Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
336 PM EDT Tue Aug 11 2026
Day 2
Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR PORTIONS
OF THE OHIO VALLEY, TENNESSEE VALLEY, AND SOUTHEAST ARIZONA...

...Portions of the Plains/Ohio Valley/Midwest/Corn Belt/TN Valley...
2000 UTC Update: Adjustments to the Slight Risk area over the OH
Valley and TN Valley were fairly minor, largely based on
persistence (i.e. the pattern), with some deference to the 12Z
guidance. Confidence in the QPF guidance remains lower than average
however, based on the verification over the past couple of days
and thus far on Tuesday. We did expand the Slight Risk into eastern
portions of the TN Valley, based on some of the QPF trends (RRFS
and ARW2 especially), which has resulted in an uptick in the 12Z
REFS maximum FFG Exceedance probabilities.

Across the Midwest, we've also expanded the Slight Risk westward
to include much of IA and parts of eastern NE and far southeastern
SD, based on the growing model consensus that more organized
convection (upscale growth into an MCS) will initiate farther
upstream than previous expectations, buoyed by favorable right-
entrance region forcing form the next upper level jet streak
traversing the Northern Plains and Upper MS Valley. The 12Z HREF
QPF and FFG exceedance probabilities in fact have come up across
this area, also supporting the extension of the Slight Risk area.

Hurley

Previous discussion below...

On Wednesday, broad cyclonic flow expected to persist across the
northern tier of the Lower 48 from the Northern Plains/Upper MS
Valley, eastward through the Midwest/Great Lakes and into the
Northeast, to the south of a closed mid-level low that has shifted
into Quebec. A frontal boundary will lie along the southern
periphery of this broad trough from portions of the Northern and
Central Plains into portions of the Ohio Valley and Appalachians.
Embedded shortwaves in this broad cyclonic flow will accentuate
lift in the vicinity of this front where PW values are expected to
rise to ~2" ahead of each set of shortwaves and low- level
confluence is expected.

The guidance continues to show a signal for heavy rainfall that at
the moment is most centered over the NE Sand Hills. Due to the
hydrology there, thought it prudent to maintain the inherited
Marginal Risk. When looking at the ingredients, organized
convection looks most likely near portions of the Ohio Valley and
the Corn Belt Wednesday into Thursday morning, where the better
1000-500 hPa thickness difluence is forecast to remain. Hourly rain
amounts to 3" and local totals to 6" remain possible. A high end
Slight Risk lies across portions of the region due to previous days
of rains further saturating soils. Spread remains in the guidance
concerning QPF magnitude, so allowed to existing risk levels to
ride, though there was some southward nudging to the Slight Risk
positioning based on the recent guidance.

...The West/Central & Northern High Plains...
A broadening upper level trough is forecast across portions of the
West and Great Basin Wednesday into Thursday morning. Broad
Marginal Risk areas exist near and ahead of this upper level trough
for portions of the Southwest and Great Basin. Increased upper
level troughing opens the door to strengthening low-level level
flow and broader convective development/coverage across the West
and High Plains when compared to previous days as the mid-level
cap further erodes, with upslope flow developing across portions of
the High Plains. Above average precipitable water values slowly
shift northward through the Intermountain West. While there
continues to be low confidence with the details, locally heavy
rains with the potential of 2.5" an hour amounts (particularly
across southeast AZ) could be problematic. Urban areas,
arroyos/dry washes, box/slot canyons, and burn scars would be most
at risk for flash flooding. A Slight Risk from continuity remains
across southeast AZ.

Roth


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