Flash Flood in Delaware, May 30 to 31, 2025

Area Forecast Discussion
National Weather Service Mount Holly NJ
925 PM EDT Fri May 30 2025

.SYNOPSIS...
A strengthening area of low pressure will cross our area tonight
into Saturday morning with a warm front lifting northward. A cold
front then sweeps across our area later Saturday afternoon and
evening. High pressure builds into the Ohio Valley later Sunday then
arrives in our area later Monday through Wednesday. A surface trough
may move through Thursday into Friday as a cold front approaches
from the west.

&&

.NEAR TERM /THROUGH SATURDAY/...
The stalled surface boundary has moved northward over the past
couple of hours now stretching across portions of southeast PA
into southern New Jersey. Overall, not expecting this boundary
too move much further as low pressure tracks along it across
southeast Pennsylvania and southern New Jersey around midnight
before moving off the coast and deepen as it lifts off into New
England. Latest guidance suggests that this low may strengthen
to sub 990mb which is quite impressive and strong for the end of
May. As a result of this dynamic low moving across the area
tonight, we are progged to have severe weather in the area
tonight (mainly confined to the south of the track of the
surface low) until roughly 12-1 AM or so. Latest RAP analysis
shows the area still lies in an area of 1000 J/kg of MLCAPE,
with 40-45 kt of bulk shear and 100-150 m2/s2 of SRH. The
greatest potential for severe thunderstorms will primarily be
across southeast Pennsylvania, central/southern New Jersey and
the Delmarva. Convection does remain low topped in nature, so it
has been a struggle to see anything other than low-level
circulations develop. As a result of the increased threat
tonight with the dynamic low, the Tornado Watch remains in
effect for portions of the area through midnight tonight.

After the severe/tornado threat concludes, the event will
transition to more of a heavy rain threat as the low pressure
system deepens over the area. Rain at times will be heavy
tonight as long skinny CAPE is shown in model soundings and
PWATs are around 1.2-1.5 inches. Overall, we are expecting
roughly 1-2 inches of rainfall for the entire area, with
localized amounts in excess of 3 inches are possible. The Flood
Watch did not change with this update and remains in effect for
the same locations as previously issued.

Winds will remain light into the evening, but then increase slowly
overnight as the low deepens and moves off to our east. This
will occur around midnight or slightly after as the low crosses
the region. Expect a gradual turning of the winds from southeast
to westerly where winds are currently expected to gust upwards
of 50 mph are possible across the Delmarva and southern New
Jersey on the backside of the low. As a result of the increased
wind threat, have now issued a short-fused Wind Advisory for
these areas between 2 AM and 8 AM. Winds will remain gusty
through dawn before diminishing as the low pulls away. Lows will
drop into the mid/upper 50s most spots.

For Saturday, the strong low pressure area will be northeast of our
CWA and pulling away. A decent pressure gradient across the Middle
Atlantic will keep gusty winds from the West over the area much of
the day. Winds gusts 25 to 35 mph are expected for the morning for
southern NJ and Delmarva. Steady rains will end from SW to NE thru
the morning with scattered showers into the afternoon. Highs temps
will mostly be in the upper 60s to low 70s, but a couple mid 70s
possible for srn Delware.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
An upper-level trough is forecast to be in place across the
Northeast and Mid-Atlantic through Sunday night, then this starts to
weaken during Monday.

Some showers and thunder possible Saturday evening which then shift
offshore or dissipate as a cold front works across the area and then
offshore. A much drier air mass will overspread the region in the
wake of the cold front, and the cloud cover is expected to decrease.
An increase in cold air advection will also deliver cooler air, with
lows Saturday night mostly in the mid/upper 40s to low 50s.

An impulse looks to move through Sunday within the upper-level
trough. Much less moisture will be in place, reflected by dew points
in the 40s to potentially dropping into the upper 30s in the
afternoon. The model forecast soundings however do show a well mixed
boundary layer occurring and this should support some cumulus
development. Cannot rule out a brief light shower or a few sprinkles
in the Poconos area. Given the deeper mixing and a tightened
pressure gradient, a notable westerly breeze is expected on Sunday.
The wind should then quickly diminish Sunday evening as the vertical
mixing weakens. High temperatures look to be below average with
highs in the 60s to low 70s.

