Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

316
FXUS61 KPHI 171054
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
554 AM EST Wed Dec 17 2025

.SYNOPSIS...
High pressure exits today and is followed by a weak cold front
that passes through the region this evening. High pressure
briefly builds in tonight before moving out through the day
tomorrow. A strong cold front approaches late Thursday night
and moves across the region on Friday. High pressure returns on
Saturday, then another cold front passes through on Sunday. High
pressure returns for the start of the new week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As the high pressure system fully exits today, we see some upper-
level troughing that moves into the northeast. At the surface, there
is a low pressure system in the Great Lakes region which moves into
Quebec through the day today. A weak cold front associated with this
low passes through the area this afternoon into the evening. After
the early morning stratus erodes, we see more mid and high clouds
that increase ahead of this cold front. Overall, the frontal passage
looks to lack precipitation. A southwest wind ahead of the cold
front will allow temperatures to rise into the 40s for many. As the
cold front passes through, we see the winds shift to be more out of
the west which will start to usher in slightly colder air which does
not last for long. There will be a tightening pressure gradient
between the departing high and the low moving into the northeast.
Also, there are stronger winds aloft but the potential for these
winds to mix down may be limited due to the milder air filtering in.
Overall, wind gusts look to be 20-25 mph on Wednesday with
occasional gusts to 30 mph in the higher elevations.

Another high pressure system moves in tonight and will be relatively
fast moving. This high will be moving offshore already by tomorrow.
Due to the high pressure system building in tonight, tranquil
conditions are expected with just the potential for some low to mid
level clouds filtering in through the night. Our pressure gradient
will lessen into tonight and upper-level winds also lower, so the
winds will be decreasing into tonight with a period of nearly calm
winds at times. Lows tonight will be in the 20s.

Once the high pressure system departs tomorrow, we see winds become
more out of the southeast to south with the return flow. This allows
warmer air to move into the region. Highs on Thursday will reach
into the mid and upper 40s north and west of the Fall Line and in
and in the low to mid 50s along the I-95 corridor, the Delaware
Valley, and southern New Jersey. Highs will be in the mid to upper
50s in southern Delmarva. These temperatures will be upwards of 10
degrees warmer compared to Wednesday in some spots. Clouds will
gradually increase throughout the day with some showers starting to
move into the area by the evening hours as our next system
approaches.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
A strong frontal system moves in Thursday Night into Friday,
bringing a period of moderate to heavy rain and gusty winds. Rain
overspreads the area as the night goes on Thursday Night, with
rainfall continuing through about midday Friday. Rainfall amounts
have trended up slightly, with around 1-1.5" expected from
Philadelphia on north with about 0.75"-1.25" anticipated south of
that line. Surface-based instability looks to be zero, so the severe
threat is low at this time even with a highly sheared setup and a
strong low-level jet moving overhead. Cannot rule out a rumble of
thunder though as some of the CAMs in range show a little bit of
elevated instability. There likely will be some impacts to the
morning commute on Friday in terms of pockets of heavy rain and
urban/poor drainage flooding, though this should mainly be a
beneficial rain. Even with a decent snowpack in place, not
currently expecting any creek or river flooding as ensemble
river forecasts at all forecast points have a less than 10%
chance of even reaching action stage.

The more noticeable impact with this system will be the winds. The
gradient tightens as Thursday night goes on, with a strong 60-80 kt
low-level jet moving overhead. However, a stout inversion will be in
place at the surface, preventing the stronger winds from mixing
down. Thinking the strongest winds will be post-frontal in the cold
advection regime, where deeper mixing will be at play. Ahead of the
front though, gusts peak around 25-35 MPH, potentially up to 40
MPH on Friday morning.

The cold front comes through Friday morning with modest cold-air
advection ramping up behind it. This will result in deep-layer
boundary mixing and wind gusts picking up out of the
west/northwest. Gusts from late Friday morning through Friday
evening should be around 30-40 MPH, up to 45 MPH. Where gusts
could be higher are along the higher elevations in the Poconos.
A Wind Advisory may need to be considered up that way, as NBM
Probability of max gust over 46 MPH hovers around 60-80%. By
midday Friday, the last of the showers move out, and
temperatures actually should be dropping through the day.
Temperatures by daybreak Friday are likely in the 50s, ending
the day in the upper 30s/low 40s.

