871
AGUS74 KWCO 261513
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SAT APR 26 2025

.Synopsis...
Flash and urban flooding possible across the Southern Plains into the
Ozarks...Isolated flooding across the Northern Plains and Upper
Midwest...Urban and small stream flooding likely in Puerto Rico and the
USVI...Rain and snowmelt induced flooding in Montana...Significant flooding
continues along the Lower Mississippi River...

.Discussion...

.Southern Plains into the Ozarks...
Ongoing slow-moving showers and storms will persist across portions of the
TX Panhandle and southern OK through the early morning hours, continuing
the threat of flash and urban flooding. While much of this region has been
dry (RSM less than 40%, 0 - 10 cm NASA SPoRT), rainfall over the past 12
hours (2 - 3", locally 5"+) has saturated soils across portions of the TX
Panhandle and southwestern OK, diminishing infiltration capacity.
Regardless of soil conditions, the intense nature of the rainfall will
quickly overwhelm any remaining capacity, producing flooding, some of which
may be locally significant. Rapid rises on quick responding creeks/streams,
flooding of low water crossings, and standing water in urban areas with
poor drainage are possible. The latest guidance from the HRRR-forced
National Water Model (NWM) has been consistently indicating small stream
responses from Childress, TX to Lawton, OK, with basin average
probabilities generally at or greater than 50%. Corresponding annual
exceedance probabilities (AEPs), per the High Flow Magnitude Forecast,
indicate the potential for significant stream rises and out of bank flows
along streams, with AEPs of 4 - 10% noted across the region.

As this activity dies out mid to late morning, redevelopment of showers and
storms are expected across the Red River Valley (southern OK/northern TX)
and move into the Ozarks and Ouachita Mountains (eastern OK/western AR)
this afternoon and evening. This activity is expected to occur over already
saturated soils, increasing the likelihood of flooding in this region. The
HRRR-forced NWM is also indicating small stream responses of similar
magnitude across this region for tonight. More significant stream rises and
subsequent flooding is possible in this region, especially where slow
moving or training storms materialize.

After a brief reprieve in heavy rainfall on days 2 - 3 (Sun - Mon),
additional convective rainfall returns to the region on days 4 - 5.
Confidence continues to increase in the likelihood of hydrologic responses
from the Red River Valley through southern MO and guidance in the
GFS-forced NWM is showing increased probabilities (25 - 75%) of small
stream responses in this region. However, the magnitude of the impacts
remain in question, due to discrepancies in the QPF amongst various models.

.Northern Plains and Upper Midwest...
Waves of showers and storms on days 2 - 3 (Sun - Mon) may bring isolated
flash and small stream flooding from portions of the Dakotas through much
of WI. Antecedent soil conditions are generally a mixed bag across this
region, with generally drier soils across the Dakotas (RSM less than 40%, 0
- 10 cm NASA SPoRT) and wetter soils across southern MN, IA, and WI
following recent rainfall earlier this week (50 - 75% RSM, 0 - 10 cm NASA
SPoRT). Regardless of the soil conditions, isolated enhanced rainfall rates
may locally overwhelm infiltration capacity and lead to flooding of
ditches, culverts, and low water crossings, as well as standing water in
fields, on roadways, and in typical flood prone areas across the region.
Guidance from both the GFS and NBM-forced NWM are only indicating isolated
small stream responses across the entire region on days 2 - 4 (Sun - Tue).
Some uncertainty exists in exactly where the highest magnitude of impacts
will occur however, the impacts are expected to remain isolated given the
overall progressive nature of the activity.

.Puerto Rico and the USVI...
Repeated rounds of heavy rainfall will likely generate urban and small
stream flooding across the region through much of next week, with locally
considerable impacts possible. The most significant rainfall and subsequent
impacts are expected today and again on day 5 (Wed). Landslides, rapid
river rises, and isolated flash flooding will become increasingly likely as
soils become more saturated, especially in areas of steep/hilly terrain.

.Montana...
A combination of rainfall (1 - 2") and snowmelt may produce areal and small
stream flooding impacts across portions of south-central MT on days 2 - 3
(Sun - Mon). The most vulnerable areas are the Bighorn, Beartooth,
Abasroka, and Crazy mountains and associated foothills, as these areas
contain anywhere from 3 - 7" of snow water equivalent in the snowpack. With
snow levels above 7500, warm temperatures will accelerate snowmelt in the
mid elevations (5500 - 7500 up to 3" of SWE loss, SNODAS). This is
expected to occur on frozen ground, which will generate immediate runoff
into nearby small streams and rivers, leading to rapid rises and flooding
of low-lying and poorly drained areas. A drier pattern for the remainder of
the workweek should largely mitigate more widespread and significant small
stream responses.

.Lower Mississippi River...
Significant river flooding along the mainstem of the lower Mississippi
River (AR/MS/LA) will continue for the foreseeable future as the flood wave
continues downstream. The floodwave is near Arkansas City, AR and
Greenville, MS, as crests in moderate flood stage are expected in these
locations this weekend. Significant rises are still expected from Vicksburg
to New Orleans over the next week, with the river in Baton Rouge expected
to crest in major flood stage mid to late next week. Showers and storms are
expected across the region over the next several days, which may briefly
delay ongoing recessions and crests however, renewed rises are not
expected.

//JDP





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