236
AGUS74 KWCO 071528
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST FRI FEB 7 2025

.Synopsis...
New and renewed river rises, along with small stream flooding impacts, are
possible across the Ohio River Valley and from East Texas into the
Southeast... River flooding continues through the weekend in portions of
Northern and Central California...

.Discussion...

.Ohio River Valleys...
Across portions of the eastern OHRV, small stream and minor river flooding
are ongoing and forecast to crest over the weekend. Locally heavy rainfall
of 1 - 3" fell in eastern KY into much of central WV over the past 24hrs
(MRMS). Much of the region is experiencing wet top soils (0 - 10cm RSM,
NASA SPoRT) and much above normal to high streamflows (USGS) which has led
to efficient runoff, and vulnerable hydrologic conditions throughout many
of the basins. Mixed precipitation and freezing temperatures are forecast
over the weekend and while not hydrologically significant regarding
impacts, this will keep ground conditions primed for new and renewed river
responses as we head into next week. An additional 2 - 4" of forecast
rainfall is possible through next week as yet another low-pressure system
moves across the area. The NWM MRF is continuing to highlight rapid-onset
flooding responses across the area with 25 - 50% probability. Ensemble
river forecasts (MMEFS) show widespread minor flooding will be possible
across the Ohio Valley, with some isolated moderate river flooding possible
across portions of eastern KY and WV starting day 6 (Wed). In particular,
the Kentucky River and Cumberland river basins are showing the strongest
responses of isolated major river flooding. Additionally, long duration
crests are forecast for the Lower Ohio River as flows exit the tributaries
and enter the mainstem. The combination of wet soils, highflow conditions,
and additional forecast rainfall suggests that this area may see more
flooding impacts by next week. While confidence is not high for the exact
location or magnitude of responses, it is increasing as model guidance has
been starting to show consistency in a well-developed wet pattern setting
up.

.East Texas into the Southeast...
Broad river crests continue down the mainstem rivers in portions of East TX
and AR, from previous rainfall events. Fortunately, many of the basins
across the regions have been able to recover from highwater flows and top
soils have had some time to dry out somewhat as it braces for another round
of moderate to heavy rainfall beginning early next week. Rainfall totals of
2 - 4", with locally higher amounts, is possible starting day 4 (Mon)
across portions of East TX through the Lower Mississippi and Tennessee
Valleys and the Southeast. With such a wide extent of forecast rainfall,
the hydrologic responses are likely going to be highly dependent on initial
conditions of streamflows, terrain enhancement, and training. Top soils are
still wet (>50% RSM, NASA SPoRT), particularly where the extent of the
heaviest rainfall is forecast. Streamflows are generally near normal in the
Mississippi Delta into the Tennessee Valley. However, as you get into the
southern extent of the Appalachian Mountains it is much more dry which may
help to mitigate the initial flooding responses. As this forecast rainfall
is expected to take place over the course of several days any beneficial
ground conditions will deteriorate over time leading to a vulnerable
hydrologic state. Ensemble guidance (HEFS, MMEFS) is suggesting that new
and renewed river rises of minor to isolated moderate flooding are
possible. The NWM MRF is also signaling widespread highwater flows and
associated rapid-onset flooding responses. Additionally, the GFS-forced
Maximum High Flow Magnitude is suggesting annual exceedance probabilities
of 20 - 4%, particularly in northern AL through eastern TN which would
suggest a reasonable worst case scenario for these stream responses. As
mentioned in a previous section, confidence is increasing in the overall
placement and magnitude of flooding impacts. However, due to the extended
nature of the forecast some uncertainties remain and will need to be
monitored over the weekend.

.Northern and Central California...
Isolated minor to moderate river flooding is ongoing this morning across
the region and is forecast to persist through the weekend. Widespread 2 -
4" of rainfall has fallen over the past 72 hours (MRMS) with pockets of 8"+
in the Upper and Lower Sacramento river basins. The National Water Model
(NWM) Medium Range Forecast (MRF) is signaling ongoing highwater flows with
peak flows over the next 48 hours. Dry weather will continue through day 4
(Mon), before another low pressure system impacts the region yet again on
day 5 (Tue).

//JAC

$$