909
AGUS74 KWCO 011517
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT TUE APR 1 2025

.Synopsis...
Considerable and locally catastrophic flooding possible for portions of the
Southern Plains and Mississippi, Tennessee, and Ohio Valleys... Isolated
flooding possible across the Upper Great Lakes...

.Discussion...

.Southern Plains and Mississippi, Tennessee, and Ohio Valleys...
Considerable flash, urban, small stream, and river flooding is expected
from ArkLaTex to the Lower OH Valley as a heavy rainfall event impacts the
regions beginning on day 2 (Wed) and continuing through day 5 (Sat).
Widespread rainfall totals for this event range from 5 - 10", with locally
higher amounts up to 15" possible from east of the Ozarks in AR to into far
southern IN. For areas along the Lower Ohio River, particularly near the
confluence of the Ohio and Mississippi rivers, this is a historically high
rainfall amount, and may lead to locally catastrophic flooding impacts.

Leading into this event, soil moisture levels across the regions are wet
but not saturated (NASA SPoRT), suggesting infiltration capacity remains
available, at least initially. However, early rounds of rainfall will prime
topsoils, increasing susceptibility to rapid runoff as the event
progresses, especially in the Lower Ohio Valley, where clay-rich soils are
less permeable, and throughout the steep terrain of the Ozarks, where
runoff is enhanced. Streamflows are generally near or below historical
normals (USGS), except in the OH Valley, where locally elevated streamflows
increase the potential for small stream and river flooding. This is
supported by river ensemble forecasts from MMEFS, which indicates the
possibility of widespread moderate and major river flooding across the
region. In addition, Southern Region PQPF and the Hydrologic Ensemble
Forecasting Service (30% chance of exceedance) projects widespread moderate
and major river flooding potential across AR, with isolated moderate to
major river flooding elsewhere.

The latest guidance from the National Water Model (NWM) Medium Range
Forecast (MRF) is signaling widespread rapid onset flooding potential
throughout the region, with the strongest probabilities (75%+) concentrated
across the Lower Ohio Basin, aligning with the forecast of  potentially
historic rainfall. The MRF Maximum High Flow Magnitude Forecast further
indicates widespread high magnitude/low annual exceedance probability (AEP)
flows across AR into the Lower OH Valley, with peak flows expected on days
4 - 7 (Fri - Mon). Combined, these signals support the potential for
widespread considerable, and locally catastrophic, flooding impacts.

.Upper Great Lakes...
Isolated urban and small stream flooding, as well as minor river flooding,
is possible on days 2 - 3 (Wed - Thu) as periods of locally heavy rainfall
affect the region. Across much of MI and WI, recent rainfall and snowmelt
have left upper-layer soils wet but not saturated, while conditions
elsewhere remain dry to near normal (NASA SPoRT). This suggests
infiltration capacity remains readily available. However, multiple rounds
of rainfall over the same areas may quickly saturate soils, increasing
runoff into area streams and rivers, some of which are already elevated,
with isolated minor river flooding already forecast in MI and WI. The NWM
MRF is signaling the potential for isolated rapid stream rises with
probabilities greater than 50%. However, corresponding AEPs from the MRF
Maximum High Flow Magnitude Forecast remain at or above 50% overall,
suggesting lower-magnitude, non-impactful responses.

//Freeman



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