529
AGUS74 KWCO 051530
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT FRI JUL 5 2024

.Synopsis...
New and renewed flooding and flash flooding threats continue across the
Upper and Middle Mississippi Valleys... Heavy tropical rainfall to begin
late this weekend across South Texas... Isolated flash and urban flooding
possible for portions of Central Plains... Increased snowmelt and glacier
runoff, heavy rainfall to continue in Alaska... Flooding possible this
weekend in American Samoa...

.Discussion...

.Upper and Middle Mississippi Valleys...
Across MN, IA, and WI, scattered thunderstorms today may cause additional
flash and urban flooding as storms continue to impact this already
saturated region. Hydrologic conditions remain primed with wet soils and
swollen streams. All forcings of the NWM indicate scattered to widespread
small stream responses with peak flows ongoing or will occur this afternoon
as runoff routes into the stream network. Corresponding annual exceedance
probabilities (AEPs) from the National Water Model (NWM) Short Range
Forecast High Flow Magnitude forecast continues to show that most rises
will be modest, although some locally significant rises cannot be ruled out
in areas of heavier rainfall. Additionally, moderate to major river
flooding continues across portions of southern MN, and much of IA, with
moderate to major flooding forecast along portions of the mainstem Missouri
and Mississippi rivers through early next week.

.South Texas...
Periods of tropical rainfall associated with Hurricane Beryl will impact
portions of the Hill Country, Deep South TX, and the Rio Grande Valley on
days 3 - 6 (Sun - Wed). Rainfall totals of 4 - 8" (with locally higher
amounts) is forecast for the aforementioned areas, and have been trending
upward over the past couple of days. Antecedent conditions are generally
dry, as relative soil moisture is low (0 - 30%, 0 - 10 cm RSM NASA SPoRT)
and streamflows are generally below normal (USGS) for this time of year.
These conditions would suggest that at least initially, there is ample
infiltration capacity and in-channel storage in the streams and rivers.
However, caution must be exercised as very dry soils can still be somewhat
hydrophobic and elicit some runoff responses. Once the rainfall begins the
antecedent conditions will transition to be more hydrologically favorable
for enhanced runoff and out-of-bank rises.

The NWM MRF is beginning to suggest some rises to high water and AEPs as
low as 4% on the smaller streams and tributaries begin on day 4 (Mon) in
southern and eastern TX. These signals would suggest that the main threat
would likely be flash and urban flooding responses, particularly in
low-lying areas and areas of complex terrain found further inland. River
ensembles (HEFS and PQPF) are also beginning to suggest some isolated
riverine responses may be possible. Nevertheless, confidence in the exact
location and magnitude of potential flooding remains uncertain and is
highly dependent on the track and intensity of Beryl.

.Central Plains...
Isolated flash and urban flooding, along with the new rises on small
streams may be possible for portions of southern KS, western MO, and
northern OK from additional rounds of rainfall (1 - 3") this weekend into
early next week. Surface and shallow soils (0 -10 cm and 0 -100 cm) are
very dry (0 - 30% RSM, NASA SPoRT) with variable climatological streamflow
percentiles for this time of year (USGS). Given the dry soils, runoff may
still be a factor if intense rainfall rates exceed infiltration
rate/capacity. However, streams look to be in rather good shape overall and
will likely have enough storage capacity available to mitigate more
widespread flooding responses. Some pluvial flooding may still occur in
typically flood prone areas and areas with poor drainage. Both the GFS and
NBM-forced NWM are signaling magnitudes of 10 - 4% AEPs with peak flow
arrival times by day 3 - 4 (Sun - Mon). While widespread flooding responses
are unlikely, it is possible that locally significant flooding responses
may occur where training thunderstorms develop, particularly in urban areas.

.Alaska...
Periods of moderate to heavy rainfall (1 - 2"+) will continue into the
weekend across much of the Brooks Range, Interior, and into the south
central portions of the state. This influx of rainfall may enhance runoff,
generate rapid rises and rivers and streams, and potentially lead to
ponding and isolated flooding. Elsewhere, elevated flows and isolated minor
flooding from increased snowmelt and glacial melt continues along the
Klutina River near Copper Center and the Nuyakuk River northeast of
Dillingham.

.American Samoa...
Rainfall from a developing trough may cause instances of isolated flash
flooding through day 2 (Sat).

//Capp




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