741
AGUS74 KWCO 161513
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SUN JUN 16 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 the next week (7-day totals of 5 - 7"), and will likely generate
flash, urban, small stream, and river flooding, some of which may be
locally considerable. 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 24 hours (widespread 1 - 3",
locally higher, MRMS) have wetted soils (35 - 90% RSM, 0 - 10 cm NASA
SPoRT) and brought area streams to much above average annual median flows
(USGS). Each additional round of rainfall will serve to further compound
the flood potential, and later in the week is when locally considerable
impacts are more likely. Day to day flooding impacts will be heavily
dependent on the intensity of rainfall rates and if storms train and/or
persist over the same area for prolonged periods of time. 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 moderate river flooding towards the end of next
week after subsequent rounds of heavy rainfall. It is worth noting that the
HEFS 10% Chance of River Flow Exceedance and PQPF guidance has widespread
moderate and major river flooding throughout central and southern MN by
late this week, and should the rainfall overperform compared to the current
forecasts, this scenario could materialize.

For today, the HRRR-forced National Water Model (NWM) signals indicate the
chance of flood impacts is minimal, and recent runs have been inconsistent
with the placement of impacts. In the medium range, both the GFS and
NBM-forced NWM remain in good agreement on the placement and magnitude of
responses by early to mid next week, as both models are indicating
widespread small stream responses across much of MN and surrounding states.
Peak flows are not expected until day 6 (Fri) and beyond. Corresponding
AEPs are showing widespread significant stream rises (AEPs of 10% and
below) across the region, adding another layer of confidence in the
potential for locally considerable flooding impacts and the potential for
river flooding across this region.

.Western and Central Gulf Coast...
Heavy rainfall is expected to begin along the central Gulf coast today then
move into southern LA and TX on days 2 - 5 (Mon - Thu). QPF totals for this
event are in the 7 - 10"+ range along coastal LA and TX, and gradually
decrease further inland. Significant flash, urban, and small stream
flooding impacts are possible along with new and renewed river flooding.

In LA, antecedent conditions are not conducive to rapid formation of runoff
conditions with soils being on the drier side (35 - 50% RSM, 0 - 10 cm,
NASA SPoRT) and streamflows generally near or below normal annual mean flow
(USGS). The rainfall expected today and tomorrow will most likely just
serve to prime conditions for the heavier rainfall on day 3 (Tue), when
flooding impacts are more likely to be seen. River ensemble guidance (HEFS,
PQPF) indicate river rises to minor flood near the coast are possible.
Given the drier antecedent conditions and the relatively slow-response of
area rivers, the highest threat for this event will likely be urban
flooding in areas of poor drainage. The NBM-forced version of the NWM is
signally widespread stream responses across southern LA, with most stream
reaches showing exceedance of the highwater threshold, but do not exceed
the 50% AEP threshold, supporting the idea of a lesser small stream
flooding threat. Signals are stronger (50% AEP exceedance) close to the
coastline, but are not low enough or widespread enough to give confidence
to significant small stream flooding occurring.

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, heavy rainfall is likely to lead to at least locally
considerable flash, urban, and small stream flooding impacts. Ensemble
guidance (HEFS 30%) indicates 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 bone-dry
and streamflows are running at normal to much below normal flows, allowing
ample infiltration and in-channel storage capacity for the initial rounds
of expected rainfall. Considering the generous buffer these dry conditions
give southern TX, flooding impacts are expected to be less significant than
further north and east, but with multiple rounds of heavy rainfall
expected, flooding is still a very real possibility. Confidence in the
occurrence of riverine flooding is low, as HEFS 30% exceedance does not
have any river gages south of Houston reaching flood status. The NBM-forced
NWM does show isolated reaches between Houston and Victoria, as well as
tributaries along the lower Rio Grande, exceeding at least the 50% AEP
threshold, but southern TX has the same ARI as eastern TX and these signals
are likely underpredicting. Flash and urban flooding, which are not as
dependent on ripe antecedent conditions, are still likely anywhere repeated
rounds of heavy rainfall occurs.

//Bliss



$$