487
AGUS74 KWCO 121515
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST WED FEB 12 2025

.Synopsis...
Considerable flooding impacts possible in portions of the Ohio River
Valley... All flooding modes possible this week across East Texas into the
Southeast and Tennessee Valley... New and renewed river rises possible in
portions of the Mid Atlantic... Potential for flooding impacts tomorrow in
portions of California...

.Discussion...

.Ohio River Valley...
New and renewed river flooding, widespread small stream flooding, and
potential for flash flooding, are likely later this week in southern
portions of the valley. Eastern KY is the most concerning area currently,
having received well above normal precipitation over the last week (2 - 5",
7-day departure from normal, NWPS), and with light rainfall and snowmelt up
to 1" expected over the next several days, will remain hydrologically
vulnerable leading into the weekend. Additional significant rainfall is
likely again on day 4 (Sat). With soils still wet (60% 0 - 10cm RSM, NASA
SPoRT), any additional rainfall will quickly convert to runoff and lead to
an increased potential for flooding impacts. Recent model trends have
shifted the higher QPF into western KY/TN where antecedent conditions are
slightly less favorable for runoff/flooding, but nonetheless there remains
a reasonable possibility for considerable flooding impacts.

The primary threat from this event appears to be river flooding, especially
as streamflows remain elevated (USGS) from recent rainfall. Minor river
flooding is ongoing along portions of the mainstem Ohio River and the Green
River (KY), and river ensemble forecasts (MMEFS) indicate the potential for
additional river flooding in the region. At the 30% exceedance probability,
GEFS/NAEFS/HEFS are all indicating potential for moderate to major river
flooding in the Sandy (KY/WV), Upper Cumberland (KY), and Kentucky (KY)
river basins, and minor to moderate river flooding in the Green River basin
(KY) later this week. Additionally, long duration crests are forecast for
the lower Ohio River as flows exit the tributaries and enter the mainstem
river. The wet antecedent conditions will also likely lead to small stream
flooding both the GFS and NBM forced versions of the National Water Model
(NWM) are signaling widespread rapid-onset flooding (ROF) across KY and WV.
The NWM highflow magnitude service is showing widespread exceedance of 50 -
20% AEP thresholds across the region, with a large pocket of robust 10 - 2%
AEP exceedance in eastern KY, supporting the enhanced risk in that area.
Flash and urban flooding also cannot be ruled out, although the lack of
more intense rainfall rates may limit that potential. Overall, considerable
flooding impacts are possible primarily due to the potential for major
river flooding.

.East Texas into the Southeast and Tennessee Valley...
Rainfall is ongoing across the region and will persist into day 2 (Thu),
bringing totals of 2"+ (WPC) on top of the 1 - 3" that has already fallen
over the last 24 hours (MRMS). Topsoils were primed by yesterdays rainfall
(60 - 90% RSM, 0 - 10 cm, NASA SPoRT), and additional rainfall will begin
turning to runoff as soils reach saturation. Streams are running above
normal, especially in northern MS/AL where most gages are reporting flows
in the 95% percentile or above (USGS, annual mean flow). Urban, small
stream and minor riverine flooding are possible primarily from northeast LA
through the northern halves of MS and AL, northern GA, and extreme western
portions of NC and SC in response to todays rainfall. With potential for
heavy convective rainfall, flash flooding cannot be ruled out. This
weekend, another round of significant rainfall (3"+, WPC) is forecast over
the lower Tennessee Valley that brings the potential for considerable
flash, urban, small stream, and riverine flooding impacts.

For today, the HRRR-forced NWM SRF is signaling widespread 25 - 75%
time-lagged ROF probability signals, especially over northern LA, MS, and
AL, and are coupled with pockets of 50 - 10% AEPs on lower order streams,
suggesting that out-bank-rises are possible. In the Southeast, widespread
minor river flooding is forecast, but river ensembles (HEFS 10-day 30%)
suggest isolated rises to moderate flood stage are also possible.

In the lower Tennessee and Ohio Valleys on day 4 (Sat), both the GFS and
NBM-forced versions of the NWM are signaling widespread responses in the
deterministic ROF service, as well as AEPs in the 50 - 10% range,
supporting the potential for considerable flash, urban, and small stream
flooding impacts.

.Mid Atlantic...
River flooding is possible in southern VA and NC later this week and this
weekend due to 1 - 1.5" of liquid equivalent precipitation forecast today
and additional rainfall expected on day 4 (Sat). River ensemble guidance
(MMEFS) indicates the potential for minor to isolated moderate river
flooding in the Chowan (VA), Cape Fear (NC), and Roanoke (NC/VA) river
basins. Small stream and urban flooding are also possible.

.California...
Moderate to heavy rainfall is expected to affect the state later this week,
primarily through day 2 (Thu), bringing the potential for all flooding
modes and locally considerable impacts. Urban and small stream flooding are
most likely, particularly along the coast and in portions of the Central
Valley. Northern CA may be more susceptible to flooding due to wetter soils
than areas further south (NASA SPoRT). Debris flows from flash flooding
near recently burned areas cannot be ruled out, especially along portions
of the Southern California coast near and along the Transverse Ranges
locally considerable impacts will be possible in these areas. Minor river
flooding is also forecast later this week along the Sacramento River and
the Laguna de Santa Rosa.

//Bliss



$$