595
AGUS74 KWCO 151527
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SAT JUN 15 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...Isolated flooding impacts
possible today across South Florida...

.Discussion...

.Central Plains and the Upper Midwest...
Repeated rounds of heavy rainfall are likely across these regions over the
next week (7-day totals of 5 - 10"), and will likely generate urban, small
stream, and river flooding, some of which may be locally significant.
Antecedent conditions are primed for hydrologic responses, as repeated
rounds of rainfall over the past 30 days (2 - 6"+ positive departures from
normal precipitation) have wetted soils (60 - 70% RSM, 0 - 10 cm NASA
SPoRT) and generated streamflow conditions that are well above annual
median flows for this time of year (USGS). 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) and PQPF
guidance suggests a potential of new and renewed minor to potentially
isolated moderate river flooding towards the end of next week after
subsequent rounds of heavy rainfall.

For today, the HRRR-forced National Water Model (NWM) is indicating
isolated small stream responses across southwestern MN later this
afternoon. Associated annual exceedance probabilities, per the High Flow
Magnitude Forecast, is indicating modest stream rises, which suggests that
widespread small stream flooding is expected with this latest round of
rainfall. Both the GFS and NBM-forced NWM continue to come into good
agreement in 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, western WI, and northern IA. Corresponding AEPs are
showing widespread significant stream rises (AEPs of 10% and below) across
the region, adding another layer of confidence of at least locally
significant flooding impacts and the potential for river flooding across
this region.

.Western and Central Gulf Coast...
Confidence continues to increase in the potential for heavy rainfall (3 -
7", locally higher) and subsequent flooding along the LA and MS coasts on
days 2 - 4 (Sun - Tue) and along much of the TX coast into the Rio Grande
Valley on days 4 - 6 (Tue - Thu).

Across southern LA and coastal MS, antecedent conditions ahead of this
rainfall are not generally conducive for hydrologic responses, given
somewhat dry soils (35 - 50% RSM, 0 - 10 cm NASA SPoRT) and streamflows
near or below annual median values for this time for year. So at least
initially, this area should be able handle the rainfall. However, this
rainfall is expected to be persistent over a  long duration, and as
conditions become increasingly saturated, all modes of flooding will be
possible, particularly on day 3 (Mon), when the heaviest and most
widespread rainfall is expected to occur. The latest guidance from both the
GFS and NBM-forced National Water Model is indicating scattered small
stream responses across much of southern LA and portions of southern MS on
days 2 - 4 (Sun - Tue), with the most robust signals across southern LA.
Corresponding annual exceedance probabilities (AEPs), per the High Flow
Magnitude Forecast, generally indicate modest stream rises (AEPs at or
greater than 50%) however, some localized AEPs down to 2% are noted on
smaller reaches in southeast LA and southwestern LA (near Lake Charles),
which suggests some of these rises may be locally significant. In regards
to river flooding, both river ensemble guidance and PQPF guidance indicate
isolated rises to minor flood stage across this region. All impacts will be
heavily driven by storm motions and hourly rainfall rates.

As this moisture and associated rainfall shifts west to portions of
southeast, southern, and coastal TX, flooding concerns are expected to
increase by mid-week, with all modes of flooding possible here as well.
Antecedent conditions are a mixed bag in this region. Southeast TX
(generally the Houston Metro and points north and east) are most at risk
for flooding impacts, as antecedent conditions remain saturated following
multiple rounds of heavy rainfall over the past month and ongoing minor to
moderate river flooding, particularly along the lower reaches of the
Trinity, Sabine, and Neches river basins. Reservoirs in this region are at
or near capacity, and additional rainfall in this region may generate new
river flooding and/or worsen ongoing flooding in the aforementioned basins.
South and west of Houston, along the middle and lower TX coasts and into
the Rio Grande Valley, antecedent conditions are very dry with moderate to
severe drought conditions in place. While initially being able to handle
the rainfall, subsequent rounds of rainfall will likely exhaust
infiltration capacity in soils and lead to rapid stream rises and
subsequent flooding in these areas as well. The latest guidance from the
GFS-forced NWM indicates widespread small stream rises across southeast TX
(primarily near Houston and areas north and east) on days 4 - 5 (Tue -
Wed). The responses from the NWM become more muted further south across the
TX coast and the Rio Grande Valley, likely due to dry antecedent conditions
and the higher annual recurrence interval (ARI) across much of southern and
eastern TX (3.2 ARI). New and renewed river flooding is possible across
southeast TX given already flooded rivers and saturated soils in this
region. River rises and isolated river flooding is also possible elsewhere
along the TX Coast and Rio Grande Valley as soils become increasingly
saturated from this heavy rainfall (HEFS, PQPF). Regardless, these impacts
will be driven by the intensity of rainfall rates.

.South Florida...
Isolated urban and areal flooding impacts remain possible today as
additional showers and storms bring locally heavy rainfall to this already
saturated region. Rainfall totals are expected to be relatively modest
(areal totals of 0.5") unlike earlier in the week, which should limit the
magnitude of the impacts. The likelihood of flooding will be greatest in
urban areas and will be heavily dependent on rates and storm motion. After
today, light amounts of rainfall over the next week should allow for areas
of residual standing water to recede.

//Pritchard



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