Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

683
FXUS61 KPHI 030720
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
320 AM EDT Thu Apr 3 2025

.SYNOPSIS...
Strong low pressure will lift north of the Great Lakes today with a
warm front moving northward across our area. A cold front then moves
through tonight and looks to stall near or over our area through the
start of the weekend. The cold front moves through Sunday night
followed by a stronger cold front Monday night into early Tuesday.
High pressure then builds in later Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Warm front continues slow progress northward. The QLCS, which as of
3 AM is getting close to western PA, will continue to shift eastward.
As mentioned by the previous shift, the remnants of this convective
system could arrive in our region later this morning (likely shortly
after sunrise), but given that it will have out-run the main
forcing, it should be weakening and may be little more than showers
by that point.

The center of the surface low will be moving northeastward into
Ontario and Quebec through the day. The warm front should continue
to progress northward from our area, Cloudy conditions will somewhat
limit heating today. However, still expecting high temperatures to
range from the upper 60s to upper 70s.

A cold front on the south side of the low is expected to approach
our region from the west as it occludes. As it does so, there will
be an increasing chance for showers and storms very late today into
tonight. The main area of forcing will remain well north of our
region. Additionally, the low will generally be weakening and
filling through the day. The combination of this, plus the potential
of the front occluding will mean that by the time showers and storms
approach our region, there will be very limited large scale forcing.

There remains a marginal risk (1 out of 5) for severe thunderstorms
due to the low cape and high shear environment.
However, model soundings (even from the NAM, which tends to be a bit
more bullish than most guidance in these situations) show very weak
lapse rates below 700 mb. In fact, there may be a robust temperature
inversion around 850 mb that never erodes until very late Thursday
night. Consequently, the severe risk will be mostly predicated on
the area seeing more warming in the low levels, than what we are
currently forecasting. Given that the front won`t be approaching our
region until after sunset, that further supports weak low level
lapse rates.

That being said, there is a persistent veering profile in model
hodographs for many locations in the region. Bottom line is that if
any updrafts can overcome the very limited instability, the wind
shear would support a severe risk, with the primary hazard being
damaging winds.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
A cold front works its way across most of our region through Friday
morning. A weak low pressure system on the front moves through the
region and departs by midday Friday. Any showers should be ending
through the morning hours on Friday. The conditions should tend to
dry out Friday afternoon as a cooler and drier air mass builds down
into the region. Highs will be in the 60s, though it will be warmer
in portions of Delmarva.

As high pressure slided to our north Friday night into Saturday, low
pressure tracks up the Ohio Valley and then moves mostly to our
north through Saturday. This will pull the boundary back north as a
warm front. Some additional showers develop Friday night and
Saturday, although the focus should become more focused farther
northward with time. Onshore flow however will keep the area rather
cloudy, and 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 into the low to mid 70s across much of Delmarva with
temperatures then quickly turning cooler northward. Again, these
temperatures will be highly dependent on how far north the warm
front gets. Some fog may begin to develop Saturday night mainly
along the immediate coast and the adjacent marine area as dew points
start to increase and rise above the chilly ocean water
temperatures.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Summary...A cold front moves through Sunday night followed by a
stronger cold front Monday night. High temperatures falling to
several degrees below average through the middle of next week.

Synoptic Overview...An upper-level trough is forecast to amplify
into and across the East during early next week, before starting to
lift out later Wednesday. At the surface, a cold front moves through
later Sunday followed by a stronger cold front Monday night. High
pressure then builds in later Wednesday before shifting offshore
Wednesday night.

For Sunday...As a significant upper-level trough slides across
central to eastern Canada, 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 mostly Sunday night.
Most if not all of our area should get within the warm sector during
the day as southerly flow increases ahead of the front and maximizes
the warm air advection. This should result in high temperatures
getting into the 70s across much of the area with even low 80s
possible across portions of Delmarva and the coastal plain. This
warmth will however depend on how much sunshine occurs as well as
the timing of the cold front and associated showers. It will feel
more humid as dew points rise through the 50s and even the low 60s
for many areas ahead of the cold front. This 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
more from the southwest. While the parent trough lags well behind
the surface cold front, height falls arriving with a greater
thickness packing also arriving from the west later in the day
should result in enough forcing for ascent for showers to develop
and move in from the west especially in the afternoon and more
likely at night. The timing of the front looks to be mostly at night
which is less ideal for stronger convection, however some thunder is
certainly possible given the strength of the incoming trough. The
intensity of any convection will depend on the available instability
and shear magnitudes in combination of any stronger forcing for
ascent.

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 some
additional rain showers with the second cold front mainly Monday
night. This could also be in the form of some wet snow across the
higher elevations of the Poconos if enough moisture remains. High
temperatures Monday should be closer to 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 before diminishing some at
night. High temperatures Tuesday look to be several degrees below
average. 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 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 at or 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).

&&

.AVIATION /07Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through today...Expect prevailing MVFR and IFR conditions (mostly
due to low ceilings) through at least 18Z. Many sites could see a
brief improvement to VFR centered around 21Z.
Isolated showers possible between 12 and 18Z. Wind either SE or S
less than 10 kt through 12Z, becoming southwest around 10-15 kt with
gusts up to 25-30 kt. Moderate confidence on the overall pattern,
but low confidence on details especially timing of flight category
changes.

Tonight...Another round of scattered showers and isolated
thunderstorms possible between 00 and 09Z (coverage of thunderstorms
is expected to be too limited to include in the TAFs at this time).
If any sites are VFR to start the evening, expect conditions to once
again lower to MVFR or IFR (again mostly due to low ceilings). SW
winds 10 to 15 kt to start the night, but should see a wind shift to
northwesterly after 06Z.
Low confidence.

Outlook...

Friday...Areas of sub-VFR and some showers possible.

Saturday...Sub-VFR conditions probable along with some possible
showers.

Sunday...IFR possible in the morning due to low clouds and/or fog,
then some improvement however sub-VFR probable at times along with
showers and possibly a thunderstorm in the afternoon and especially
at night. Southwesterly wind gusts up to 25 knots possible during
the day.

Monday...Sub-VFR and some showers possible.

&&

.MARINE...
A Small Craft Advisory remains in effect for the Atlantic Coastal
Waters through tonight. On the Delaware Bay, the SCA will be in
effect during the day time hours today.

Winds and seas have been a bit slower to increase overnight than
previously expected, but with the warm front now in our region,
expect winds to increase over the next few hours. Thus, no changes
made to the SCA, but will keep an eye on trends over the next few
hours.

Winds should begin to diminish late this afternoon, and should
generally be below 25 kt by late this evening. It may take time for
seas to diminish, but should drop below 5 ft especially late tonight
as winds shift to northwesterly.

Outlook...

Friday...The conditions are anticipated to be below Small Craft
Advisory criteria.

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

Sunday...Small Craft Advisory conditions possible. Visibility
restrictions possible due to some morning fog, then showers and
isolated thunderstorms late in the afternoon and especially at
night.

Monday...Small Craft Advisory conditions possible.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ430-
     431.
     Small Craft Advisory until 6 AM EDT Friday for ANZ450>455.

&&

$$

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