687
AGUS74 KWCO 081532
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
932 AM CST FRI NOV 8 2024

.Synopsis...
Flash flood potential this weekend across the Lower Mississippi and
Tennessee Valleys...Rain and flooding continues across the Southern
Plains...River flooding across the Southeast and Ozark Plateau
(MO/AR)...Periods of rainfall, with isolated river flooding possible across
the Northwest...Daily flood potential continues for Puerto Rico and the
USVI...Flooding possible late this weekend across the Mariana Islands...

.Discussion...

.Lower Mississippi and Tennessee Valleys...
Tropical moisture from Rafael in the central Gulf, coupled with an
approaching frontal boundary will likely generate periods of heavy rainfall
across the region on days 2 - 3 (Sat - Sun) and bring with it the potential
for flash, urban, and small stream flooding impacts. On a regional scale,
antecedent conditions are variable, as much of LA, the MS Delta, and
western TN have wetter soils in the wake of recent rainfall. There is some
uncertainty in the magnitude and placement of potential impacts as this is
heavily dependent on how close the center of Hurricane Rafael moves to the
coast. Nevertheless, rainfall with locally intense rates are becoming
increasingly likely, and flooding impacts, especially in low-lying and
urban areas are possible, some of which may be locally significant.

Guidance from both the GFS and NBM-forced National Water Model (NWM)
indicate isolated to scattered small stream responses across north-central
LA and across portions of western/middle TN on day 2 (Sat). Corresponding
annual exceedance probabilities from both the GFS and NBM aren`t
particularly impressive, as most AEPs are at or greater than 50%. River
ensemble guidance from HEFS indicates some for river flooding, as the 10%
exceedance suggests areas of isolated minor flooding across LA and western
TN.

.Southern Plains...
The threat for urban and small stream flooding, as well as river flooding,
will continue through tonight across north-central and west TX, western OK,
and southwestern KS as an area of low pressure continues to generate
persistent moderate to locally heavy rainfall across the region. Heavy
rainfall over the past 12 - 18 hours has resulted in several instances of
flooded roadways and quick rises to minor and moderate flood along small
streams between Abilene, TX and San Angelo, TX, and additional rainfall
through early afternoon may generate new/renewed flooding impacts. Further
north across portions of the Red River Valley, western OK, and southwestern
KS, rainfall overnight has saturated soils (60 - 95% RSM, 0 - 10 cm NASA
SPoRT) and given the potential for more robust rainfall rates, infiltration
capacity across this region will likely be at least locally overwhelmed,
leading to rapid runoff into nearby streams, creeks, and urban drainages,
and generate localized flooding impacts. Rapid stream and river rises are
expected, as a scattered action stage to minor river flooding is forecast
across the region over the next 24 - 36 hours. The HRRR-forced NWM is
indicating scattered small stream responses across the entire region, with
the greatest probability of hydrologic responses from San Angelo to the
KS/OK border. Corresponding AEPs, per the High Flow Magnitude Forecast, are
generally near around 20%, which suggests an increased confidence for small
stream flooding, although widespread significant impacts are not expected.
Flooding impacts in urban areas, low water crossings, and small streams
will linger into this afternoon.

The threat of flooding will gradually shift eastward into portions of
central OK/northern TX (including Dallas-Ft. Worth) this afternoon, and
some flooding impacts are possible here as well as antecedent conditions
remain somewhat wet from recent rainfall. Impacts will be dependent on the
intensity and the persistence of the rainfall later today.

.Southeast...
Isolated areas of significant river flooding are expected to continue
across southeast GA and the Lowcountry of SC through the weekend in the
wake of recent heavy rainfall. Record flooding occurred along the North
Fork Edisto River at Orangeburg, SC, where areas of significant flooding
were observed yesterday. This area has crested and will continue to recede
unimpeded through the weekend. Elsewhere, crests are generally expected
within the next 24 hours, and minimal rainfall through the next 7 days
should allow for rivers to recede through next week. Slower draining rivers
near the Southeast Coast will slowly rise over the next week as runoff from
recent flooding/rainfall routes downstream.

Across central FL, minor to moderate flooding will continue for the
foreseeable future along the Withlacoochee and St. Johns Rivers.


.Ozark Plateau (MO/AR)...
Minor to major river flooding continues along the Black (AR) and Meramec
(MO) rivers in the wake of widespread heavy rainfall that occurs earlier
this week. Rainfall is expected to return to the region this weekend, and
while totals are lower than forecast in previous days (up to 0.25"), given
the sensitive antecedent conditions across this region, isolated instances
of renewed/new flooding impacts may occur in areas of heavy rainfall. Both
the GFS and NBM-forced NWM support this idea, as this guidance suggests
only isolated small stream rises on day 2 (Sat).

.Northwest...
Several rounds of moderate to locally heavy rainfall is expected to impact
western WA and OR over the weekend and continue through mid next week,
bringing an isolated risk of flash and urban flooding to the region,
including near burn scars. With relative soil moisture of 50 - 65% (0 - 10
cm RSM, NASA SPoRT), streamflows running near normal, and possible lower
snow levels, all this will help mitigate any widespread flooding impacts,
especially on rivers. However, isolated river flooding is possible,
especially on rivers draining the Olympics and Cascades early next week
(HEFS), and isolated moderate flooding is now possible along the Skokomish
River in northwest WA.

.Puerto Rico and the USVI...
Daily rainfall will continue to bring the risk of flash, urban, and small
stream, and river flooding to the islands through this weekend, especially
across eastern portions of the island that have seen 5- 15" of rain over
the past week (RFC QPE), and have saturated soils and above normal stream
flows (USGS). Mudslides are also possible in areas of steep terrain.

.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 3 - 4 (Sun - Mon).

//JDP

$$