Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Mon Aug 17 2026
Day 1
Valid 16Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY AND THE CENTRAL APPALACHIANS...

...OH/TN valley into the Appalachians...
Regional radar mosaic this morning shows convection already
blossoming across the Ohio Valley and into Pennsylvania, as well as
draped as far southwest as Arkansas along the slowly advancing
cold front. Weak impulses moving within broad eastern CONUS
troughing will help enhance ascent through this evening, and the
12Z CAMs indicate scattered to widespread thunderstorms will
develop through peak heating. Modest bulk shear of 20-25 kts within
a plume of 1.75 to 2.0 inch PWs will support clusters of
thunderstorms with rainfall rates of 1-2"/hr, briefly even heavier
than that, with the most concentrated heaviest rates focused from
the Ohio Valley into the Central/Southern Appalachians. This area
is also where the FFG is significantly compromised due to recent
heavy rain, reaching generally 0.75-1.5"/3hrs. Fortunately, mean
0-6km winds of 10-15 kts with similarly paced propagation vectors
aligned perpendicular to the boundary should prevent significant
training, although repeating rounds of heavy rain are possible in
some areas. The inherited SLGT risk encompasses the most vulnerable
areas today, and was tailored only cosmetically from inherited to
match the highest FFG exceedance probabilities from both the REFS
and the HREF.

...Southwest...
Expansive west-to-east oriented mid-level ridge draped from the Low
Deserts of California through the Southern Plains will maintain a
favorable pattern for at least some monsoonal convection today.
While airmass convection will be the primary mode, a potent
shortwave noted in GOES-E WV imagery lifting out of West Texas will
pivot W/SW through the day, providing at least locally enhanced
ascent through southern NM and southeast AZ. Subtly enhanced inflow
will help draw PWs to as high as 1-1.25", lesser farther north
into the Four Corners, to support rainfall rates for which the
HREF, REFS, and UA WRF suggest will reach 0.5-1"/hr. While the CAMs
are not extremely aggressive with coverage, mean 0-6km winds of
just around 5 kts indicate slow movement for any storms that
develop today, posing a flash flood risk over vulnerable terrain
features or urban areas. For this reason the MRGL risk remains
supported, with a subtle extension south from inherited to capture
the advancing shortwave.

...Central Plains to Upper Midwest...
Amplifying mid-level trough over Northern Manitoba will shed pieces
of energy through its base and into the Northern Plains today. One
of these impulses is progged to be potent as it drops NW to SE from
the Dakotas into Iowa this evening. This impulse will interact with
a surface front, leading to locally accelerated and backed inflow
within a modest warm sector to the south. This should result in
increasing coverage of thunderstorms from South Dakota into
Nebraska this evening, with organization into one or multiple MCS
likely overnight within a region of 25-35 kts of bulk shear. As the
LLJ backs and amplifies to 25-30 kts, it will draw PWs above 1.5
inches northward, overlapping with a plume of 1000+ J/kg MUCAPE to
produce thermodynamics capable of supporting 1-2"/hr rainfall rates
(20-40% chance of 1+"/hr from both HREF and REFS guidance).
While storm motions may be progressive on 20 kts of 0-6km winds,
Corfidi vectors aligned to both this mean wind and the elevated
front could support training from west to east, enhancing the
potential for excessive rainfall. The CAMs continue to be highly
variable in their footprint for tonight, but there appears to be a
focus from the Sandhills of NE into western IA, as well as a
secondary maxima across the Coteau des Prairies of SD/MN. Any
development farther southeast would impinge into lower FFG, but
otherwise the exceedance probabilities within the most likely
footprint tonight suggests the MRGL risk remains supported, and
only minor cosmetic adjustments were made with this issuance.

Weiss


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt