Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Aug 12 2026
Day 1
Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL PLAINS THROUGH THE TENNESSEE VALLEY, AND FOR THE DESERT
SOUTHWEST..

...Central Plains southeast to the Tennessee Valley...
Extremely active convective weather continues Wednesday as a "ring
of fire" type setup continues with mid-level waves rotating atop a
ridge to bring convective clusters to the region.

The ridge responsible for this setup amplifies across the Southern
Plains but does retreat subtly in the east resulting in pinched
flow around its northern periphery. Within this flow, multiple
shortwaves will lift out of the Four Corners and then race E/SE
from the Central Plains through the Ohio Valley in a pattern very
repetitive from previous days. While the exact placement of any of
these shortwaves is difficult to determine even on D1, each ripple
interacting with a stationary front draped across the region will
result in clusters of thunderstorms moving repeatedly across the
region, with ascent additionally enhanced by a jet streak
positioned near the Canada/U.S. border leaving its diffluent RRQ
overtop this area. While storms will generally track NW to SE
within the region of greatest thickness gradient, small variations
in mesoscale features can play a significant role in where the
heaviest rainfall will occur.

This activity will develop within robust thermodynamics as a plume
of elevated PWs reaching +1.5 to +2 sigma arcs around the ridge and
pools along the front, collocated with impressive MUCAPE driven by
an EML noted in regional forecast soundings lifting out of Mexico
and around the ridge. Although lapse rates weaken farther east into
the Ohio Valley than the Central Plains, this results in deeper
column saturation to support intense rainfall rates, for which the
HREF neighborhood probabilities suggest will exceed 2"/hr at times.
The most intense rain rates will occur within any MCS development
as 0-6km bulk shear of 40-60 kts drives strong convective
organization, which with mean 0-6km winds aligned to both the front
and the Corfidi vectors, indicates a strong likelihood for periods
of training. While there continues to be uncertainty as to the
exact footprint of the heaviest rainfall, anywhere prolonged
training of these intense rain rates can occur, more than 5 inches
of rain is possible.

As noted above, confidence in the exact placement of the heaviest
rainfall remains uncertain due to model variations in the placement
of these, already notoriously difficult to forecast, mid-level
impulses. However, both the REFS and HREF depict a region of higher
probabilities from Iowa through Kentucky, with a subtle SW trend
noted in the accompanying deterministic guidance. Some of this rain
will fall atop recently saturated soils from prior heavy rain, but
in general the axis of heaviest rain may just miss those areas. For
this reason changes to the inherited ERO were modest and cosmetic.

...Southwest/Great Basin/Four Corners...
The subtle eastward retreat of the expansive mid-level ridge over
the southern tier of the CONUS in response to a trough diving along
the Pacific Coast will allow for more intense moisture and
thermodynamic return into the region today. This evolution will not
only allow for elevated PWs to stream northward (widespread +2 to
+2.5 sigma and above the 97th percentile according to NAEFS), but
will also weaken the cap as mid-level temps fall to enhance
available instability. Additionally, forcing for ascent will be
maximized as multiple weak impulses lift northward from Mexico
around the ridge periphery, acting upon the favorable
thermodynamics to drive widespread convective development.

Coverage of thunderstorms today should be quite widespread,
potentially one of the more active days of the entire monsoon
season so far. Simulated reflectivity from the high-res CAMs
suggest storms will develop as far west as the Sierra, while
covering as well most of the Four Corners, Great Basin, and into
the High Plains. Mean 0-6km winds of 10-15 kts will allow for at
least some more progressive movement than previous days, but bulk
shear of 20-30 kts and persistent southerly moist inflow will help
to organize convection resulting in periods of possible
training/backbuilding. With rain rates likely reaching 1"/hr at
times, this could produce locally 3" of rain, but even where total
rainfall is less, the intensity of the rain could still cause flash
flood related impacts. The greatest threat will be across
vulnerable features like burn scars, dry washes, canyons, and urban
areas, but activity is expected to be widespread today, especially
where any morning cloud cover does not limit instability later
today. After coordination with the affected WFOs, a SLGT risk was
expanded to include most of Arizona, the slot canyon region of
Utah, and far southern Nevada.

Weiss


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