955
AGUS74 KWCO 091522
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SAT AUG 9 2025

.Synopsis...
Considerable flash and riverine flooding possible for portions of the Upper
Midwest... Isolated flash flood potential to continue across the
Southeast...

.Central Plains through the Upper Midwest...
Confidence continues to increase in the potential for considerable flash,
urban, and riverine flooding impacts beginning later today and persisting
through early next week as QPF guidance continues to suggest two
consecutive days of heavy rainfall (daily totals of 2 - 4"+, totals of 5 -
10" possible) across eastern IA, far-northern MO, northwestern IL, and
southern WI. The best potential for overlap of consecutive days of heavy
rain continues to be across eastern IA where basins are already vulnerable
to additional rainfall given ongoing elevated flows and minor river
flooding. On a regional scale, streamflows are running above normal to much
above normal for this time of year (76th - 90th percentile, USGS),
indicating a limited channel storage capacity for runoff. Convection
currently in progress across the region will likely overwhelm (at least
locally) current infiltration capacity in soils and produce rapid runoff
into already elevated streams and urban drainages, potentially generating
rapid rises on small streams and flash flooding in typical flood-prone and
poor drainage areas. The HRRR-forced National Water Model (NWM) is
indicating medium probabilities (25 - 75%) of rapid-onset flooding across
northern IA, southern MN, and western WI through the morning hours, with
peak flows expected within the next 3 - 6 hours. This will act as a primer
to the main event later tonight and on day 2 (Sun), as the expected
activity will train/repeat over the same areas and generate widespread
heavy rainfall.

Significant river flooding continues to be possible, as guidance from both
the GFS and NBM-forced NWM continues to show widespread small stream
responses through day 5 (Tue) across northeastern KS, northern MO, much of
IA, northwestern IL, and southwestern WI. Corresponding annual exceedance
probabilities (AEPs) indicate the potential for significant stream rises,
particularly across eastern IA, as AEPs as low as 2% are noted across this
region (per the GFS-forced NWM). River ensemble guidance (HEFS, 10%
exceedance) indicates the potential for scattered moderate to major river
flooding across the entire region, with the most significant flooding
concentrated across eastern IA.These factors, in conjunction with a
consistent placement/increasing magnitude QPF forecast from WPC, continues
to provide an increased confidence in potential for significant flooding
impacts beginning tonight and through early next week.

The potential for at least locally considerable flooding is increasing
across eastern KS and northwestern MO on day 2 (Sun) as the axis of
heaviest rainfall shifts further south into this region. Although
antecedent conditions are drier here (near normal streamflows (USGS 10 -
50% RSM, 0 - 10 cm NASA SPoRT), the expected training nature of the
rainfall, coupled with the intensity, will likely quickly overwhelm current
capacities and generate rapid runoff into nearby streams and urban
drainages. There is still some uncertainty in the exact placement and
magnitude of impacts given the modeled QPF differences between various
models however, confidence is increasing for the potential for flash and
riverine flooding impacts, some of which may be significant.

.Southeast...
The potential for isolated flash flooding across FL and along the coasts of
GA/SC will continue through early next week as a stalled frontal boundary
provides a focus for daily afternoon/evening convection. Soils in these
areas are wet following recent rainfall (50 - 90% RSM, 0 - 10 cm NASA
SPoRT), which would generally suggest an increased vulnerability to rapid
runoff and subsequent hydrologic responses. However, urban flooding and
areal ponding are the greatest threats here, given the prevalence of karst
topography/significant underground storage and seepage across FL, and well
draining/sandy soils across much of the Southeast Coastal Plain, helping to
mitigate widespread flooding impacts. The latest guidance of all forcings
of the NWM (HRRR, GFS, and NBM) supports this idea, as isolated small
stream responses are noted across the FL Peninsula and along the immediate
coastline of GA/SC from near Brunswick, GA to Charleston, SC. Impacts will
be greatest in low-lying, urban, and poorly drained areas. Elevated flows
and isolated minor river flooding will continue across GA and the Carolinas
through early next week as runoff from earlier this week routes downstream.

//JDP





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