FOUS30 KWBC 150836
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Sat Aug 15 2026

Day 1  Valid 12Z Sat Aug 15 2026 - 12Z Sun Aug 16 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
OHIO VALLEY...

...Ohio Valley...
A Moderate risk has been expanded to stretch from portions of
eastern IL through IN and across much of OH. An MCS is forecast to
be ongoing at 12z this morning tracking across IL and IN. While
this complex should generally exhibit a weakening trend as the
morning progresses, it will be traversing a corridor of saturated
ground conditions. Consequently, scattered to numerous instances of
flash flooding remain expected into the morning hours.

Another threat will materialize by this evening into the overnight
hours, driven by the remnant MCV from the morning convection. This
feature will help focus additional convective development over OH,
eventually spreading east southeastward into WV and far southwest
PA by late tonight and early Sunday. A very moist airmass
characterized by PWs around 2.3" will support intense hourly
rainfall rates. A training and backbuilding signature again
exists, with deep layer westerly flow and strong low level
westerly moisture transport resulting in weak Corfidi vectors.
HREF neighborhood FFG exceedance probabilities range from 50-80%,
with HREF PMM QPF exceeding FFG thresholds by 200-300% in spots
and overall FFG exceedance spatial coverage topping 50%. These
impressive metrics support numerous instances of flash flooding
tonight over central and southern OH, with the threat spreading
into WV and southwest PA by early Sunday. Some of this flash
flooding will likely be considerable in nature.

...High Plains...
A Slight risk is maintained from eastern CO into central and
western KS, reflecting a southward shift relative to continuity.
The overall setup remains similar to recent days, with convective
forcing along the periphery of the upper ridge interacting with a
moist, unstable airmass.

A cold front will serve as a primary convective focus, with
activity developing north of the boundary where the better
moisture/instability will be pooling. Convection initiating over
the higher terrain will track eastward and may grow upscale
along/north of the front as it moves from CO into KS, aided by
upper level jet right entrance dynamics. Isolated to scattered
flash flooding is expected, particularly across eastern CO where
recent heavy rainfall has lowered FFG thresholds.

...Southeast...
A broad Marginal risk continues across the Southeast on the
periphery of the strong upper ridge centered over the southern
U.S. Anomalous moisture with PWs approaching the the percentile
for mid August will be present, and HREF neighborhood probabilities
support localized 3" rainfall totals However, overall convective
coverage is expected to remain low with a minimal HREF EAS signal.
While sea breeze boundaries may provide localized focus, any flash
flooding impacts should remain isolated.

Chenard
 

Day 2  Valid 12Z Sun Aug 16 2026 - 12Z Mon Aug 17 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA...

...OH Valley into the Mid-Atlantic...
A Moderate risk upgrade has been issued for portions of WV.
Persistent and strong westerly 850mb moisture transport impinging
into the complex terrain is concerning, favoring convective
training and/or repeat activity. This setup, combined with PW
values forecast to exceed 2", supports the potential for numerous
instances of flash flooding. An initial round of convection should
be ongoing at 12z Sunday with an active training/backbuilding
threat. While a relative lull in coverage may occur after this,
activity is expected to ramp back up by afternoon into the evening
along and ahead of an approaching cold front. While this latter
activity may lean more progressive the high PW airmass could
easily yield a quick 1-3" of additional rainfall. Given that soil
conditions will likely be quite saturated by this time,
considerable flash flooding impacts are a distinct possibility in
and near the terrain of WV. While some uncertainty with the details
exist, the 00z Gem reg, RRFS, REFS hit this area with over 3" of rain.

A Slight risk extends eastward across PA, MD, NJ, DE and northern
VA. A mid level shortwave/MCV, alongside the synoptic cold front,
will provide ample forcing for convection across the region. PWs
increasing towards 2.3" and strong low level moisture transport
will create an environment conducive to heavy rainfall rates. An
MCS approaching from the west at 12z Sunday may tend to weaken as
it outruns the primary instability axis, but additional convective
initiation is expected in its wake as instability and moisture
increase ahead of the front. While the cold front will eventually
push the convection along, the environment ahead of the front will
be favorable for backbuilding/training with weak Corfidi vectors.
Any training will pose a flash flood risk, particularly over
sensitive urban corridors. The Slight risk was adjusted southward
to align with guidance trends, with the 00z RRFS and REFS matching
well with the AIGFS/AIFS axis.

The Slight risk was also expanded westward into the OH Valley to
account for additional convection over saturated soil conditions.
Additional convection is expected along/ahead of the advancing
cold front within an airmass characterized by CAPE exceeding 3000
J/kg and PWs near or just over 2". While individual cells should
move more quickly compared to previous days, the saturated
conditions mean even progressive heavy rates will produce scattered
flash flood impacts.

...Southwest...
A Slight risk continues from northeast NM into southeast CO,
focused along and north of a cold front where low level convergence
and easterly upslope flow are maximized beneath upper level right
entrance jet dynamics.

While PW values are not extremely high, they remain slightly above
average, and ample instability will compensate to support isolated
to scattered flash flooding from slow moving cells and cell
mergers. Guidance from the AIGFS, RRFS, and REFS is in good
agreement with this spatial placement.

Chenard
$$
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