Mesoscale Precipitation Discussion 1099
NWS Weather Prediction Center College Park MD
333 PM EDT Thu Aug 20 2026

Areas affected...Coastal Mid-Atlantic

Concerning...Heavy rainfall...Flash flooding possible

Valid 201930Z - 202200Z

SUMMARY...Thunderstorms developing ahead of the approaching cells
farther west are capable of producing rainfall rates up to 3"/hr.
Flash flooding is possible this afternoon, especially in poor
drainage areas and within more urbanized communities.

DISCUSSION...Doppler Radar and GOES East satellite show a cluster
of storms have flared up north of a coastal frontal boundary
located over the northern DelMarVa Peninsula and into the Delaware
Valley. The environment is ripe for highly efficient rainfall
rates. RAP mesoanalysis shows MLCAPE has risen between 1,000-2,000
J/kg from the heart of the Chesapeake Bay to southern NJ. PWs are
quite high, ranging between 2.0-2.2" over the coastal
Mid-Atlantic. Brisk SWrly 850mb winds will transport this pool of
anomalous moisture aloft into the rest of southern NJ, where
instability is increasing and dew points are reaching the low-mid
70s. These storms also have exceptional vertical wind shear aloft,
as effective bulk shear values are >40 kts and effective SRH
values are between 100-200 m2/s2. This environment does favor
potential mesocyclones within developing storms, which does help
to further enhance rainfall rates.

MRMS 15-minute rainfall totals are topping out around 0.75" in
some parts of northern DE and southern coastal NJ. This suggests
ongoing thunderstorms are capable of producing 3"/hr rainfall
rates. The concern for flash flooding comes from the potential for
discrete cells over southern NJ and the northern DelMarVa ahead of
the approaching storms from northern VA, central MD, and
south-central PA. The 18Z HRRR shows the potential for additional
cells over the DelMarVa Peninsula that may flare up along the sea
breeze boundary. These storms could briefly back-build before
heading north and east towards the Delaware Bay. Farther north,
the increase vertical wind shear and upper-level forcing ahead of
an approaching 500mb vort max will foster excellent ascent aloft,
allowing for additional storms from the west to reach southern NJ.

Flash flooding is possible this afternoon and into this evening,
given the highly efficient rainfall rates that are both ongoing
and are still to come in areas currently receiving heavy rainfall.
Poor drainage areas and locations with greater influences from
urbanization are most at-risk.

Mullinax

ATTN...WFO...AKQ...PHI...

ATTN...RFC...RHA...NWC...

LAT...LON 40487409 40457393 40347376 40157374 39817387
39277405 38877425 38427460 38347504 38407552
38477590 38577613 38697627 38837637 39047638
39157613 39247591 39457558 39567536 39727507
39887486 40097460 40277443 40437424