-- Discussion containing changed information from previous version are highlighted. --
802
FXUS61 KPHI 191226
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
826 AM EDT Wed Jun 19 2024

.SYNOPSIS...
High pressure remains in control. A back door cold front may
pass through the region Friday night. A stronger cold front will
pass through the region Sunday night through Monday, and that
front may hang over the area through Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
The Heat Advisory continues for today for most of the region,
and although temperatures have come down a touch, the impacts
remain the same. Limit outdoor time today and if you have to be
outside, take frequent breaks and stay hydrated!

A strong 597-598 dam ridge will remain overhead for today. The
result will be large-scale subsidence and hot temperatures
continuing. Guidance has come down slightly, and it does look
like dew points will mix out a bit, resulting in lower heat
indices. However, the cumulative effect of the heat will
continue, and no changes to the headlines were made through
tonight. Other than some morning cloud cover, especially near
the Philadelphia Metro area, skies will be mostly sunny with
temperatures ranging in the upper 80s/low 90s. With dew points
mixing out, heat index values will be close to the air
temperature. Max apparent temperatures are currently forecast to
be in the low to mid 90s.

For tonight, there actually could be some relief as clear skies
and light winds will result in favorable radiational cooling
conditions. Temperatures will get down into the low to mid 60s.
The exception being within the urban corridor, where the urban
heat island effect will keep temperatures in the upper 60s/low
70s.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Overall, the trend is for somewhat lower temperatures and
somewhat lower humidity levels. In fact, highs will generally be
in the upper 80s to low 90s on Thursday along with surface dew
points starting out in the mid to upper 60s, lowering to the low
to mid 60s in the afternoon. This results in max heat index
values in the low to mid 90s, which would be just under Heat
Advisory criteria. However, given that this will be the third
day of heat, feel comfortable converting the Excessive Heat
Watch that is in effect for Thursday to a Heat Advisory based on
the cumulative impacts of the daytime heat along with the lack
of relief that due to relatively warm overnight low
temperatures.

By Friday, Bermuda high pressure becomes entrenched off the
Mid- Atlantic coast. Increasing southerly flow will usher a
warmer and more humid air mass into the region. Highs on Friday
should get into the low to mid 90s, perhaps in the upper 90s
along the I-95 corridor, including Philadelphia. Surface dew
points rise into the upper 60s to low 70s, resulting in max heat
index values in the upper 90s to low 100s. Based on current
forecast, the max heat index values look to fall just below
Excessive Heat Warning criteria, but since this would now be the
fourth consecutive day of high heat and now increasing
humidity, will keep the Excessive Heat Watch in effect for
Friday, but it may be converted to a Heat Advisory.

The other aspect for Friday is that a back-door cold front will
sag in from the north, sparking off afternoon and evening
showers and thunderstorms. These storms may have an impact on
high temperatures Friday, but it is far too soon to determine
timing and placement of those storms and what impacts those
storms would have on the temperature forecast. Will follow NBM
and carry low-end likely PoPs for the southern Poconos and far
northern New Jersey, and chance PoPs down to just south of the
Fall Line, down to Philadelphia. For areas north and west of the
Fall Line, PWATs will be up around 1.75 inches and 0-6 km Bulk
Shear will be less than 20 kt. SB CAPE values should range from
1500 to 2000 J/kg. Localized flooding possible due to
potentially slow-moving thunderstorms producing heavy rain.

Warm and muggy both Thursday night and Friday night, though
given any thunderstorms, humidity will be elevated Friday night.
Lows will be in the upper 60s to low 70s, though lows will be a
couple of degrees warmer Friday night compared to Thursday
night. Patchy fog may develop Friday night as well.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Excessive Heat Watch will remain in effect for Saturday and
Sunday, however, the overall trend is for slightly lower high
temperatures. As a result, it is looking more and more likely
that the Watch could be converted to Heat Advisories, and even
in some cases, Heat headlines might not be needed at all for
some locations. Will keep the Excessive Heat Watch in effect for
now, mainly based on the cumulative impacts have based on
several days of heat and humidity with little, if any, relief at
night due to warm overnight low temperatures have.

While high temperatures seem to be trending a bit lower than
originally forecast, surface dew points are trending on the
higher side given persistent southerly flow allowing low level
moisture to build into the Northeast and Mid-Atlantic.

Back-door cold front through the northern half of the forecast
area Friday night remains nearly stationary on Saturday. As a
result, highs will be in the upper 80s to low 90s for the
northern portion of the forecast area, and even along the
coasts, highs will be in the low to mid 80s due to flow coming
off the cooler ocean waters. Just inland, including the I-95
corridor from Trenton to Wilmington and areas west, as well as
Delmarva, highs will get in the low to mid 90s. Surface dew
points will be in the upper 60s to low 70s. Max heat index
values in these areas will be in the upper 90s to around 100.

