Special Marine Warning 151, NWS Philadelphia/Mount Holly

Area Forecast Discussion
National Weather Service Mount Holly NJ
423 PM EDT Thu Aug 20 2026

.WHAT HAS CHANGED...
Severe thunderstorm and tornado watches now in effect for most
of the forecast area.

&&

.KEY MESSAGES...
1. A cold front will cross the region this evening, resulting
in widespread showers and thunderstorms. Flash flooding,
damaging wind gusts and tornadoes are possible later today.

2. Conditions look to remain unsettled through the
weekend, with daily chances of showers and storms each day.

3. Fair weather is expected to return for early next week.

&&

.DISCUSSION...
KEY MESSAGE 1...Broad troughing will be the main driving force
in the sensible weather into the early to middle part of next
week across much of the Eastern Seaboard. A shortwave will round
the base of the trough today into this evening providing some
mid level synoptic forcing. At the surface a warm front has
lifted north into the Philadelphia metro area with a cold front
approaching from the west.

The trend for increasing QPF and increasing severe weather
potential is being realized this afternoon. Severe thunderstorm,
tornado, and flood watches are in effect for most of the
forecast area.

Mesoanalysis fields this afternoon show moderate to strong
instability centered roughly around the Delmarva peninsula, but
extending north into southern NJ and southeast PA. Deep layer
shear of 40-60kt along with backed low level flow and strongly
curved hodographs will support a tornado risk, most notably from
the Philadelphia metro area and points south. Primary mode at
the moment is supercellular with all attendant risks, but more
multicell clusters may become the primary mode along the cold
front.

The other threat this evening is the potential for flash
flooding. Guidance shows very moisture rich airmass surging into
the region with PWAT values raching 2-2.5 inches. Warm cloud
depths suggest warm rain processes and high precipitation
efficiency into this evening and WPC has introduced a moderate
risk of execessive reainfall from Philadelphia north and east.

In general, expecting anywhere between 1 to 2 inches of rain to
fall, with localized higher amounts up to 3 to 5 inches.

The severe and flooding rain threat will diminish by the late
evening and overnight hours.

KEY MESSAGE 2...Conditions look to remain unsettled through the
weekend, with daily chances of showers and storms each day.

Mid to long range NWP and AI guidance suggests that the pattern
could become unsettled Friday night and into the weekend.
The cold front crossing the area tonight will stall in the
offshore waters. waves of low pressure will ride along the
stalled front resulting in unsettled weather through the
weekend. The main periods to watch are early Saturday through
Saturday evening, with periods of rain increasingly likely, and
again on Sunday. The severe threat appears low at this time but
there could be additional flooding concerns, although no day
has more than a Marginal (1/5) Risk for excessive rainfall.

Temperatures will be seasonable, but humidity will be
uncomfortable.

KEY MESSAGE 3...Fair weather is expected to return for early
next week.

Weak high pressure looks to bring quieter and drier weather in
the early part of the week, with more unsettled conditions
expected later in the week with seasonable temperatures.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Widespread showers and thunderstorms are likely at all
locations this afternoon and evening. Any storms will be
capable of producing very heavy rainfall and LIFR conditions.
Winds will vary depending on location, with northern terminals
experiencing northeast winds 5 to 10 kt where southern
terminals will encounter southeasterly winds around 10 kt. Winds
will be gusty and erratic near any storm. Bulk of the shower and
storm activity ending around 01-02z. Moderate confidence
overall, but low confidence on exact timing details.

Tonight...Sub-VFR conditions probable at times with showers and
thunderstorms continuing into the evening hours. Fog and/or low
stratus possible at night after the precipitation moves out.
Conditions should lift back to VFR near or not long after
daybreak. North/northeast winds around 5 to 10 kt. Moderate
confidence.

Outlook...

Friday through Friday night...Primarily VFR.

Saturday through Sunday...Periods of VFR and sub-VFR at times.
Restrictions expected with daily chances for showers and
thunderstorms.

Monday...VFR. No significant weather expected.

&&

.MARINE...
A cold front will move through tonight, with winds becoming
more north/northeasterly around 10-15 kt. Seas of 2 to 3 feet.
Showers and thunderstorms expected on the waters this evening
into tonight.

Outlook...

Friday through Monday...No marine headlines expected. Showers
and thunderstorms possible during the day Friday through
Sunday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch until 2 AM EDT Friday for PAZ054-055-060>062-070-
     071-101>106.
NJ...Flood Watch until 2 AM EDT Friday for NJZ001-007>010-012>022-
     025>027.
DE...Flood Watch until 2 AM EDT Friday for DEZ001.
MD...None.
MARINE...None.

&&

DISCUSSION...Manning
AVIATION...Manning
MARINE...Manning

Hazardous Weather Outlook
National Weather Service Mount Holly NJ
512 AM EDT Thu Aug 20 2026

MDZ012-015-019-020-210915-
Kent MD-Queen Annes-Talbot-Caroline-
512 AM EDT Thu Aug 20 2026

This Hazardous Weather Outlook is for northeast Maryland.

.DAY ONE...Today and tonight.

Numerous thunderstorms are expected between about 2 PM and 8 PM. These
storms may contain damaging wind gusts and potentially a couple of
tornadoes over the Eastern Shore. Storms will be capable of producing
heavy rainfall, with rainfall rates of 2 to 4" per hour possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

Hoeflich

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Preliminary Local Storm Report
National Weather Service Mount Holly NJ
727 PM EDT Thu Aug 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0720 PM     Flash Flood      West Norriton           40.13N 75.38W
08/20/2026                   Montgomery         PA   Emergency Mngr

            Extensive flash flooding occurred in various
            locations throughout West Norriton and Lower
            Providence townships in Montgomery County,
            PA. Many streets not passable and many
            streams well out of their banks.

&&

PF