198
AGUS74 KWCO 211602
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST THU NOV 21 2024

.Synopsis...
Considerable flooding impacts possible in the Pacific Northwest and
Northern California...

.Discussion...

.Pacific Northwest and Northern California...
Considerable flash and urban flooding continues to be possible through
tonight in portions of northwestern CA as a prolonged atmospheric river
event continues to bring moderate to heavy rainfall over the region. Small
stream and river flooding is also possible, particularly across Northern CA
and southwestern OR, through the end of the week. Rainfall totals of 10-15"
(locally higher) continue to be forecast across coastal areas of OR and
Northern CA through the end of the week, with higher amounts possible in
the coastal ranges dependent on snow levels. Rainfall over the past 24
hours (amounts of 2-5", locally higher, MRMS) have saturated soils (75-90%,
10-40 cm RSM, NASA SPoRT) as far south as Eureka, CA, and streamflows are
near to above normal for this time of year (USGS), indicating little to no
infiltration capacity for runoff, increasing susceptibility to flooding
impacts. Flooding in low-lying urban areas and in smaller, flashier basins
is still expected. Isolated debris flows near recently burned areas and
landslides/rockslides are also possible in areas of steep terrain.

The latest RFC forecasts indicate isolated minor to moderate river flooding
for portions of Northern CA, as well as additional rises to action stage.
The Eel River at Fernbridge, CA is forecast to reach a moderate stage and
crest on day 2 (Fri).

The National Water Model (NWM) Medium Range Forecast (MRF) rapid onset
flooding (ROF) probabilities are generally between 25-50% in areas of
Northern CA. Corresponding annual exceedance probabilities (AEPs) south of
Eureka, CA, specifically near the Eel River at Fernbridge, CA, are in
agreement with ROF with probabilities as low as 2%, suggesting an increased
confidence in small stream flooding.

//Kirkpatrick



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