Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Thu Jul 16 2026
Day 3
Valid 12Z Sat Jul 18 2026 - 12Z Sun Jul 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM WEST TEXAS AND
SOUTHERN NEW MEXICO TO SOUTHERN UTAH, AND OVER PORTIONS OF THE
NORTHERN MID-ATLANTIC...

20Z Update...

...Northern Mid-Atlantic...

A deepening area of low pressure in southern Ontario will work in
tandem with high pressure south of Bermuda to cause an acceleration
in southerly low-level winds over the Northern Mid-Atlantic
Saturday morning and into the afternoon hours. PWs will jump to
above 2.0" Saturday morning for areas such as northern MD,
southeast PA, all of NJ, and northward through NYC/Long Island and
along the southern New England coast. In the northern DelMarVa and
Delaware Valley, some guidance shows >2.2" PWs which tops the 97.5
climatological percentile. Strong low-level flow and rising dew
points will support pre-frontal thunderstorm activity Saturday
afternoon; some cases firing off the higher terrain to the west,
and in other cases through strong daytime heating and mid-70s dew
points simply breaking the capping inversion. Sufficient wind shear
will also be present to support potential mesocyclones, which
would further enhance excessive rainfall rates.

Surface winds out of the south to start the day would help direct
high theta-e air over the Chesapeake into these areas initially and
provide a more buoyant air-mass to support hourly rainfall rates
approaching 3"/hr. Additionally, various model guidance shows a
corridor of >2,000 J/kg MLCAPE moving from northeast MD into the
Delaware Valley Saturday afternoon. Given the region in the Slight
also sports a larger urbanized population, rainfall rates to this
caliber would be a recipe for flash flooding with such a high
concentration of hydrophobic surfaces. Parts of the Lehigh and
Susquehanna Valleys may be a little more resilient initially, but
additional thunderstorms look to come through Saturday evening as
the cold front approaches. In collaboration with PHI/CTP/LWX, a
Slight Risk was issued for portions of northeast MD, northern DE,
eastern PA, and much of NJ, while also including the greater
Philadelphia metro area. Should more guidance come into better
consensus on the positioning of heaviest rainfall over the I-95
corridor, the Slight Risk could see additional expansion in future
forecast cycles.

Mullinax

---Previous Discussion---

...Western U.S....

An expansive monsoonal convective pattern will continue, prompting
the inclusion of a Slight Risk stretching from West Texas and
southern New Mexico northwestward across Arizona and into southern
Utah. Overnight guidance indicated a notable uptick in low level
moisture transport into these region on Saturday. This surge will
act to revitalize the potential for additional rounds of high
rainfall rate convective cores capable of producing flash flooding.

Once again, the combination of anomalous moisture and instability
will support highly efficient rainfall rates within the stronger
storms. The threat of rapid runoff and flash flooding will be
elevated across the complex terrain, normally dry washes, and slot
canyons, as well as over highly sensitive soils and wildfire burn
scars.

...Ohio Valley/Central and Southern Appalachians to the Mid
Atlantic and Northeast...

The core of the high-PW airmass will continue to shift east, with
moisture pooling ahead of a strong cold front dropping south
across the Northeast. Strong synoptic forcing, driven by an
amplifying trough over the Northeast and favorable upper jet
dynamics, will support an axis of organized storms from the upper
Ohio Valley into the Northeast. Given the high PWs and strong
forcing, heavy rainfall rates producing at least localized runoff
concerns are expected. Given the uncertainty regarding where the
axis of heaviest rains will occur, maintained a Marginal Risk for
now. However, a targeted Slight Risk may be introduced in future
updates if confidence increases.

Pereira

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt