Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

660
FXUS61 KPHI 220038
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
838 PM EDT Sat Jun 21 2025

.SYNOPSIS...
Hot and dry high pressure across the western Atlantic will build
west into the Appalachian region early next week. This will
result in a significant heat wave for much of the Eastern US
with near record breaking temperatures forecast. The high
pressure system will begin to weaken and retreat to the south by
the middle to end of the week, and temperatures will return
closer to normal along with increasing chances for showers and
storms.

&&

.NEAR TERM /THROUGH SUNDAY/...
A multi-day dangerous heat event begins Sunday.

The center of a strong upper-level ridge positioned across the Ohio
and Tennessee Valleys will start to expand northeastward some late
tonight and especially during Sunday. As this occurs, surface high
pressure will build westward from the far western Atlantic. This
will all direct a hotter and more humid air mass across our area
through Sunday night.

Overnight, some cirrus from upstream convection continues to dive
southeastward across our area. A shortwave trough however is forecast
to slide across New England later tonight and Sunday morning. Shortwave
energy associated with this along with some convectively enhanced
energy will drive one or more convection clusters around the
northern periphery of the building ridge. The guidance has generally
been settling in on a solution that keeps the convection mainly
to our north through Sunday. However, there are signals in some
of the guidance as the convection moves across parts of New England
a trailing outflow may settle into portions of our region from
the north. There is a low potential that some renewed convection
occurs along this lingering boundary during the day Sunday.
This is very low confidence at this time as much of the guidance
is dry. We will need to watch this potential though and also if
some storms can initiate over the terrain in the Pocono region
and then dive southward. Given all of this, opted to keep the
forecast dry on Sunday.

Sunday is the start of a multi-day dangerous heat event, with high
temperatures rising into the mid 90s (some upper 90s) across much of
the area. The dew points will also increase into the upper 60s to
low 70s, although these could mix out a little during peak heating
especially across the coastal plain. Heat indices to around 100
degrees are expected for nearly the entire region, with these a
little lower in the higher elevations and closest to the coast. As a
result, the Extreme Heat Warning and Heat Advisory both begin Sunday
morning. For far southern New Jersey and most of our Delmarva zones,
the criteria here is higher and therefore continued to not include
them in a heat related headline for Sunday. A stronger breeze
develops on Sunday, although given ample heating over land the wind
direction may stay more southwest or south-southwest for the coastal
areas keeping the New Jersey beaches especially cooler than a few
miles inland.

It will be very warm and muggy Sunday night as dew points rise some
and low temperatures drop only into the 70s. It may struggle to drop
below 80 degrees in the highly urbanized centers.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
Significant heat wave peaking Monday and Tuesday with
dangerously hot and humid conditions expected.

A strong ridge of high hot and dry pressure will build across
the eastern US Monday and Tuesday. Mid level heights,
thicknesses, and 850 mb temperatures contributing to the heat
wave will all peak during this time frame. The center of the
ridge will be positioned to our southwest, which should maximize
heat potential locally. At the surface, winds will be modest
from the west or northwest. This will add a component of
downsloping into the temperature equation, helping to boost
temperatures by a couple degrees. The wind direction will also
lessen the influence of the sea breeze, or lack thereof, for
coastal areas. The pattern is checking a lot of boxes for a set
up capable of producing fairly widespread 100 degree
temperatures in our region, perhaps one of the most favorable
patterns in a few years.

Fortunately, the pattern will favor dewpoints mixing out
reasonably well into the afternoon hours, but the high
temperatures will still result in heat indices of around 100-110
degrees across the board both Monday and Tuesday. At this
point, Monday looks to be the most humid of the 2 days, and thus
highest heat index day with heat indices around 105-110 degrees
in many areas. Slightly drier air, slightly stronger northwest
flow, and perhaps deeper mixing on Tuesday will favor slightly
higher temperatures, but heat indices should be near or perhaps
a few degrees lower than Monday. Regardless of those details,
it`s going to be very hot both days!

