Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Wed Aug 12 2026
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 SOUTHWEST AND CENTRAL GREAT BASIN, CENTRAL ROCKIES INTO THE
CENTRAL-NORTHERN HIGH PLAINS, AND MIDWEST INTO THE OHIO VALLEY...

1930 UTC Update: Notable changes were made to the Day 2 ERO,
particularly over the Midwest-OH Valley and across the Southwest
and Lower Great Basin where we expanded the Slight Risk areas. Over
the Midwest-OH Valley, guidance may be starting to come to a
consensus, at least when it comes to the orientation of the outlook
areas into the OH Valley along the north-northeast periphery of
the mid level ridge, essentially aligning within the 588-594DM 500
mb heights. Across the Southwest and Lower Great Basin, per
collaboration with the WFOs have connected the gap between the
Slight Risk areas in NV and AZ-NM, based on the expanded area of
higher (>15%) probabilities of QPF exceedance over current 1/3/6
hour Flash Flood Guidance (FFG) values per the 12Z HREF and
especially REFS.

We also maintained the Enhanced Slight Risk area (>25%
neighborhood probability) across much of IA into northern IL --
where several CAMs (ARWs, HRRR, and NAM CONUS-Nest) show pockets
of 3-6+ inches of rain with the next MCS. This area is also
supported with last night's 00Z ECMWF EFI, along with maximum FFG
exceedance probabilities of 25-40% per the HREF.

Hurley

0800Z 8/12 Discussion..

...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


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt