418
FXUS61 KPHI 202113
AFDPHI

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

.WHAT HAS CHANGED...
Updated rip currents discussion within the marine section.

Updated coastal flooding key message.

&&

.KEY MESSAGES...
1. A cold front moves through the region late today into
tonight, bringing rain at times through the day.

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 late this
afternoon into tonight, bringing rain at times through the day.

A warm front associated with the main surface low has generally
progressed across the region bringing light warm advection
rainfall. Accumulations were relatively minor however with the
bulk of the heavier rainfall coming later this evening
associated with a cold front late today into this evening.

Low level flow will keep an onshore component through much of
the day. As such, low level moisture has increased with surface
dew points rising well into the 60s to lower 70s and
precipitable water values increasing to near 2 inches.
Precipitable water values that high are well above the 90th
percentile for this time of year.

However, with little instability, there will generally be a lack
of embedded convection within the areas of rain. Instead, this
will primarily be a more steady, stratiform rain event. As a
result, the risk for excessive rainfall leading to flash
flooding continues to be low. Given the high PWATs, there may
still be some pockets of moderate rainfall at times,
particularly for areas northwest of I-95. But the risk for
flooding remains low (less then 5 percent) across the entire
region.


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 /21Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today through Tonight...Prevailing MVFR ceilings expected to
continue through around 00z with periods of light rain possible.
Between 00z and 03z expect a brief increase in cigs to
potentially VFR before a cold front brings MVFR cigs back across
the region. Between 21-00z guidance does show some isolated
potential for convection in the form of an isolated thunderstorm
or two mainly for terminals PHL and south.Southeast winds
around 5-10 kts gradually shifting more SSE. Southwest winds
abruptly shifting to northwest with a frontal passage from west
to east between 21 and 09Z. Low confidence on specific timing
but moderate confidence on overall pattern.

Monday...Sub-VFR conditions possible due to precipitation, low
stratus, and fog.


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. Winds around 10-15 knots gradually transition
from east- southeast tonight to southerly Sunday afternoon and
evening. A front will shift winds to the northwest tonight.

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/WFO PHI
MARINE...Cooper/Fitzsimmons/WFO PHI