with scattered showers and thunderstorms. Temperatures
look to rise back well into the 90s with possible heat headlines
on Wednesday before returning to the 80s on Thursday.

&&

.AVIATION /10Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Primarily VFR. Scattered showers and thunderstorms
possible north of I-78 may affect KABE and bring brief
restrictions, but the probability of that is low (only around
30%), so have kept prevailing VFR for the entire TAF period.
Quiet elsewhere with winds out of the southwest around 5-10 kt.
Winds more southerly at KACY/KMIV with the sea-breeze and at
KILG with the bay-breeze. Moderate confidence.

Tonight...Primarily VFR, though some patchy fog possible at
KRDG/KABE/KACY/KMIV. Probabilities are low however, only around
15-20%, and prevailing VFR was kept with the 12z TAF package.
South/southwest winds around 5 kt or less. Moderate confidence.

Outlook...

Saturday through Saturday night...Generally VFR.
Afternoon/evening SHRA/TSRA could result in brief sub-VFR
conditions. VSBY restrictions in fog possible at night.

Sunday through Sunday night...VFR during the day, then sub-VFR
in SHRA/TSRA in the afternoon and at night.

Monday...Mainly VFR, but scattered SHRA/TSRA with sub-VFR
conditions from time to time. VFR at night.

Tuesday...VFR.

&&

.MARINE...
No marine headlines expected through tonight. South/southwest
winds 10-20 kt and seas 2 to 4 feet.

Outlook...

Saturday...Generally sub-SCA conditions, however, sustained 15
to 20 kt winds will develop on the ocean waters.

Saturday night through Sunday night...SCA conditions likely
with 25 to 30 kt gusts, especially on the ocean waters.

Monday through Tuesday...Sub-SCA conditions.

Rip Currents...

Today...Southerly winds around 10-15 mph and breaking waves around
2-3 feet. Portions of the southern NJ coast will keep an
onshore component of the wind, in addition to the occurrence of
the Full Moon. For this reason, have maintained a MODERATE Risk
for all New Jersey beaches except Monmouth County. For the
Delaware Beaches and Monmouth County, maintained the LOW Risk of
dangerous rip currents.

Saturday...Similar weather conditions are expected on Saturday.
With this in mind, saw no reason to change Rip Risks for NJ/DE
beaches from what is currently posted for today.

Rip currents occur often in the vicinities of jetties and
piers. Utilize any guarded beaches if venturing out into the
water. For specific beach forecasts, visit weather.gov/beach/phi

&&

.CLIMATE...
Record high temperatures for today. Trenton is the only site
currently forecast to be within 3 degrees of their record. As a
result, not really expecting any records to fall today. No daily
record high minimum temperatures will be set for today as
temperatures this morning have all fallen below the record high
minimums.

Location          Record High (6/21)

Philadelphia, PA      99/1923
Allentown, PA         100/1923
Reading, PA           99/1923
Mount Pocono, PA      90/1953
Trenton, NJ           97/1923
AC Airport, NJ        97/1988
AC Marina, NJ         94/2012
Wilmington, DE        98/2012
Georgetown, DE        99/2012

Record high temperatures Saturday.

Location          Record High (6/22)

Philadelphia, PA      100/1988
Allentown, PA         95/1941
Reading, PA           96/1921
Mount Pocono, PA      90/1908
Trenton, NJ           99/1988
AC Airport, NJ        100/1988
AC Marina, NJ         92/1949
Wilmington, DE        98/1988
Georgetown, DE        97/2012

Record high temperatures Sunday.

Location          Record High (6/23)

Philadelphia, PA      97/1888
Allentown, PA         95/1965
Reading, PA           96/1908
Mount Pocono, PA      90/1908
Trenton, NJ           97/1894
AC Airport, NJ        98/1988
AC Marina, NJ         91/1909
Wilmington, DE        100/1894
Georgetown, DE        100/1988

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory until 6 AM EDT Saturday for PAZ070-071-102-104-
     106.
     Excessive Heat Watch from Saturday morning through Sunday
     evening for PAZ070-071-102-104-106.
     Excessive Heat Watch from Sunday morning through Sunday
     evening for PAZ054-055-060>062-101-103-105.
     Heat Advisory until 6 AM EDT Sunday for PAZ054-055-060>062-101-
     103-105.
NJ...Heat Advisory until 6 AM EDT Saturday for NJZ010-012-015>019.
     Excessive Heat Watch from Saturday morning through Sunday
     evening for NJZ010-012-015>019.
     Excessive Heat Watch from Sunday morning through Sunday
     evening for NJZ001-007>009-013-014-020>027.
     Heat Advisory until 6 AM EDT Sunday for NJZ001-007>009-013-020-
     027.
DE...Heat Advisory until 6 AM EDT Saturday for DEZ001.
     Excessive Heat Watch from Saturday morning through Sunday
     evening for DEZ001.
     Excessive Heat Watch from Sunday morning through Sunday
     evening for DEZ002>004.
MD...Excessive Heat Watch from Sunday morning through Sunday
     evening for MDZ012-015-019-020.
MARINE...None.

&&

$$

SYNOPSIS...MPS/Wunderlin
NEAR TERM...Hoeflich
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...Hoeflich/MPS
MARINE...Hoeflich/MPS
CLIMATE...NWS PHI