Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

966
FXUS61 KPHI 041244
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
844 AM EDT Fri Apr 4 2025

.SYNOPSIS...
A cold front settles to our south today and tonight as high pressure
builds across portions of southeastern Canada and New England. As
low pressure moves up the Ohio Valley and then across the Great
Lakes tonight and Saturday, the front will move northward some as a
warm front. A cold front then moves through Sunday followed by a
stronger cold front Monday night into early Tuesday. High pressure
then builds in later Wednesday before shifting offshore into
Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
The severe threat has ended, and showers continue to taper off.
Outside of a few lingering showers over the next couple of
hours, there will be a lull through the rest of the day.

The cold front is through southeast Pennsylvania and half of New
Jersey. The front will continue to sag southward and will get
hung up over Delmarva and extreme southern New Jersey. Cold air
advection will lag well behind the front, so most areas will
still be warmer than normal with highs ranging from the upper
50s to near 70. Dew points will slowly drop through the day as
well.

Otherwise, as the next low pressure system lifts through the
Ohio Valley overnight, the stalled front will begin to slowly
work its way back north as a warm front. This will lead to
another chance for rain overnight. Predominant mode is expected
to be showers, but if the front starts lifting north early
enough, a few rumbles of thunder are possible in Delmarva during
the evening hours. As with the previous low pressure system,
the main forcing with this system will stay well north of our
region. Therefore, rain amounts overnight are expected be light,
generally less than a quarter of an inch of rain.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As high pressure slides to our north Saturday, low pressure
tracks up the Ohio Valley and then moves mostly to our north
through Saturday. This will start to pull the stalled boundary
just south of Delmarva back north as a warm front. Some
additional showers develop Saturday, although the focus should
become more focused farther northward with time. Onshore flow
Saturday however will keep the area rather cloudy and much
cooler with even some morning drizzle possible. The location of
the front and associated warm sector to its south will impact
how warm the area gets. As of now, it should get the warmest
during the day Saturday across much of our Delmarva zones with
temperatures then quickly turning cooler northward. Again, these
temperatures will be highly dependent on how far north the warm
front gets however with high pressure sliding by to our north
and an onshore flow the warm front should be slow to move
northward. Some fog may begin to develop Saturday night mainly
along the coast and the adjacent marine area as dew points start
to increase and rise above the chilly ocean water temperatures.

As we go through Sunday, a significant upper-level trough
slides across central to eastern Canada and it is forecast to
amplify southward with its leading edge starting to move into
the Northeast Sunday night. This trough will drive low pressure
well to our north, however a trailing cold front will arrive in
our area Sunday. The existing warm front across the area looks
to surge northward Sunday morning however the cold front looks
to be not to far behind. How far north and inland the warm
sector gets ahead of the cold front will determine how warm
areas get. The dew points will rise through the 50s in areas
that get more into the warm sector for a time. The warm and more
moist air mass may also result in marine fog that could impact
the coastal areas as well, before it gets pushed out as the low-
level flow veers from the southwest to west. While the parent
trough lags well behind the surface cold front, height falls
arriving with a greater thickness packing also arriving from the
west should result in enough forcing for ascent for showers to
develop and move in from the west especially in the afternoon
and at night. The timing consensus of the cold front looks a bit
faster and this resulted in temperatures a bit lower especially
north of Delmarva. Some thunder is certainly possible given the
strength of the incoming trough, however the intensity of any
convection will depend on the available instability and shear
magnitudes in combination of any stronger forcing for ascent.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Summary...A stronger cold front moves through Monday night
followed by high pressure arriving Wednesday into Thursday. High
temperatures falling to several degrees below average through
the middle of next week, then some warming begins on Thursday.

Synoptic Overview...An upper-level trough is forecast to
amplify into and across the East during early next week, before
starting to lift out on Wednesday. The next upper-level trough
then looks to begin to amplify across the eastern U.S Wednesday
night and Thursday. At the surface, a stronger cold front moves
through Monday night. High pressure then builds in during
Wednesday before shifting offshore into Thursday. A warm front
well ahead of low pressure may then arrive later Thursday.