As we go through Monday, the upper-level trough is forecast to
weaken and start to lift out. As high pressure starts to build
closer, the pressure gradient weakens and the cold air advection
also weakens. There is a weak impulse to our south though rounding
the slowly retreating trough which could toss some more clouds for a
time especially across Delmarva. Otherwise, a warmer afternoon is
expected with more in the way of sunshine for the bulk of the area.
High temperatures in the low to mid 70s for most.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Summary...Warmer and dry to start, then more humid with a chance for
some convection late in the week.

Synoptic Overview...As an upper-level trough departs to start
Tuesday, mid to upper level ridging is forecast to build across the
East through Wednesday. The ridge may then flatten some Thursday and
especially Friday. It should be noted that some guidance shows a
closed low across the Mid-Atlantic to the Northeast through about
Thursday. The ECMWF was most notable with this, with only one or two
GFS ensemble members showing this potential.

For Tuesday and Wednesday...In the wake of an upper-level trough to
start Tuesday, mid to upper level ridging is forecast overall to
build over much of the East. This will result in increasing
temperatures, with highs getting into the low 80s for much of the
area Tuesday followed by mid to some upper 80s Wednesday. As
mentioned above, much of the guidance is in favor of the ridge
however a few of the model guidance showed a closed low moving
across the area. The guidance that is showing the closed low seems
to be an outlier with a stronger signal in the guidance/ensembles
more toward the ridge. As a result, this forecast follows the warmer
solutions (National Blend of Models). The air mass is forecast to be
drier Tuesday then the dew points increase Wednesday which will
start to make it feel more humid. The flow is forecast to be light
given the presence of the ridge aloft and high pressure at the
surface, therefore sea/bay breeze circulations are anticipated both
days. No precipitation is anticipated given the ridge scenario.

For Thursday and Friday...The mid/upper level ridge looks to flatten
some Thursday and especially on Friday as shortwave energy slides
across Canada to the Midwest and Great Lakes. This will send a
surface cold front toward our area, however it may end up slowing or
stalling to our west. It is forecast to still be very warm, and on
the humid side, during this time frame with perhaps some cooling
Friday given the ridge flattening more. Some mostly diurnally driven
convection cannot be ruled out each day, especially tied to any
surface troughs that develop, however coverage and intensity this
far out is less certain.

&&

.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR at all terminals until roughly 02-03Z when
ceilings rapidly descend to MVFR/IFR with periods of rain. Rain
may be heavy at times with embedded thunderstorms likely.
South-southeast winds around 5-10 kt will rapidly shift and
become westerly after 05-06Z and increase to around 15 kt with
gusts up to 20-30 kt possible. Overall, moderate confidence in
what will occur, but lower confidence regarding to timing of
lowering ceilings and visibilities.

Saturday...Lingering MVFR ceilings in the morning should improve
to VFR at most terminals by the afternoon. Remaining showers
will cease in the morning, with a second round of showers and
isolated thunderstorms possible in the afternoon. Moderate
confidence overall.

Outlook...

Saturday night and Sunday...VFR. West-northwest winds diminishing
Saturday evening, then westerly wind gusts to around 20 knots
Sunday.

Monday through Wednesday...VFR.

&&

.MARINE...
A Gale Warning is now in effect for all marine zones until 11 AM
Saturday with the exception of the zone from Sandy Hook to
Manasquan Inlet where a Small Craft Advisory is currently in
effect until 6 PM Saturday.

Wind gusts in the SCA area will average around 30 kt and closer
to 35 to 40 kt for the Gale area. The periods of Gales will not
last as long as the warning, but we've added time on both sides
due to normal forecast variations which may develop. Showers
and tstms tonight and into Saturday morning. On Saturday, SCA
flags will be needed after the gale is taken down. West winds
late tonight and Saturday with showers or a few tstms mostly
through noon.

Outlook...

Saturday night...The conditions improve as winds and seas diminish.

Sunday through Wednesday...The conditions are anticipated to be
below Small Craft Advisory criteria.

Rip Currents...

For Saturday...West-southwest winds around 15-25 mph with
breaking waves around 1-3 feet and a period of 6-7 seconds. As
a result, a MODERATE risk for the development of dangerous rip
currents is expected for both New Jersey and Delaware Beaches.

For Sunday...Southwest winds will diminish to around 10-15 mph
with breaking waves around 1-2 feet and a period of 6-7 seconds.
As a result, have opted to go with a LOW risk for rip currents
for all beaches.