Things quiet down on Friday Night as high pressure begins to move in
and the gradient weakening. Winds decrease as Friday Night carries
on with skies clearing out. Temperatures get down into the low to
mid 20s.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
The long term looks rather benign. High pressure moves in on
Saturday, resulting in a pleasant day with temperatures in the upper
30s/low to mid 40s. High pressure moves offshore on Saturday Night,
with warm air advection picking up ahead of another frontal system.
Lows on Saturday Night should be in the low to mid 30s.

A cold front moves through on Sunday, though the only impact will be
breezy conditions ahead of and behind the front. The front looks
moisture starved, resulting in little to no precipitation. Modest
south/southwesterly flow will allow temperatures to climb into the
mid to upper 40s and low 50s.

High pressure works in behind the front, bringing temperatures back
to near or just below normal for the end of December to start next
week. Seeing some signal for another system for the middle of next
week, but too early to dive into specific details.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Wednesday...VFR with areas of mainly mid to high level clouds.
Southwest winds increasing to 5-10 knots with some gusts to
around 20 knots mainly during the afternoon. Low-level wind
shear near 40 knots mainly at KRDG and KABE through this morning.
Low confidence regarding how much wind mixes down to the
surface and results in frequent gusts.

Wednesday night...VFR. Low to mid level clouds look to filter
in. West-northwest wind around 5 knots or less which may go
light and variable at times. Moderate confidence on how low the
ceilings get.

Outlook...

Thursday Night through Friday Morning...IFR conditions expected with
periods of moderate to heavy rain moving through and low clouds. Low-
level wind shear likely with a strong low-level jet moving
overhead. Wind gusts at the surface around 20-30 kt out of the
south/southwest.

Friday Afternoon...Conditions likely lift to VFR by the afternoon,
with wind gusts 25-35 kt out of the west/northwest expected.

Friday Night...VFR. West/northwest wind gusts 20-25 kt for the first
half of the night, diminishing as the night goes on.

Saturday through Sunday..VFR. No significant weather.

&&

.MARINE...
A Small Craft Advisory is in effect for all of the Atlantic
coastal waters from 7 AM to 7 PM today. The winds should remain
below advisory criteria on the Delaware Bay.

Southwest winds increase to 15-20 kt this morning with gusts as
high as 25 kt. Seas will increase to 4-6 feet. By this evening,
winds become westerly while decreasing to 10-15 kt. Seas
decrease to 2-4 feet.

No marine headlines are expected for Wednesday night and
Thursday as winds and seas stay below SCA criteria.

Outlook...

Thursday Night through Friday...A Gale Watch was issued for all
marine zones. Winds will pick out of the south-southwest, with gusts
upward of 35-40 kt expected by early Friday morning. A strong cold
front comes through by late Friday morning, with winds picking up
out of the west/northwest in its wake. This will result in another
period of 35-40 kt wind gusts. Seas of 8 to 12 feet expected.

Friday Night...The Gale Watch goes until 03z Saturday for Delaware
Bay and 06z for the ocean zones. SCA conditions likely follow but
conditions quiet down quickly with winds falling below SCA levels by
daybreak Saturday. Seas quickly diminishing as well but hang
around 4 to 7 feet.

Saturday...No marine headlines expected.

Saturday Night through Sunday...SCA conditions expected with wind
gusts near 30 kt and seas 4 to 7 feet.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Thursday evening through Friday evening for
     ANZ430-431.
     Small Craft Advisory until 7 PM EST this evening for
     ANZ450>455.
     Gale Watch from Thursday evening through late Friday night for
     ANZ450>455.

&&

$$

SYNOPSIS...Guzzo/MPS
NEAR TERM...Guzzo
SHORT TERM...Hoeflich
LONG TERM...Hoeflich
AVIATION...Guzzo/Hoeflich
MARINE...Guzzo/Hoeflich