Extreme Heat Warnings and Heat Advisories remain in effect as
previously issued. Around the Delaware Valley, Lehigh Valley,
and northern New Jersey, temperatures could be the hottest
observed in over a decade. There will be little relief overnight
as temperatures Monday night and Tuesday Night are expected to
be around 75 to 80 degrees. Heat tends to have cumulative
effects on the body and we are currently forecasting 4
consecutive days of 95+ with Monday and Tuesday likely to
feature high temperatures around 100 degrees and heat index
values pushing 105-110 degrees. Take frequent breaks if outside,
and drink plenty of fluids. Not much else to say to highlight
the threat and main story of the upcoming week.

For details regarding the last 100 degree day and temperature
records during the upcoming heat wave, see the Climate section
below.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
For Wednesday and Thursday...The upper-level ridge weakens a
little bit and flattens out/erodes, especially on the northern
periphery. While the heat and humidity looks to continue, it
should decrease at least some by Wednesday, and especially
Thursday and Friday with continued erosion of the northern side
of the ridge. In addition, a surface trough should become better
defined and introduce a bit more clouds and also the potential
for at least some diurnally driven convection. There is a cold
front that tries to slide southward on Wednesday, but there is a
decent amount of uncertainty with how far south the cold front
makes it given the strength of the ridge. Due to this front,
there is a slight chance to chance (15-30%) of showers and
thunderstorms late Wednesday from the Philadelphia metro on
north and west. Increased cloud cover will keep temperatures a
touch cooler on Wednesday, but the Extreme Heat Warning remains
in for northern Delaware, the Philadelphia metro, Lehigh Valley,
and central/northern NJ. Temperatures will be a few degrees
cooler compared to Monday/Tuesday, but still in the mid to upper
90s, marking the 4th consecutive day of highs greater than 95
degrees. Heat indices will be near 100 degrees on Wednesday too,
so a slight improvement, but still hot.

As mentioned above, the ridge erodes a bit towards the end of
the week with a signal for some troughing building in from the
north that could lead to showers and thunderstorms both days to
end the week. The temperatures look to still hold near 90
degrees on Thursday and in the 80s by Friday, with heat indices
still in the 90s but this will be lower than earlier in the
week.

&&

.AVIATION /01Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. West to Southwest winds less than 10 knots,
becoming light and variable at some terminals. High confidence.

Sunday...VFR. Light and variable winds becoming west to west-
southwest 5-10 knots in the morning then 10-15 knots with some gusts
up to 20 knots in the afternoon. The winds then diminish in the
evening. Moderate confidence.

Outlook...

Monday through Tuesday Night...VFR. No significant weather.

Wednesday and Thursday...Prevailing VFR, though chances of a
shower/thunderstorm (20-40%) for the I-95 and Lehigh Valley
terminals, especially Thursday.

&&

.MARINE...
The conditions are expected to remain below Small Craft Advisory
criteria through Sunday night, despite southwesterly winds
increasing some tonight and Sunday.

Outlook...

Monday through Thursday...No marine headlines expected. Winds
5-15 kts and seas 1-2 feet. Fair weather, but chances for
thunderstorms return by Thursday.

Rip Currents...

On Sunday, SW winds increase to 10 to mph with breaking waves
of 2 to 3 feet. This will result in a MODERATE risk for the
development of dangerous rip currents for the Jersey Shore
counties of Ocean, Atlantic, and Cape May. For the Jersey Shore
county of Monmouth, as well as Delaware Beaches, flow will be a
bit more offshore, resulting in a LOW risk for the development
of dangerous rip currents.

For Monday, W to NW winds of 5 to 10 mph will turn S in the
afternoon behind developing sea breezes. Since winds will be
fairly light, there will be a LOW risk for the development of
dangerous and life threatening rip currents for the Jersey Shore
and for Delaware Beaches.

For specific beach forecasts, visit weather.gov/beach/phi

&&

.CLIMATE...
Near record breaking heat is expected across the area through
the middle of next week. Below is a look at the last 100 degree
day, daily record high maximum and minimum temperature for all
of our climate sites.