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

&&

.HYDROLOGY...
A Flood Watch remains in place for northern Delaware, the Philadelphia
metro area and surrounding suburbs in New Jersey, the Lehigh Valley,
and northwest New Jersey. The watch is valid from 7 PM tonight
through 8 AM Saturday.

Even with all the rain this Spring, groundwater recharge has been
slow. And that's why state drought declarations remain in place. But
area creeks, streams, and rivers have fully rebounded and are running
normal to above normal. The upper levels of our soils are also
moist/saturated. And the latter items support the Flood Watch.

As a result of the recent rains and saturated soils, we've seen our
FFG values drop. So there is a concern tonight if a quick 1 to 3
inches of rain materializes, especially if the majority of it falls
in the 3 to 6 hour timeframe.

No river flooding is currently forecast, but keep an eye on our
SE PA creeks and streams, as well as northern New Castle County
in Delaware. Bodies of water like Brandywine Creek, the Neshaminy
Creek, the Perkiomen Creek, Red and White Clay Creeks, and the
Christina. This is not an all inclusive list.

As for the mainstems, we're basically looking at 1/2 to 3/4
bankfull, although the Passaic River at Pine Brook could come close
to bankfull. Most of our forecast points will crest sometime on
Saturday, although the Passaic won't crest until Sunday.

Until recently, it's been quite dry across the region, going back to
last fall. And we haven't been dealing with a lot flooding. So it's
time again to remain vigilant. Most flooding injuries and fatalities
occur in vehicles. So if you're out on the road tonight and
encounter a flooded road, turn around. Flooding at night, and the
severity of it, is harder to pick up vs. during the day. And check
your cell phone to make sure your Wireless Emergency Alert feature
is turned on. This is one of the best ways to get flash flood
warnings.

You can also visit our website for the latest warnings:

weather.gov/phi

To stay on top of the flooding potential, visit the National Water
Prediction Service (NWPS) website:

water.noaa.gov

To view river observations and our forecasts in table form, check
out our dashboard:

weather.gov/phi/hydrodashboard

If you want to see the flooding threat in terms of probabilities,
check out this Hydrologic Ensemble Forecast link:

weather.gov/erh/mmefs

&&

.TIDES/COASTAL FLOODING...
One more tidal cycle of some minor flooding comes tonight. With
onshore flow setting up later this evening as a surface low
crosses the heart of our area, high tide tonight may be a bit
higher than the previous two nights. This may result in more
widespread minor tidal flooding, especially given the
freshwater contribution from tonight's expected heavy rain. In
fact for this reason, we've issued a Coastal Flood Advisory that
runs overnight from 2-7am for the tidal Delaware. Elsewhere,
any minor tidal flooding that occurs should be more spotty in
nature. Tidal levels finally look to decline from Saturday Night
onward and coastal flooding concerns ceasing.

For the Chesapeake Bay, no tidal flooding is expected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch until 8 AM EDT Saturday for PAZ060>062-070-071-
     101>106.
     Coastal Flood Advisory from 2 AM to 7 AM EDT Saturday for
     PAZ070-071-106.
NJ...Flood Watch until 8 AM EDT Saturday for NJZ001-007>010-012-
     015>019.
     Wind Advisory from 2 AM to 8 AM EDT Saturday for NJZ016>027.
     Coastal Flood Advisory from 2 AM to 7 AM EDT Saturday for
     NJZ017>019.
DE...Flood Watch until 8 AM EDT Saturday for DEZ001.
     Wind Advisory from 2 AM to 8 AM EDT Saturday for DEZ001>004.
MD...Wind Advisory from 2 AM to 8 AM EDT Saturday for MDZ012-015-
     019-020.
MARINE...Gale Warning until 11 AM EDT Saturday for ANZ430-431-451>455.
     Small Craft Advisory from 1 AM to 6 PM EDT Saturday for ANZ450.

&&

SYNOPSIS...Gorse
NEAR TERM...DeSilva/OHara
SHORT TERM...Gorse/Hoeflich
LONG TERM...Gorse
AVIATION...DeSilva/Gorse/OHara
MARINE...DeSilva/Gorse/OHara
HYDROLOGY...PHI
TIDES/COASTAL FLOODING...PHI