951
AGUS74 KWCO 041605
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST TUE FEB 4 2025

.Synopsis...
Atmospheric river will continue impacting California today... Potential for
isolated flooding across the Ohio River Valley...

.Discussion...

.California...
The atmospheric river will continue across northern California throughout
today, bringing moderate, and at times locally heavy rainfall that may lead
to flooding impacts (1 - 4", locally higher, WPC). Recent rainfall of 3 -
8" (72-hr QPE, MRMS) has moistened topsoils to near saturation (65 - 85%
RSM, 0 - 10 cm, NASA SPoRT), allowing additional rainfall to efficiently
convert to runoff. Streamflows are already running high, with many gages
reporting flows above the 95th percentile for annual flow (USGS),
suggesting streams may be nearing in-channel storage capacity. Snow levels
are currently above 6,000 ft in the Sierra Nevadas, but are expected to
lower throughout the day, which will help mitigate the duration and
magnitude of the downstream flooding. Moderate flooding is ongoing or
forecast in the Russian, Upper Pit, and Lower Eel basins, in addition to
minor flooding along the Sacramento River south of Redding. Most rivers are
expected to crest today or tomorrow. Out-of-bank rises on small streams and
some rivers is possible, and even flash flooding cannot be ruled out with
the potential for heavy rainfall rates. Shallow landslides and isolated
debris flows near recently burned areas may also be possible.

Signals from the National Water Models (NWM) Short Range Forecast (SRF)
services have been consistently growing more robust, with rapid-onset
flooding (ROF) probabilities from 25-50% between Sacramento and Redding.
The NWM highflow magnitude service is signalling widespread annual
exceedance probabilities of below 50% all across northern California.
Pockets of more robust AEPs (10 - 2%) can be found in the Redding/Chico
area, but those seem overdone given the rainfall amounts. Peak flows can be
expected to arrive on lower order streams generally by the late afternoon
for the coastal ranges and over the course of the evening for the Central
Valley.

.Ohio River Valley...
Rainfall on days 2 - 3 (Wed - Thu) and again this weekend will bring the
potential for isolated flooding impacts to the region with 7-day precip
totals of 1 - 3" (WPC). Some of that precip across the northern portions of
the region will be mixed precip. Recent snowmelt of  1 - 2" in eastern WV
over the past 24 hours (SNODAS) appears to have had minimal impact on
streamflows based on analysis of many of the USGS streamgages near the snow
covered areas. SNODAS suggests that the tap is quickly being shut off with
very little melt if any expected over the next couple days. Soils across
KY, WV and OH are wet throughout the profile (65 - 95% RSM, 10 - 40 cm,
NASA SPoRT) and streamflows are above normal to much above normal (USGS),
indicating a hydrologic system primed for quick responses. Both the NBM-
and GFS-forced NWM Medium Range Forecast (MRF) suggest modest responses
during the first round this week and the second round this weekend with NWM
retrospective annual exceedance probabilities hovering near 50%. The models
appear to diverge in the extended range on days 7 - 8 (Mon - Tue), with the
GFS-forced NWM showing more significant responses. With this divergence,
the uncertainty is greater in the day 7 to 8 timeframe.  Ensemble river
forecasts (MMEFS) show a similar story with generally a low chance (less
than 30%) for isolated minor river flooding. The NAEFS is a bit more
aggressive showing more widespread river responses. Overall, the threat for
flooding may be minimal with the first round and increasing this weekend
with the additional rounds of expected rainfall. With that being said, the
Ohio Valley will need to be closely monitored as we approach the weekend.

//Bliss



$$