583
AGUS74 KWCO 211518
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST FRI FEB 21 2025

.Synopsis...
Rain and snowmelt induced flooding possible in the Intermountain West,
Northern Rockies, and Northern Plains... River flooding continues across
portions of the Lower Mississippi into the Ohio Valley and Mid-Atlantic...
Heavy rainfall with potential for flooding impacts in the Pacific
Northwest... 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 as early as day 2 (Sat) and continuing at least
through day 4 (Mon) across eastern WA, eastern OR, much of ID and MT, and
north central WY. Potential impacts include areal ponding, 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 (HFMF) are
indicating 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 Mississippi Valley into the Ohio Valley and Mid-Atlantic...
Widespread minor to moderate and isolated major river flooding continues or
is forecast across much of these regions this morning, primarily on
mainstem rivers. With no significant rainfall in the forecast through day 5
(Tue), flows will continue to route downstream and recede relatively
unimpeded. The Green River at Calhoun Lock and Dam (KY) is forecast to
crest right at major flood stage late before the end of day 2 (Sat) before
beginning a slow recession that is expected to continue into next week.
Moderate river flooding continues elsewhere on the Green and lower Ohio
rivers.

.Pacific Northwest...
An atmospheric river is expected to affect western portions of WA and OR
through day 4 (Mon), bringing the potential for urban and river flooding
impacts. Rainfall amounts of 3 - 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 6000 ft. With the high
snow levels, snowmelt is also expected to be a contributor to runoff
(around 1" of SWE). The bulk of the rainfall is expected on days 2 - 4 (Sat
- Mon), making these days the most likely possibilities for urban and small
stream flooding to occur. Landslides are also possible in areas of steep
terrain, 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 possible as early as day 3 (Sun) and continuing into next week. The
Skokomish River near Potlatch (WA) is forecast to rise above moderate flood
stage on day 3 (Sun), and the Snoqualmie River near Carnation (WA) is
forecast to rise above minor flood stage on day 4 (Mon). River ensemble
forecasts (HEFS) indicate further potential for river flooding at a 30%
chance of exceedance in the Snoqualmie River basin (WA).

.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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