573
AGUS74 KWCO 221529
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST SAT FEB 22 2025

.Synopsis...
Rain and snowmelt induced flooding possible in the Intermountain West,
Northern Rockies, and Northern Plains... Heavy rainfall with potential for
flooding impacts in the Pacific Northwest... River flooding continues
across portions of the Lower Mississippi into the Ohio Valley and
Mid-Atlantic... Flash flooding and mudslides in American Samoa through
today...

.Discussion...

.Intermountain West, Northern Rockies, and Northern Plains...
Flooding from snowmelt (around 1" of SWE) and/or moderate lowland rainfall
is possible beginning today and continuing through day 3 (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, along with clogged drains and culverts.
There is also some potential for out-of-bank rises and ice-jam flooding on
small streams and rivers. 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.

.Pacific Northwest...
An atmospheric river is expected to affect western portions of WA and OR
through day 3 (Mon), bringing the potential for urban and river flooding,
especially in western WA. Rainfall amounts of 4 - 5" are forecast (WPC),
with amounts up to 8" possible in favored upslope areas on the Olympic
Peninsula and in the Cascades as snow levels are expected to be above 7000
ft. With the high snow levels, low and mid-elevation snowmelt is also
expected to be a contributor to runoff (around 1" of SWE). Urban flooding
is possible each day through day 3 (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
ruled out.

Rises on area rivers are expected across the region, and river flooding is
also forecast as early as day 2 (Sun) and continuing into next week. The
Skokomish River near Potlatch (WA) and the Snoqualmie River near Carnation
(WA) are forecast to rise above moderate flood stage on day 2 (Sun), and
other rivers in western WA are forecast to rise above minor flood stage.
River ensemble forecasts (HEFS) indicate further potential for minor river
flooding at a 30% chance of exceedance in the Willamette (OR) and North
Oregon Coastal (OR) basins.

.Lower Mississippi Valley into the Ohio Valley and Mid-Atlantic...
Widespread minor to moderate and isolated major river flooding continues or
is forecast on mainstem rivers across much of these regions this morning.
With no significant rainfall in the forecast through day 7 (Fri), flows
will continue to route downstream and recede relatively unimpeded. The
Green River at Calhoun Lock and Dam (KY) will crest just above major flood
stage today and is forecast to fall below major flood stage on day 2 (Sun)
and subsequently begin a slow recession that is expected to continue into
next week. Moderate river flooding continues elsewhere on the Green and
lower Ohio rivers.

.American Samoa...
Heavy rainfall is expected to cause flash flooding through today, as soils
are already saturated from previous rainfall this week. Additionally,
landslides, mudslides, and/or rockslides are also possible along areas of
steep terrain.

//Watson

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