FOUS30 KWBC 120822
QPFERD

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 2  Valid 12Z Thu Aug 13 2026 - 12Z Fri Aug 14 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST THROUGH THE OHIO VALLEY, THE CENTRAL AND NORTHERN HIGH
PLAINS, PORTIONS OF THE SOUTHWEST, AND THE EASTERN SIERRA/WESTERN
GREAT BASIN...

...Midwest/Ohio Valley...
The active pattern from Wednesday continues almost without respite
into Thursday. A wavering stationary front positioned north of an
impressive mid-level ridge (500mb heights climbing above the 90th
percentile over the Gulf Coast/Southeast) will again be the focus
for waves of thunderstorms. As is the case on D1, and even moreso
on D2, ripples of energy moving within the pinched westerlies and
atop this boundary will help focus ascent for thunderstorm
development, and the upstream connection to the Mexican EML and
Pacific moisture will provide favorable thermodynamics for intense
rainfall. In fact, PWs on Thursday may be higher than any other day
this period, with a plume of 2.25" PWs progged to align near the
front, reflected in the NAEFS with an area exceeding the 99th
percentile of the CFSR climatology. This is focused in a stripe
from central IA through southwest OH, which is also where the ECMWF
EFI reaching 0.7/SoT of 1-2.

This is a more cohesive signature among the models for focused
heavy rainfall, although the greatest risk area may occur on the
upstream portion of this region as reflected by REFS indicating a
high risk (>60% chance) for at least 1"/hr rates, with the
environment supporting the training of 2"/hr rates at times also.
This is also where the AI (both GFS and ECMWF) focus the heaviest
rainfall and is also where the greatest REFS/GEFS/ECENS
probabilities for 3+" of rainfall exist. Conceptually this could be
the region of greatest training as convection develops over NE/IA
and backbuilds along the boundary before organizing into
clusters/MCS and moving more progressively eastward. While anywhere
across this region could experience flash flooding, especially
since the vulnerability will be increased due to antecedent
rainfall, the inherited SLGT (and enhanced SLGT) was expanded to
the west with this update.

...Four Corners through the Northern High Plains...
Widespread convection is again expected in a broad area from the
Four Corners into the Northern High Plains as moisture streams
around a ridge centered over the Gulf Coast. This re-centering of
the mid-level ridge will continue the evolution which begins on
Wednesday of allowing more open 700-500mb flow out of Mexico and
the Pacific to support elevated PW anomalies across a broad region
of the Western CONUS. This is reflected by NAEFS PW anomalies
above the 90th percentile covering most of the this region, with a
focused area of anomalies above the 99th percentile spreading into
the Northern High Plains.

Within this impressive thermodynamic plume, forcing for ascent
will intensify thanks to broad height falls as a trough digs in
from the Pacific, with several mid-level impulses rotating up
through the flow to locally enhance deep layer lift. At the same
time, a stationary front will be draped from Montana through the
Central High Plains, with additional ascent provided via upslope
mechanisms in its wake, especially within the High Plains. As bulk
shear increases to 35-45 kts, the setup will become increasingly
supportive for clusters of thunderstorms containing rain rates
above 1"/hr traversing generally SW to NE (W to E farther north),
and repeating through multiple rounds. The REFS/SREF/GEFS all
indicate regions of 3+" of rain occurring, with the general focus
matching the locations of highest EFI, which also concurs with the
inherited ERO and first guess fields. For these reasons, changes to
the inherited risk areas were modest and cosmetic due to overall
consistency for these areas.

...Southwest/Great Basin...
Broadening upper level trough moving along the Pacific Coast will
bring expanding height falls to the Southwest, with pinched flow
between this trough and a ridge over the Gulf Coast/Southeast
driving pronounced thermodynamic advection northward. PWs will
again exceed 1 inch all the way into Utah (+1 to +2 sigma), but
this will be actually slightly less anomalous than what is
anticipated to exist on Wednesday /D1/. Still, with these elevated
PWs overlapping MLCAPE of 500-1000 J/kg, another active day of
convection is anticipated, with locally elevated bulk shear of
20-25 kts helping to organize development, especially near the
AZ/NM border. Mean winds will be generally light at 5-10 kts across
AZ/NM and from the south, but any organization could locally result
in some training/merging to enhance rainfall. This would be most
problematic over any urban areas, dry washes, canyons, and burn
scars, and the inherited SLGT risk was only modified cosmetically
for new guidance.

However, a second SLGT risk area was added for portions of the
Great Basin/western Nevada where the overlap of high PWs and
significant CAPE will support multiple rounds of thunderstorms.
Both the HREF and REFS guidance indicate an elevated risk of flash
flood guidance exceedance over this area, so after coordination a
targeted SLGT risk was added here as well.

Weiss
$$
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