568
AGUS74 KWCO 231509
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST SUN FEB 23 2025

.Synopsis...
Heavy rainfall with potential for flooding impacts in the Pacific
Northwest... Rain and snowmelt induced flooding possible in the
Intermountain West, Northern Rockies, and Northern Plains... River flooding
continues in the Lower Ohio Valley...

.Discussion...

.Pacific Northwest...
An atmospheric river will continue to affect western portions of WA and OR
through day 2 (Mon), bringing the potential for urban, small stream, and
river flooding, especially in western WA. Over the past 24 hours, a
widespread 1 - 2" of rainfall/liquid equivalent precipitation fell across
these areas, with local amounts up to 3" in the higher elevations.
Additional amounts of 1 - 3" are forecast through day 2 (Mon), and locally
higher amounts possible in mid to high elevations with snow levels expected
to be above 7000 ft. The high snow levels are also expected to cause low
and mid-elevation snowmelt, which will contribute to runoff (around 1" of
SWE with higher amounts possible). Urban flooding is possible each day
through day 2 (Mon), and landslides are also possible in areas of steep
terrain. The chances for small stream flooding will become more likely as
time progresses and soils saturate, and localized flash flooding causing
debris flows near recently burned areas cannot be completely ruled out.

Rises on area rivers are expected across the region, and river flooding is
also forecast as early as today and continuing early this week. The
Skokomish River near Potlatch (WA) is forecast to rise above moderate flood
stage today and remain above flood stage through at least day 3 (Tue).
Other rivers in the region, such as the Snoqualmie (WA), Grays (WA), and
Clackamas (OR), are forecast to rise above minor flood stage.

.Intermountain West, Northern Rockies, and Northern Plains...
Flooding from snowmelt (around 1" of SWE) and/or moderate lowland rainfall
is possible, primarily through day 2 (Mon), across eastern WA, eastern OR,
much of ID and MT, and north central WY. Potential impacts include areal
ponding and overland flow, especially as the ground remains frozen in those
areas, and urban flooding impacts such as seepage into basements cannot be
ruled out. There is also potential for out-of-bank rises and ice-jam
flooding on small streams and rivers as the warm conditions promote the
movement of any ice present. The GFS- and NBM-forced NWM Medium Range
Forecast (MRF) High Flow Magnitude Forecasts indicate the potential for
flows exceeding 10% annual exceedance probability in eastern portions of WA
and OR and western ID, supporting the potential for small stream flooding
in those areas. Rock and mudslides are also possible in areas of steep
terrain.

.Lower Ohio Valley...
Widespread minor to isolated moderate major river flooding continues on the
Green River (KY) and the lower Ohio River this morning. With no significant
rainfall in the forecast through day 7 (Sat), flows will continue to route
downstream and recede relatively unimpeded. The Green River at Calhoun Lock
and Dam (KY) is forecast to fall below major flood stage on day 2 (Mon) and
subsequently begin a slow recession that is expected to continue through
this week.

//Watson

$$