697
AGUS74 KWCO 171527
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT MON JUN 17 2024

.Synopsis...
Flood potential increasing across portions of the Central Plains and the
Upper Midwest...Heavy rainfall and associated flooding impacts likely
across the Western and Central Gulf Coast...

.Discussion...

.Central Plains and the Upper Midwest...
Repeated rounds of heavy rainfall are likely across these regions
throughout this week (7-day totals of 5 - 7"), and will likely generate
flash, urban, small stream, and river flooding. This region has already
received well above average rainfall over the past month (2 - 6"+ positive
departures from normal precipitation) and rainfall over the last 48 hours
(widespread 1 - 3", locally higher, MRMS) have wetted soils (60 - 90% RSM,
0 - 10 cm NASA SPoRT) and brought area streams to above average annual
median flows (USGS). Each additional round of rainfall will serve to
further compound the flood potential. Day-to-day flooding impacts will
heavily depend on the intensity of rainfall and whether storms train and/or
persist over the same area for prolonged periods, potentially causing
locally considerable flood impacts. River flooding is increasingly becoming
a concern as well, especially across MN and western WI, where stream flows
are the most elevated and where river flooding is ongoing. Current river
ensemble guidance (HEFS) suggests the potential of new and renewed minor to
major river flooding towards the end of this week after subsequent rounds
of heavy rainfall.

For today, the short range HRRR-forced National Water Model (NWM) indicates
a chance of rapid-onset flooding (ROF). The past few runs have been
consistent, showing high ROF probabilities (50 - 75%), mostly situated in
southeastern MN. Corresponding annual exceedance probabilities (AEPs), per
the High Flow Magnitude forecast, have continually indicated significant
stream rises (AEPs as low as 2%) southwest of Minneapolis, increasing
confidence that this area may experience hydrologic impacts. In the medium
range, the GFS and NBM are indicating the potential for ROF, with the GFS
showing more widespread signals. Furthermore, both models are indicating
widespread small stream responses across much of the region with lower AEPs
concentrated in eastern MN and into northwestern WI. Peak flows are not
expected until day 5 (Fri) for smaller streams, and day 8 (Mon) and beyond
for larger mainstem streams.

.Texas and Coastal Louisiana...
Tropical moisture is bringing heavy rainfall and the threat of considerable
flash, urban, small stream, and riverine flooding impacts to TX and coastal
LA. Rainfall is ongoing along the LA coast, and is expected to shift into
TX on days 2 - 4 (Tue - Thu). Widespread QPF totals for this event are in
the 7 - 10" range along coastal TX, and decrease further inland, but even
as far inland as the Austin/San Antonio area is still forecast to receive
up to 5". Considerable flash, urban, and small stream flooding impacts are
expected in response to this rainfall, along with new and renewed river
flooding. Isolated rainfall amounts of 15 - 20 inches may be possible in
southern TX by the end of this event.

In LA, antecedent conditions are becoming more conducive to efficient
runoff due to the ongoing rainfall. NASA SPoRT is beginning to respond to
the rainfall that fell this morning with current topsoil RSM in the 35 -
65% range. Streamflow responses are lagging, as would be expected, and are
still near or below normal annual mean flow (USGS). Recent forecast trends
have shifted most of the day 2 (Tue) and beyond QPF out of LA into TX, thus
the hydrologic threat is greatly reduced. Nonetheless, heavy rainfall today
could still lead to locally significant flash and urban flooding impacts
anywhere heavy rainfall persists.

In TX, antecedent conditions vary, with east TX being on the wetter side
after an abnormally wet April and May (5 - 8"+ positive departure from
normal precipitation). The area has been spared significant rainfall over
the last week, allowing soils to dry to near-normal conditions (40 - 50%
RSM, 0 - 10 cm, NASA SPoRT), but streamflows remain well above average
climatological norms (USGS). Reservoirs across east TX are at or near
capacity, and river flooding is ongoing in the lower reaches of the
Trinity, Sabine, and Neches basins. With the current hydrologic sensitivity
of eastern TX, persistent heavy rainfall is likely to lead to at least
locally considerable flash, urban, and small stream flooding impacts.
Thankfully, recent forecasts have shifted the highest QPF totals to the
southwest, and current 7 day totals for the area are at worst 3 - 4" (WPC).
Ensemble guidance (HEFS 30%) continues to indicate the potential for minor
to moderate flooding on rivers north and east of Houston. The NBM-forced
version of the NWM is showing muted small stream responses despite the high
QPF totals, but is not surprising due to the high annual recurrence
interval (ARI) set for TX and small stream flooding impacts are still
likely.

On the other hand, south and west of Houston, soil conditions are dry and
streamflows are running at normal to much below normal flows, allowing some
infiltration and in-channel storage capacity for the initial rounds of
expected rainfall. This will help buffer any flooding, but with high
forecasted rainfall totals and likely high rainfall rates, considerable
flooding impacts are likely. Texas Hill Country will be an area to watch,
as basins are flashier due to the high relief and the area is currently
forecast to get 3 - 5"+ over the duration of the event (Isolated rainfall
amounts of 15 - 20 inches may be possible in southern TX by the end of this
event.). As for river flooding, ensemble guidance has been slow to pick up
on the southwest shift in QPF, with only the HEFS 10% exceedance showing
any river flooding in south TX. Regardless, rises on rivers in south TX to
at least minor flood status are likely. The NBM-forced version of the NWM
is showing scant signals considering the amount of rainfall forecast, which
is due to the high annual recurrence interval (ARI) value from the NWM in
TX. The GFS-forced version of the NWM is much more aggressive, with
widespread AEPs less than 20% from Victoria, through San Antonio and into
Texas Hill Country. These signals are likely overdone in their wide areal
coverage, but isolated occurrences of these extreme signals is possible
should the rainfall exceed current forecasts. These signals support the
possibility of locally considerable flooding impacts. Flash and urban
flooding, which are not as dependent on ripe antecedent conditions, are
still likely anywhere repeated rounds of heavy rainfall occurs.

//Bliss/Wood



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