For Monday and Tuesday...This time frame starts as more of a
day in transition on Monday as the first cold front is offshore
and we await a secondary cold front tied to the amplifying
upper- level trough. This second front may end up moving through
our area Monday night with little in the way of precipitation
as the deeper moisture is shoved offshore with the earlier cold
front. Given the strength of the incoming upper-level trough
however, cannot ruled out a few rain showers with the second
cold front mainly Monday night or early Tuesday. The arrival of
the colder air aloft along with cyclonic flow may result in a
few rain/snow showers across portions of far northwestern zones.
High temperatures Monday should be several degrees below
average. The second cold front should be offshore to start
Tuesday with strong cold air advection underway. There will be a
tightened pressure gradient in place between departing low
pressure well to our northeast and incoming high pressure from
the Midwest to the Ohio Valley. This will translate to a gusty
wind on Tuesday with peak gusts in the 25-35 mph range before
diminishing some at night. High temperatures Tuesday look to be
well below average as much of the area may not get above the 40s
during the day. It will be cold Tuesday night with most if not
all areas dropping below freezing. There may be to much wind for
frost formation, however a freeze warning may be needed where
the growing season has started (most of our Delmarva zones).

For Wednesday...The strong upper-level trough is currently
forecast to be lifting out with time. This will result in
surface high pressure building into our area before it starts to
shift offshore at night. There may still be a gusty wind in place
before a tightened pressure gradient relaxes as the center of the
surface high builds closer. High temperatures will be several
degrees below average once again, and the wind will determine how
cold it gets at night despite the air mass especially aloft starting
to modify. A clear sky with dew points below freezing would
typically yield a cold night especially if the wind is able to
completely decouple. As a result, there is some risk for frost
and/or freeze (the growing season has started for most of our
Delmarva zones).

For Thursday...The next upper-level trough looks to begin amplifying
into the Eastern U.S. A surface low is forecast to be tracking
toward the upper Ohio Valley with a warm front approaching our area
from the southwest. This will be dependent on the strength of the
developing low relative to the departing high, so much of Thursday
may end up remaining dry. Temperatures do look to begin to moderate
though as southerly flow returns in advance of this system.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...TSRA have ended, and SHRA will continue to taper off.
Though VSBYs are now VFR, any MVFR CIGs will lift from to VFR
throughout by 15Z. LGT/VRB winds this morning will turn NW 5 to
10 kt by late morning and will continue through the afternoon.
High confidence.

Tonight...Mostly VFR, though some brief MVFR will be possible
as showers move into the region from the south. Initially with
sunset, expect winds to go light and variable, but eventually
will settle out of the east near or below 10 kt. Moderate
confidence.

Outlook...

Saturday...Sub-VFR conditions probable along with some showers.
Some drizzle possible in the morning.

Sunday...Sub-VFR conditions due to low clouds and/or fog should
improve some during the day from south to north. Showers and
possibly a thunderstorm especially in the afternoon and at night
will result in visibility restrictions. Southwest to west and
northwest winds could gust up to 25 knots during the day.

Monday...Sub-VFR conditions possible lingering showers ending.

Tuesday...VFR. Northwest wind gusts up to 30 knots.

&&

.MARINE...
Seas on the ocean continue to subside to less than 5 feet, so
will cancel the Small Craft Advisory that was in effect until
noon. Sub-SCA conditions then on tap for the rest of today and
tonight.

Outlook...

Saturday...The conditions should be mostly below Small Craft
Advisory criteria. Some visibility restrictions possible due to
some fog mostly at night.

Sunday...Small Craft Advisory conditions possible. Visibility
restrictions possible due to some morning fog, then showers and
possibly a few thunderstorms mainly in the afternoon and at
night.

Monday...Small Craft Advisory conditions possible especially in
the morning.

Tuesday...Small Craft Advisory conditions probable.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$

SYNOPSIS...AKL/Gorse
NEAR TERM...Johnson/MPS
SHORT TERM...AKL/Gorse
LONG TERM...AKL/Gorse
AVIATION...Gorse/Johnson/MPS
MARINE...Gorse/Johnson/MPS