A thermal trough sets up over central Pennsylvania and some
approaching shortwave energy may spark off some afternoon and
evening showers and thunderstorms, with the highest PoPs north
and west of the Fall Line. Again, heavy rain and localized
flooding possible.

A cold front approaches on Sunday. Southerly flow ahead of the
front ushers warm and humid air with highs in the low to mid 90s
and surface dew points in the lower 70s. Max heat index values
look to be in the upper 90s to low 100s, but for now look to
remain below 105. Will keep an eye on potential severe weather
on Sunday. Ahead of the front, SB CAPE values rise to around
1500 J/kg and 0-6 km Bulk Shear will rise to 35 to 40 kt in the
afternoon and evening. DCAPE values will be up around 600 to 800
J/kg. PWATs will be up around 2 inches. Damaging winds and
heavy rain possible Sunday afternoon and evening.

Cold front comes through on Monday and may get hung up over the
area into Tuesday. With the front over the area, additional
scattered showers and thunderstorms possible Monday afternoon.
Highs will be in the upper 80s to around 90, with surface dew
points in the mid and upper 60s. Max heat index values will not
be much higher than the actual surface temperatures. Tuesday
looks to be dry with temperatures a few degrees above normal,
topping off in the upper 80s to low 90s.

&&

.AVIATION /13Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Updated the TAFS this morning as an area of lower clouds
have developed due to morning mixing across southeast
Pennsylvania, and will likely spread south and eastward across
southern New Jersey and Delaware over the next couple of hours
through 14z/15z. Once these clouds dissipate, VFR will develop
again for the rest of the day.. Winds out of the south/southwest
around 5-10 kt. South/southeast winds later in the day at KILG
with wind funneling up the Delaware Bay and at KACY/KMIV with
the sea- breeze moving through. High confidence.

Tonight...VFR. South/southwest winds around 5 kt. High
confidence.

Outlook...

Thursday through Saturday...Overall, VFR. Afternoon evening
SHRA/TSRA with sub-VFR conditions possible at
KRDG/KABE/KTTN/KPNE/KPHL Friday and Saturday.

Sunday...Generally VFR. Scattered afternoon SHRA/TSRA may
result in sub-VFR conditions.
-- End Changed Discussion --

&&

.MARINE...
No marine headlines expected through tonight. Winds will be out
of the south around 10-20 kt with 2 to 3 foot seas.

Outlook...

Thursday through Saturday...Sub-SCA conditions, though
persistent 15 to 20 kt winds may develop at northern NJ ocean
waters each afternoon.

Sunday...SCA conditions may develop with S winds 15 to 20 kt
with 25 to 30 kt gusts and Seas building to 3 to 5 feet.

Rip Currents...

Wednesday...With winds around 10 MPH or less and breaking waves
around 2 feet or less, all coastal zones have a LOW risk for the
development of rip currents.

Thursday...Winds will be a bit stronger on Thursday, with waves
slightly higher. Part of the NJ coast will have an onshore
component to the wind. For this reason, kept a MODERATE Risk
for all New Jersey beaches except Monmouth County. For the
Delaware Beaches and Monmouth County, continued with a LOW risk
of dangerous rip currents.

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. Reading and Mount Pocono
are the only climate sites with a forecast high within 2 degrees
of the record. Highly unlikely any other sites match or break a
record.

Location          Record High (6/19)

Philadelphia, PA      100/1994
Allentown, PA         96/1994
Reading, PA           95/1929
Mount Pocono, PA      86/1929
Trenton, NJ           96/1994
AC Airport, NJ        96/1994
AC Marina, NJ         93/1952
Wilmington, DE        100/1994
Georgetown, DE        96/1952

Record high temperatures Thursday.

Location          Record High (6/20)

Philadelphia, PA      98/1931
Allentown, PA         100/1923
Reading, PA           101/1923
Mount Pocono, PA      89/2012
Trenton, NJ           98/1923
AC Airport, NJ        95/2012
AC Marina, NJ         90/1908
Wilmington, DE        97/2012
Georgetown, DE        98/2012

Record high temperatures Friday.

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 Friday for PAZ054-055-060>062-070-
     071-101>106.
     Excessive Heat Watch from Friday morning through Sunday
     evening for PAZ054-055-060>062-070-071-101>106.
NJ...Heat Advisory until 6 AM EDT Friday for NJZ001-007>010-012-013-
     015>020-027.
     Excessive Heat Watch from Friday morning through Sunday
     evening for NJZ001-007>010-012-013-015>020-027.
DE...Heat Advisory until 6 AM EDT Friday for DEZ001.
     Excessive Heat Watch from Friday morning through Sunday
     evening for DEZ001.
MD...None.
MARINE...None.

&&

$$

SYNOPSIS...MPS
NEAR TERM...Hoeflich/Robertson
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...Hoeflich/MPS/Robertson
MARINE...Hoeflich/MPS
CLIMATE...