327
AGUS74 KWCO 031524
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT THU APR 3 2025

.Synopsis...
Considerable to catastrophic flooding expected for portions of the Southern
Plains and Mississippi, Tennessee, and Ohio Valleys... Isolated flash
flooding possible across the Southeast... Ongoing river flooding in
Michigan...

.Discussion...

.Southern Plains and Mississippi, Tennessee, and Ohio Valleys...
Confidence in catastrophic flash and urban flooding along with moderate to
major river flooding continues to increase across the Mississippi and Ohio
valleys from northeast AR into western KY and IN through day 3 (Sat).
Rainfall totals of 1 - 4" have already fallen across the region in the past
24 hours (MRMS), and with an additional 7 - 10"+ (WPC) on the forecast,
historic rainfall totals are expected. Considerable flash, urban, small
stream, and river flooding is also expected for portions of southwest AR
through southwest OH, including the Ozarks, where widespread forecast
rainfall totals range from 5 - 10". Elsewhere throughout the region,
locally considerable flooding impacts will be possible through day 4 (Sun).

Soil moisture levels across the regions have increased significantly in
response to the past 24 hours of rainfall, with soils in the 0 - 40 cm
depth range approaching saturation (NASA SPoRT), leaving little
infiltration capacity to help alleviate runoff. Expect efficient runoff
processing to begin 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. Depth to bedrock decreases
drastically outside of the Mississippi River flood plain, going from 60 ft
to 6 - 3 ft and significantly limiting infiltration capacity. Streamflows
are elevated, but are generally running in the 50 - 75% range for mean
annual flow (USGS), although isolated reaches in TN and KY have already
exceeded the 50 - 20% annual exceedance probability (AEP) threshold
according to the National Water Model (NWM) Analysis and Assimilation (AnA)
Highflow Magnitude service, suggesting that significant streamflows have
already materialized, and with several additional rounds of rainfall
expected, much worse is yet to come. RFC forecasts are beginning to capture
most/all of the QPF for this event, showing widespread minor to moderate
river flooding throughout the region and pockets of major river flooding in
AR, MO and IL. River ensembles (MMEFS, HEFS 30%, PQPF) indicate potential
for widespread rises above major flood status across much of the Lower
Mississippi and Ohio Valleys, with isolated record flooding possible.

The latest guidance from the NWM Short Range Forecast (SRF) is capturing
more of the heavy QPF and is showing strong trends (50%+) of rapid-onset
flooding (ROF) in an axis from Memphis, through western TN and KY.
Corresponding highflow magnitude signals of 20 - 2% AEPs along the same
axis indicate significant and locally considerable small stream flooding
impacts are possible.

Both the GFS and NBM-forced versions of the NWM Medium Range Forecast (MRF)
are indicating widespread exceedance of the 20 - 2% AEP thresholds,
supporting the potential for considerable to catastrophic flooding across
northeast AR, eastern MO, and through much of the Ohio Valley. The Peak
Flow Arrival Times services forecast lower order streams to crest in the 48
- 72 hour timeframe (Sat - Sun).

In summary, confidence in catastrophic flash and urban flooding for
northeast AR through southwest IN and western KY continues to increase as
the event unfolds. Widespread considerable flash, urban, and small stream
flooding is likely and moderate to major river flooding is forecast across
much of the Lower Mississippi and Ohio valleys, with river ensembles
suggesting more widespread major and possibly isolated record river
flooding possible. Significant to locally considerable flooding is possible
elsewhere throughout the region.

Michigan...
Isolated minor to moderate river flooding is ongoing or forecast across MI
in response to 1 - 3" of rainfall over the past 24 hours (MRMS). With only
1 - 1.5" of rainfall forecast for southern MI over the next 7 days (WPC),
rivers should be able to route and recede relatively unimpeded. Most rivers
will have crested by the end of the weekend.

.Southeast...
Isolated flash flooding is possible across the region on days 4 - 5 (Sun -
Mon). The heaviest QPF totals (2"+, WPC) are over parts of AL and northern
GA into SC/NC. Topsoils are near normal (50% RSM, 0 - 10 cm, NASA SPoRT),
allowing for better absorption of additional rainfall into the system.
Combined with fast-moving storms, this will help reduce the overall flood
threat, though periods of higher rainfall rates could still lead to
isolated flash flooding.

//Bliss



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