702
AGUS74 KWCO 091523
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST SAT NOV 9 2024

.Synopsis...
Considerable flash and urban flooding impacts expected across
Louisiana...locally considerable flooding impacts possible across Puerto
Rico and the USVI...Periods of rainfall, with isolated river flooding
possible across the Northwest...Flooding possible Sunday and Monday across
the Mariana Islands...River flooding continues across the Southeast and the
Ozark Plateau (MO/AR)...

.Discussion...

.Louisiana...
Considerable flash and urban flooding impacts are likely across
southwestern and central LA this afternoon and tonight as tropical moisture
from Rafael, currently in the central Gulf, interacts with an approaching
frontal boundary and generates heavy to locally intense rainfall. Despite
near normal antecedent conditions ahead of this rainfall, the locally
intense nature of the rainfall will overwhelm current infiltration
capacity, generating rapid runoff into nearby streams and urban drainages.
Areas particularly at risk for considerable flooding impacts are areas from
the TX/LA border to near Alexandria, LA as this region has higher soil
moisture and more urban areas. River ensemble and PQPF guidance continue to
suggest the potential for isolated minor river flooding across this region,
particularly near bayous draining into the Calcasieu Basin in southwestern
LA. Similar to yesterday, the exact placement of impacts still remains
somewhat unclear as it is heavily dependent on how close the center of
Rafael gets to the coast though intense rainfall rates should be expected
and significant flooding impacts are likely in low-lying and urban areas.

Guidance from both the GFS and NBM-forced National Water Model (NWM)
continue to indicate isolated small stream responses across central LA over
the next 24 hours. Corresponding annual exceedance probabilities (AEPs),
per the High Flow Magnitude Forecast, aren`t particularly impressive, with
the majority of the AEPs at or above 50%. However, some AEPs as low as 20%
are noted near Alexandria, LA, which somewhat increases confidence in the
potential for significant stream rises/flooding in this region.

.Puerto Rico and the USVI...
Locally considerable flooding impacts are likely through day 2 (Sun) across
the islands as a tropical wave continues to bring periods of moderate to
heavy rainfall. While soils across the entire region are saturated (65 -
90% RSM, 0 - 100 cm NASA SPoRT), the eastern portions of Puerto Rico, as
well as the USVI, are most vulnerable to additional heavy rainfall, given
the 5 - 15"+ of rainfall received over the last week (RFC QPE). Guidance
from the NWM 48-Hour Forecast is indicating scattered small stream
responses across the northern and eastern portions of Puerto Rico within
the next 24 hours. Corresponding AEPs are generally between 10 - 4%,
suggesting significant stream rises possible. Given these factors,
additional rainfall across this region will likely lead to flash, urban,
small stream, and river flooding, some of which may be locally significant.
Mudslides are also possible along areas of steep terrain.

.Northwest...
A series of atmospheric rivers bringing periods of moderate to locally
heavy rainfall to WA and OR may elicit isolated flooding impacts (including
near burn scars) through mid next week. While current relative soil
moisture values aren`t particularly high (50 - 65%) and anticipated lower
snow levels (3500 - 4000`), this should initially mitigate any widespread
flooding impacts. However, as soils become more saturated, the potential
for flooding will likely increase, especially near recently burned
locations. River rises and isolated river flooding are likely, as the
latest RFC forecasts indicate isolated rises to action stage along rivers
draining the western Cascades and isolated moderate flooding on the
Skokomish River (WA).  Flooding impacts will be largely determined by
hourly rainfall rates.

.Mariana Islands...
A tropical disturbance is expected to move south of the islands this
weekend, bringing heavy rainfall and localized flooding impacts to Guam and
Rota on days 2 - 3 (Sun - Mon).

.Southeast...
Isolated areas of moderate to major river flooding are ongoing across
southeast GA and the Lowcountry of SC in the wake of recent rainfall.
Record flooding along the North Fork Edisto River at Orangeburg, SC and the
Canoochee River near Claxton, GA has crested, and these areas are expected
to slowly recede over the next several days. Elsewhere across the region,
most locations are at or near crest and minimal rainfall over the next week
should allow for unimpeded recessions. Slower draining rivers near the
Southeast Coast will slowly rise next week as runoff from recent
rainfall/flooding routes downstream.

.Ozark Plateau (MO/AR)...
Minor to major river flooding continues along the Black (AR), White (AR)
and Meramec (MO) rivers in the wake of widespread heavy rainfall that
occurred earlier this week. All locations along the Meramec River have
either crested or are near crest and will slowly recede below flood stage
over the next several days. Flooding along the Black and White rivers will
persist well into next week.

//JDP





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