Most Recent 100 Degree Day

Site                              Date/Temperature
Allentown (ABE)          July 22, 2011/104
AC Airport (ACY)         July 21, 2019/100
AC Marina (55N)          July  5, 1999/101
Georgetown (GED)         July 21, 2019/100
Mount Pocono (MPO)       July  3, 1911/103
Philadelphia (PHL)       July 18, 2012/100
Reading (RDG)            June 22, 2024/101
Trenton (TTN)            July 18, 2012/100
Wilmington (ILG)         July 18, 2012/101

Record High Temperatures
                          June 23
Site                  Record/Year
Allentown (ABE)           96/2024
AC Airport (ACY)          98/1988 & 2024
AC Marina (55N)           91/1909
Georgetown (GED)         100/1988
Mount Pocono (MPO)        90/1954
Philadelphia (PHL)        98/2024
Reading (RDG)             98/2024
Trenton (TTN)             97/1894
Wilmington (ILG)         100/1894

Record Warmest Low Temperatures
                          June 23
Site                  Record/Year
Allentown (ABE)           71/2017 & 2024
AC Airport (ACY)          75/2024
AC Marina (55N)           74/2010
Georgetown (GED)          76/1988
Mount Pocono (MPO)        69/2024
Philadelphia (PHL)        76/1888
Reading (RDG)             77/2024
Trenton (TTN)             74/2017 & 2024
Wilmington (ILG)          75/2024

Record High Temperatures
                          June 24
Site                  Record/Year
Allentown (ABE)           98/1923
AC Airport (ACY)          99/2010
AC Marina (55N)           95/2002
Georgetown (GED)          97/2010
Mount Pocono (MPO)        92/1914
Philadelphia (PHL)        99/1923
Reading (RDG)             98/1908 & 1923
Trenton (TTN)             98/1894
Wilmington (ILG)         102/1894

Record Warmest Low Temperatures
                          June 24
Site                  Record/Year
Allentown (ABE)           71/2010
AC Airport (ACY)          72/1951, 2013, 2017 & 2024
AC Marina (55N)           72/1924, 2002, 2010, 2011, 2013 & 2020
Georgetown (GED)          74/2010
Mount Pocono (MPO)        66/1960, 1964 & 2002
Philadelphia (PHL)        75/1909, 1921, 1994 & 2020
Reading (RDG)             73/1909, 1994 & 2010
Trenton (TTN)             76/1894 & 2010
Wilmington (ILG)          74/1994

Record High Temperatures
                          June 25
Site                  Record/Year
Allentown (ABE)           98/1923
AC Airport (ACY)          96/1997
AC Marina (55N)           95/1952
Georgetown (GED)          96/1952
Mount Pocono (MPO)        91/1952
Philadelphia (PHL)        98/1952
Reading (RDG)             99/1943
Trenton (TTN)             99/1997
Wilmington (ILG)          98/1894

Record Warmest Low Temperatures
                          June 25
Site                  Record/Year
Allentown (ABE)           74/1923 & 1976
AC Airport (ACY)          80/1950
AC Marina (55N)           75/1997 & 2002
Georgetown (GED)          75/2019
Mount Pocono (MPO)        69/1952
Philadelphia (PHL)        76/1909, 1976 & 2002
Reading (RDG)             75/1943 & 1949
Trenton (TTN)             75/1976
Wilmington (ILG)          75/1976

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Extreme Heat Warning from 8 AM Sunday to 8 PM EDT Wednesday
     for PAZ060>062-070-071-101>106.
     Heat Advisory from 8 AM Sunday to 8 PM EDT Tuesday for PAZ054-
     055.
NJ...Extreme Heat Warning from 8 AM Sunday to 8 PM EDT Wednesday
     for NJZ001-007>010-012-013-015-017>020-027.
     Heat Advisory from 8 AM Monday to 8 PM EDT Tuesday for NJZ014-
     016-021>026.
DE...Extreme Heat Warning from 8 AM Sunday to 8 PM EDT Wednesday
     for DEZ001.
     Heat Advisory from 8 AM Monday to 8 PM EDT Tuesday for
     DEZ002>004.
MD...Heat Advisory from 8 AM Monday to 8 PM EDT Tuesday for MDZ012-
     015-019-020.
MARINE...None.

&&

$$

SYNOPSIS...Staarmann
NEAR TERM...Gorse/po
SHORT TERM...Hoeflich/Staarmann
LONG TERM...Guzzo/Hoeflich/Staarmann
AVIATION...Gorse/Staarmann/po
MARINE...Gorse/MPS/Staarmann
CLIMATE...Hoeflich/Staarmann