Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

103
FXUS61 KPHI 271857
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
257 PM EDT Fri Jun 27 2025

.SYNOPSIS...
A front well to our south and west will gradually lift northward as
a warm front Saturday, followed by a weak cold front stalling nearby
on Sunday. The front then shifts northward as a warm front Monday
followed by a cold front later Tuesday. High pressure builds closer
later Wednesday into Thursday before settling over our area on
Friday.

&&

.NEAR TERM /THROUGH SATURDAY/...
Satellite imagery and surface obs indicate the back door cold
front has stalled in eastern Virginia. Despite the front being
well south, spotty showers have developed across the region
thanks to elevated instability, warm/moist advection aloft and
weak upper-level impulses. Expect this to more or less continue
thru tonight.

The front is expected to start pushing back northward as a warm
front ahead of another cold front tonight, possibly causing
some fog to develop particularly across southern/coastal areas
as it does so later tonight into early Saturday. Temperatures
won`t change much, perhaps even warming late tonight as the
front approaches from the south.

The warm front should then push north of the area toward
midday Saturday, resulting in clouds breaking for some sun with
a very warm and humid day across the region. With temps likely
to approach 90 and dew points likely in the 70s, heat indices
will likely reach our early-season heat advisory thresholds
across the metro, so have issued a metro-only heat advisory for
heat indices of 96-99. Heat indices likely pass 100 across parts
of the Delmarva, but the all-year criteria there is 105, so no
advisories necessary.

Some spotty sea-breeze showers and storms may develop across NJ
and DE during the midday and afternoon Saturday, but the main
focus is the next cold front which approaches from the
northwest late in the day. With decent CAPE and shear but
nothing too crazy, for now we remain in a marginal outlook for
severe weather, with damaging winds being the main threat.
Depending on guidance trends, wouldn`t be surprised if this ends
up as a slight with a severe thunderstorm watch at some point
later tomorrow, but confidence isn`t there yet. Gotta see how
destabilized we get after we break out of this marine air mass
first.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
A more zonal flow aloft from the Midwest to the Northeast is
forecast to be in place Saturday night into Sunday. A shortwave
trough embedded in this flow looks to slide by to our north into
Saturday evening. Surface low pressure tied to this feature tracks
well to our north, however a weak cold front associated with it
arrives into our area Saturday evening. This front should tend to
stall in our vicinity Sunday as the flow remains mainly zonal aloft.
Some showers and thunderstorms should accompany the front into
Saturday evening, a few which could produce locally damaging wind
gusts. Some drier air will also start to arrive from the west in the
wake of the front later Saturday night, although much of the area
will remain on the muggy side Saturday night.

Some additional drying is forecast to take place on Sunday with dew
points dropping into the 60s for much of the area, although a more
humid feel may linger across parts of Delmarva. The extent of the
drying will depend on where the surface front sets up. There does
not appear to be a real strong focus for convective development
during Sunday, although a few showers or thunderstorms cannot be
ruled out mainly across Delmarva where deeper moisture may remain
longer. High temperatures are forecast to be well into the 80s for
the majority of our area, and with lower dew points the heat indices
are forecast to be lower Sunday afternoon compared to Saturday
afternoon.

As we go through Monday, an upper-level trough across the Midwest
and adjacent Canada is forecast to shift eastward and amplify some
as it approaches the Northeast. A front warm lifts north across our
area on Monday. A few showers or thunderstorms are possible with the
northward movement of the warm front, with the greatest chance (40-
50 percent) mainly across southwestern parts of our forecast area.
Temperatures will make a run at 90 degrees for many areas and with
dew points on the rise, heat indices will also be on the increase.
As of now, the forecast heat indices are below Heat Advisory
criteria (even for the the I-95 urban corridor, Wilmington to
Philadelphia to Trenton, where the criteria is lower through June
30th).

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Summary...Some showers and thunderstorms to start, otherwise no
extreme heat forecast.

Synoptic Overview...An upper-level trough is forecast to move into
much of the East by later Tuesday. This trough looks to become
reinforced as it remains across much of the East through the end of
next week. At the surface, a cold front moves through later Tuesday,
then high pressure builds closer later Wednesday into Thursday
before settling over our area on Friday.

