018
AGUS74 KWCO 061520
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
10:15 AM CDT TUE MAY 6, 2025

.Synopsis...
Locally considerable flooding responses possible across the Southern Plains
into the Central Gulf Coast... Lingering rainfall may cause isolated
flooding impacts in Puerto Rico and the USVI... Potential urban flooding
and small stream rises in the Northeast... River ice breakup increasing in
Alaska... Flash and small stream flooding, along with landslides, are
possible this week in American Samoa...

.Discussion...

.Southern Plains into the Central Gulf Coast...
Ongoing rain will continue today in portions of eastern NM, the TX
panhandle, and OK, bringing the potential for additional flash flooding
before spreading east across the Gulf states through this evening. This
region remains vulnerable to additional localized flooding given rainfall
over the last week has left soils wet and streams elevated. Given these
antecedent conditions, small stream flooding may linger into day 2 (Wed)
before conditions improve enough to allow for recessions. Additionally,
steeper terrain combined with potentially heavy rainfall rates may elicit
localized flash flooding near the foothills of the Colorado Rockies.

Multiple rounds of rainfall are expected in East TX through the Central
Gulf Coast today with widespread 3 - 5", locally higher possible, through
day 3 (Thu). With limited soil infiltration capacity and above normal
streamflows (NASA SPoRT and USGS), the antecedent conditions are nearly
primed for enhanced runoff. As such, locally considerable flash and urban
flooding impacts are possible primarily over LA and southern MS but may
extend as far west as East TX. The HRRR-forced National Water Model is
signaling many tributaries and mainstems across the Southern Plains into MS
with annual exceedance probabilities of 50 - 20%, down to as low as 4% in
some localities. This would suggest that new and renewed rises are possible
with some basins experiencing significant rises. Additionally, the peak
flow arrival times are generally over the next 72 hours especially along
smaller order streams. Basins in the MS Delta may take a few extra days to
recede due to flatter terrain.

Ongoing moderate and major river flooding in the Red River Valley has
generally crested and will continue to recede before additional rainfall
across the area through today causes some recessions to stall. River
ensembles suggest that new and renewed minor to scattered moderate river
flooding is possible, particularly if training were to occur across East TX
through MS.

.Northeast...
Localized urban and small stream flooding is possible today due to
persistent rainfall across portions of the Northern Appalachians and New
England. Rapid onset flooding probabilities range between 25 - 50% for much
of the area. Recent rainfall has primed the soil moisture for the region,
showing saturated conditions of 75 - 95% (0 - 100cm RSM, NASA SPoRT). While
widespread hydrologic responses are unlikely, flooding in areas of steep
terrain or poorly drained urban areas should not be ruled out.

.Puerto Rico and the USVI...
Locally heavy rainfall will continue to be possible across much of the
island, in particular the interior sections, through day 2 (Wed) which may
continue to cause isolated urban and small stream flooding. While the most
significant impacts may have peaked yesterday, antecedent soil conditions
and stream flows are still vulnerable to additional rainfall. This may
result in isolated fluctuations to river stages with a rise from non-flood
to flooding status remaining possible. By midweek, atmospheric conditions
improve and drier conditions take hold, providing some much needed reprieve
from the flooding threat.

.American Samoa...
Flash, urban, and small stream flooding may begin today and continue
through the weekend due to prolonged rainfall across the island. The most
significant flooding impacts are likely to occur in low-lying areas and
areas that may be poorly drained. Previous weeks have also seen some active
weather causing saturated soil conditions. Most of the island is
mountainous topped with thin erodible soils, making them now favorable for
landslides at the base of steep slopes. While there is still some
uncertainty with the peak timing of the forecast, current forecast trends
suggest that significant flooding may occur late this week into this
weekend.

.Alaska...
River breakup is underway in portions of AK, with minor to moderate
flooding possible on portions of the Yukon River as well as sections of the
Lower Kuskokwim river. In addition, several rivers north of the Alaska
Range and south of the Brooks Range have been characterized by steady stage
rises with intermittent rapid spikes indicative of ice jamming and
releasing. Flood potential on these rivers is expected to continue through
this week as breakup fronts continue to push downstream.

//DPL/WW

$$