200
FXUS61 KPHI 201117
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
717 AM EDT Sun Sep 20 2026

.WHAT HAS CHANGED...
Added more detail on the coastal/tidal flooding threat later
this week.

&&

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

2. A stationary front south of the region could keep rain chances
around, especially for Delmarva through mid week.

3. A closed mid-level low ejecting from the Ohio Valley could help
to spin up surface low pressure off our coast by Thursday,
potentially bringing another chance for rain.

4. There will be an increasing risk for coastal/tidal flooding
through the second half of the week.

&&

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

The warm front associated with the main surface low will
briefly move northward into the region early this afternoon,
but will be quickly followed by 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 is expected to increase
through mid day, with surface dew points rising well into the
60s 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 stationary front south of the region could keep
rain chances around, especially for Delmarva through mid week.

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 south will be a big factor in how much
rain we see through the week. The further south it stalls, the
less likely our region will see continued rain chances through
the mid week period. At least for Tuesday, multiple models
trended slightly further north with the front, and as such, rain
is likely on Tuesday generally for areas along and south of the
PA Turnpike/195 corridors.

Regardless, our region will be on the cool side of the front
through this period keeping temperatures below normal.
Confidence is higher here.


KEY MESSAGE 3...A closed mid-level low ejecting from the Ohio Valley
could help to spin up surface low pressure off our coast by
Thursday, potentially bringing another chance for rain.

As a mid level low digs in from the northwest, models are showing
surface cyclogensis by Wednesday off the Carolinas. There remain
significant differences in the track and strength of the low.
The closer the track is to the coast, the more likely our region
will see rain and gusty winds on Thursday and Friday. Further
complicating the picture, now multiple models depict the low
initially progressing northeast before retrograding westward
towards the coast at the end of the week. All that to say, the
rain and wind forecast from Thursday onward remain very
uncertain at this time.


KEY MESSAGE 4...There will be an increasing risk for
coastal/tidal flooding through the second half of the week.

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.

It is still uncertain, but we could see minor tidal flooding as
early as the late Wednesday tide cycle.

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

Today...The prevailing MVFR ceilings that have already
developed at most TAF sites are expected to continue for most,
if not all of the TAF period. For areas that are seeing more
marine layer stratus, especially KACY and KMIV, could see the
prevailing VFR conditions continue until very late in the day
(after 21Z). Brief IFR conditions possible at KRDG/KABE with
more moderate rainfall. Southeast winds around 5-10 kts
gradually shifting more SSE. Moderate confidence overall, low
confidence in timing of IFR conditions.

Tonight...Sub-VFR conditions likely due to rain, stratus, and
fog. Southwest winds abruptly shifting to northwest with a
frontal passage from west to east between 21 and 09Z. Low
confidence.

Outlook...

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

Tuesday through Thursday...Sub-VFR conditions will remain possible
due to showers/rain. Watching a potential coastal low. Breezy
northeast winds could develop, but they`ll be dependent on the
the track and strength of the potential low.

&&

.MARINE...
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.

Outlook...

Monday...On Monday, winds will gradually shift to northeasterly
and begin to increase. An SCA has been issued for the New Jersey
coastal waters during the day on Monday, where there is higher
confidence of SCA conditions. However, this may need to be
expanded later.

From Monday Night onward, there is at least a chance of gale
conditions for part of the period. However, differences in model
timing and track of a coastal low in this period make it
difficult to pinpoint when gale conditions will be the most
likely.

Rip Currents...

Sunday...The winds shift to south/southeast but remain around
15 to 20 mph. Breaking wave heights in the surf zone look to be
around 3 to 4 feet for the Jersey shore and 2 to 4 feet for the
Delaware Beaches. There is a forecast of a MODERATE risk of dangerous
and life threatening rip currents along the NJ and DE coast.

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 3 feet mostly, but could end
up a bit higher as winds remain moderate and gusty.

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 to 6 PM EDT Monday for
     ANZ450>453.

&&

$$

DISCUSSION...WFO PHI
AVIATION...WFO PHI
MARINE...WFO PHI