258
FXUS61 KPHI 202347
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
747 PM EDT Sun Sep 20 2026

.WHAT HAS CHANGED...
Updated first key message and associated discussion.
Updated Aviation discussion for 0z TAFs.

&&

.KEY MESSAGES...
1. A cold front moves through the region tonight bringing some
rain.

2. A frontal boundary south of the region will keep rain
chances around through mid-week, especially across southern
portions of the area.

3. A closed mid-level low ejecting from the Ohio Valley is
expected to lead to the development of a surface low off the
Eastern Seaboard during the middle and latter parts of this
week. Depending on the track of the low, it could bring
increased rainfall chances to the region.

4. Multiple rounds of tidal flooding are expected later this
week, with tidal flooding potentially beginning as early as
Tuesday evening.

&&

.DISCUSSION...
KEY MESSAGE 1...A cold front moves through the region tonight
bringing some rain.

A cold front will move south and eastward through the area this
evening into the overnight bringing some rain/showers with it. A
few rumbles of thunder are not out of the question either. There`s
not much instability to work with but PWATs are quite high at
around 2 inches. Also, there are some upstream reports towards
central PA of a quick .50 to 1.0 inches of rain with these
showers as they`ve moved through. Generally not expecting any
issues but if the line of showers currently moving through were
to slow down and train near the urban corridor that could result
in some localized flooding issues this evening. Expect showers
to wind down overnight as the front continues to push south.

KEY MESSAGE 2...A frontal boundary south of the region will
keep rain chances around through mid-week, especially across
southern portions of the area.

The front will be south of the region on Monday and remain
there through mid week. Confidence is not the highest pertaining
to all aspects of sensible weather, namely precipitation. How
far south that front sags will be a big factor in how much rain
we see through the week. The farther south it stalls, the less
likely our region will see continued rain chances through the
mid week period. Guidance this afternoon continues to indicate
the front may try to lift briefly northward as a warm front on
Tuesday, which looks to feature the highest rain chances through
mid-week. PoPs currently range from around 50% across far
northern NJ to around 80% across portions of southeastern PA,
far southern NJ, and most of Delmarva. For Wednesday into
Thursday, the front should again be on the move southward,
lessening rain chances across the area.

Regardless of exact frontal placement or rain chances, the
entire area will be on the cool side of the boundary. Therefore,
confidence is high that temperatures will be below normal. In
fact, temperatures will likely struggle to get out of the 60s
areawide through most of this week.


KEY MESSAGE 3...A closed mid-level low ejecting from the Ohio
Valley is expected to lead to the development of a surface low
off the Eastern Seaboard during the middle and latter parts of
this week. Depending on the track of the low, it could bring
increased rainfall chances to the region.

Model guidance this afternoon continues to indicate that a mid-
level low will track eastward out of the Ohio Valley region
towards the Mid-Atlantic by Thursday. From there, it may become
cutoff and meander off the Eastern Seaboard into late this week
and potentially even into this weekend. As the mid-level low
tracks eastward, surface cyclogensis will become favored off the
coast of the Carolinas. This could begin as early as Tuesday
night as synoptic- scale ascent increases. The surface low is
then likely to track northeastward into late this week.

There is considerable uncertainty in how close the low will
track to our area, and therefore, what impacts it may bring.
Most guidance features a track well offshore, but the
deterministic ECMWF and GFS continue to feature a westward
deviation of the low late this week, which would lead to
increased rain chances across the region. However, when looking
at their ensemble members, the deterministic runs actually lie
on the western edge of the overall envelope.

All this being said, the forecast for the latter half of this
week into this weekend features considerable uncertainty. For
now, have PoPs generally in the 20-40% range across the eastern
two-thirds of the area Friday through Sunday. However, if a
westward deviation of the low does occur these could certainly
trend up, or if the low remains farther out to sea the region
could be mostly dry for the latter part of the week.


KEY MESSAGE 4...Multiple rounds of tidal flooding are expected
later this week, with tidal flooding potentially beginning as
early as Tuesday evening.

Regardless of the exact track of the coastal low, it looks as
if we will be in persistent onshore flow from at least Tuesday
onward for the Atlantic Coast, Delaware Bay, and Raritan Bay.
This long of an onshore flow component increases the risk for
tidal flooding, especially for back bays. For the later half of
the week, the track, strength of the low, and fetch will all
play a role with tidal departures. In addition, the full moon is
on Saturday, meaning astronomical high tides are increasing as
we get towards the end of the week.

Minor tidal flooding could begin as early as Tuesday
afternoon/evening`s high tide with potential Moderate tidal
flooding on Wednesday.

For the Chesapeake Bay, the expected wind direction tends to
limit tidal flooding concerns, but it will need to be watched if
persistent easterly flow at the mouth of the Chesapeake.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...Mainly MVFR predominating however showers moving
through this evening could produce brief IFR conditions. Expect
that by around 09z conditions improve to VFR for TAF sites from
the I-95 corridor and points north/west as drier air moves in.
Winds mainly light and variable this evening becoming northerly
around 5-8 knots overnight. Moderate confidence.

Monday...MVFR cigs may linger at ACY and MIV but VFR should
predominate through the day north of these sites. Northeast
winds generally around 5-10 knots. Moderate confidence.


Outlook...

Monday night through Tuesday night...Sub-VFR conditions likely
in periods of rain and low clouds.

Wednesday through Friday...While uncertainty is high, sub-VFR
conditions will remain possible at times in rain or low clouds.
Gusty northeast winds are possible.

&&

.MARINE...
A Small Craft Advisory is in effect from 11 AM Monday through 11
AM Tuesday for all Atlantic coastal waters.

Conditions are expected to remain below SCA criteria
through tonight with winds mainly southerly this evening shift
to northwest and then northerly overnight.

Winds will become northeast on Monday and increase to 15-20 kt
with gusts 25-30 kt and seas 4-7 feet.

Outlook...

Monday night...Advisory conditions expected with northeast
winds of 15-20 kt and gusts of 25-30 kt. Seas 4-7 feet.

Tuesday through Friday...Advisory conditions expected. Gales
are possible for at least part of this timeframe, but
uncertainty is high at this point in time.

Rip Currents...

Monday... We`ll continue with the MODERATE risk for dangerous rip
currents on Monday as winds will switch around to Northeast and
increase to 15 to 20 mph during the day. Breaking waves in the surf
zone will be around 2 to 4 feet.

Tuesday...Northeast winds strengthen to 20 to 30 mph with wave
heights in the surf zone building to 3 to 5 feet. As a result, there
is a HIGH risk of rip currents both for the New Jersey shore and the
Delaware Beaches.

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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 11 AM Monday to 11 AM EDT Tuesday
     for ANZ450>455.

&&

$$

DISCUSSION...Cooper/Hoeflich/WFO PHI
AVIATION...Cooper/Fitzsimmons/WFO PHI
MARINE...Cooper/Fitzsimmons/WFO PHI