For Tuesday...An upper-level trough is forecast to shift eastward
and amplify some as it overspreads the Northeast and Mid-Atlantic.
This will drive a cold front across our region later Tuesday. An
uptick in the shear is forecast, especially across Pennsylvania and
New Jersey where the stronger mid level flow is forecast to be.
Moderate instability may develop and if this overlaps with the
greater shear, a favorable area for severe thunderstorms (damaging
winds) may develop Tuesday afternoon and/or early evening. The
details will depend on the magnitude of the shear and instability as
well as the timing of the cold front and upper-level trough axis.
Temperatures are forecast to top out between 85-90F for much of the
region, although if cloud cover and convection hold off long enough
the highs could be a little higher than forecast. While it will be
rather humid, extreme heat is currently not forecast.

For Wednesday and Thursday...The upper-level trough across the
Northeast and Mid-Atlantic may tend to sharpen again as additional
energy rounds its base. The cold front should be south and east of
our area to start Wednesday, and while temperatures do not look to
drop much, the dew points are forecast to lower some in the wake of
the cold front. Despite the presence of the upper-level trough, less
in the way of deeper moisture should keep the instability lower as
well. Given surface high pressure building closer to our area with
time, the chance for any showers and thunderstorms at this time is
little to none.

For Friday...The upper-level trough is forecast to still be in place
with weak surface high pressure becoming more established over our
area. The return flow up the Ohio Valley will increase the low-level
warm air advection with a northeastward moving warm front. The
forcing for this, as of now, looks to remain well to our west and
therefore little in the way of shower chances. Daytime temperatures
in the 80s for most, and the dew points are forecast to be in the
low to mid 60s.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of Today...Spotty showers could reduce vsby in spots below
VFR, with some mist along the southern NJ coast (ACY).
Otherwise, mostly MVFR due to cigs. Winds mostly easterly 5-10
kts with a few gusts up to 20 kts. High confidence.

Tonight...A continuation of low CIGS with scattered showers. IFR
likely with LIFR possible, perhaps intervals of VLIFR. Some fog
could also develop, most likely at ACY. Moderate confid.

Saturday... Lower CIGS and perhaps vsby early Sat then VFR
expected to return toward midday. Scattered showers/tstms with
lower CIGS/VSBYs in the afternoon, mainly late. Low confidence.

Outlook...

Saturday night...Local sub-VFR conditions at times possible with a
few showers or thunderstorms around, otherwise VFR.

Sunday...VFR overall.

Monday...A few showers and thunderstorms, with lower ceilings and
visibilities at times, are possible.

Tuesday...Showers and some thunderstorms probable with local
restrictions, especially in the afternoon and evening.

Wednesday...VFR overall.

&&

.MARINE...
Winds and seas across Delaware Bay and the southern ocean zones
have diminished, but remain up across the northern zones. We`ll
continue the SCA flag accordingly til midnight, by which point
they likely diminish, but remain easterly 10-15 kts with seas
3-4 ft. Scattered showers and possibly an isolated t-storm thru
tonight, with some patchy marine fog also possible.

Sub-SCA expected on Saturday with winds turning southerly around
10 kts with seas 3-4 ft. Patchy marine fog possible early, spotty
thunderstorms possible mainly later.

Outlook...

Saturday night through Wednesday...The conditions are anticipated to
be below Small Craft Advisory criteria. A few gusty thunderstorms
possible mainly Saturday night and again Tuesday
afternoon/evening.

Rip Currents...

For today, onshore flow with east winds around 15 to 20 mph
will continue with seas off the coast building to around 3 to 4
feet. For this reason, there is a MODERATE risk for the
development of dangerous rip currents for both the Jersey Shore
and the Delaware Beaches.

For Saturday, the flow t urns southerly so there will be less
of an onshore component however the seas will continue to be
around 3 to 4 feet. For this reason we`ll continue with a
MODERATE risk for the development of dangerous rip currents for
the NJ Shore with a LOW risk for the Delaware Beaches.

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

&&

.TIDES/COASTAL FLOODING...
One round of widespread minor tidal flooding occurred last night
into early this morning with the onshore flow and recent New
Moon. These factors begin to decrease today, so no further
widespread minor tidal flooding is expected. However, spotty
minor tidal flooding will be possible with the high tide tonight
into early Saturday along much of the coast of New Jersey and
Delaware, including the Delaware Bay and the tidal Delaware
River.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from noon to 8 PM EDT Saturday for PAZ070-071-
     102-104-106.
NJ...Heat Advisory from noon to 8 PM EDT Saturday for NJZ015-
     017>019.
DE...Heat Advisory from noon to 8 PM EDT Saturday for DEZ001.
MD...None.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450-451.

&&

$$

SYNOPSIS...Gorse
NEAR TERM...RCM
SHORT TERM...Gorse
LONG TERM...Gorse
AVIATION...Gorse/RCM
MARINE...AKL/Gorse/RCM
TIDES/COASTAL FLOODING...PHI