686
AGUS74 KWCO 041517
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT THU JUL 4 2024

.Synopsis...
New and renewed river flooding and flash flooding threats continue across the
Central Plains and the Upper to Middle Mississippi Valley...Heavy rainfall
possible this weekend across South Texas...Isolated heavy rainfall today
across portions of Puerto Rico... Increased snowmelt and glacier runoff,
heavy rainfall to continue in Alaska...

.Discussion...

.Central Plains and the Upper to Middle Mississippi Valley...
Moderate to major river flooding continues across portions of extreme
southeastern SD, 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. Scattered thunderstorms expected today and
again on day 3 (Sat) may elicit flash and urban flooding, as well as new
and renewed river flooding in already flooded areas.

Across portions of the Central Plains and Mid-Mississippi Valley, rainfall
overnight and early this morning has resulted in rapid rises to minor and
moderate flood across portions of eastern KS. This complex of storms will
persist across eastern MO and central/southern IL through at least midday,
and may generate isolated incidences of flash and urban flooding, and rapid
stream rises, especially in areas that received heavy rainfall yesterday.
The latest HRRR-forced National Water Model (NWM) indicates scattered small
stream responses across much of northeastern MO and through central IL
through mid-morning, with peak flows expected within the same time frame.
Corresponding annual exceedance probabilities, per the High Flow Magnitude
forecast, suggest that most stream rises in this region should remain
within the channel however, some locally significant rises are possible
across eastern MO, as AEPs in this region are as low as 2%. Additional
rainfall this afternoon across this same region (KS/MO/IL) may generate new
and renewed flooding as soils and streamflows will have limited recovery
time in advance of this next round of storms. Impacts will be highly
dependent on storm motions and the intensity of rainfall.

Further north across SD, MN, IA, and WI, flash and urban flooding is
possible through early day 2 (Sat) as storms impact this already saturated
region. Despite a brief reprieve in rainfall in recent days, much of this
area still has extensive river flooding and there is very little
infiltration capacity due to wet soils, so it will not take much rainfall
to generate hydrologic responses. All forcings of the NWM indicate
scattered to widespread small stream responses across this region within
the next 24 hours, with peak flows expected to begin on day 2 (Fri) runoff
routes into the stream network. Corresponding AEPs from the High Flow
Magnitude forecast shows that most rises will be modest, although some
locally significant rises cannot be ruled out in areas of heavier rainfall.

South Texas...
Periods of tropical rainfall associated with Hurricane Beryl may impact
portions of the Hill Country, Deep South TX, and the Rio Grande Valley on
days 4 - 6 (Sun - Tue). Ahead of this expected rainfall, antecedent
conditions are generally dry, as relative soil moisture (per NASA SPoRT) is
low (less than 30%, 0 - 10 cm) and streamflows are generally below annual
median values for this time of year, which suggests that at least
initially, there is ample infiltration capacity for this rainfall. However,
confidence in the exact location and magnitude of potential flooding
remains uncertain and is highly dependent on the track and intensity of
Beryl.

.Puerto Rico and the USVI...
Urban and small stream flooding is possible across the islands as a
tropical wave brings periods of heavy rainfall through early day 2 (Fri).

.Alaska...
Periods of moderate to heavy rainfall (1 - 2"+) will continue into the
weekend across much of the Brooks Range and Interior, which may enhance
runoff, generate rapid rises and rivers and streams, and potentially lead
to 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.

//Pritchard



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