168
AGUS74 KWCO 061517
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SUN APR 6 2025

.Synopsis...
Widespread river flooding and ongoing flooding impacts continue across the
Southern Plains and Mississippi, Tennessee, and Ohio Valleys... Locally
considerable flash and urban flooding possible in the Southeast...

.Discussion...

.Southern Plains and Mississippi, Tennessee, and Ohio Valleys...
Likely historic rainfall has caused catastrophic flooding across the
region, leading to widespread flooding impacts that will persist well
beyond today. Areas of standing water will be slow to recede over the next
few days, making recovery efforts difficult in the hardest-hit areas.
Additionally, isolated river flooding exceeding record levels is forecast
in the Lower OH Valley, along with exceedingly rare, widespread major river
flooding occurring across the Lower OH Valley through the Lower MS Valley.
Widespread minor to moderate river flooding is also extending from the
Middle OH Valley to northeast TX. Flooding on most rivers will continue
over the week, with some smaller streams and rivers receding over the next
few days. While the heavy rainfall has ended, scattered showers will
continue throughout the day, however, with rainfall amounts expected to
remain less than 0.5", no further exacerbation of the ongoing flooding is
anticipated.

.Southeast...
Locally considerable flash and urban flooding will be possible through the
overnight hours for portions of AL into northern GA in response to periods
of heavy rainfall. Isolated flash and urban flooding will also be possible
for the remainder of the region through day 2 (Mon). In northern AL, recent
and ongoing rainfall have left upper-layer soils wet but not saturated,
while conditions elsewhere remain dry to near normal. This suggests that
infiltration capacity is still available. However, training rainfall over
the same areas could quickly saturate soils, increasing runoff into area
streams and rivers. While flash and urban flooding are the primary threat,
some small stream and minor river flooding cannot be ruled out. Minor to
isolated moderate river flooding is currently forecast in AL and GA.

The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) has
been signaling the potential for rapid-onset flooding across north and
central AL with probabilities mostly at or below 50%. However, these
signals may be underdone due to the HRRR being slow to capture the ongoing
convection and responses may be more significant than depicted by the NWM.
Corresponding annual exceedance probabilities, per the SRF High Flow
Magnitude, remain mostly at or above 50% overall, except in northern AL,
where higher magnitude/lower AEPs (between 10% and 2%) on smaller streams
suggest the possibility of significant hydrologic responses. As the event
continues to unfold and the bulk of the QPF moves into the forecast window,
the magnitude of the SRF signals is expected to increase, potentially
expanding further south in AL and east into northern GA.

//Freeman



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