601
AGUS74 KWCO 221523
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
915 AM CST FRI NOV 22 2024

.Synopsis...
Flooding impacts continue in the Pacific Northwest and Northern
California... Localized flooding and water rises in Southcentral Alaska...

.Discussion...

.Pacific Northwest and Northern California...
Locally considerable flash and urban flooding, as well as small stream and
river flooding, continue to be possible through today in portions of
Northern CA and southwestern OR as a prolonged atmospheric river event
continues to bring moderate to heavy rainfall over the region. Rainfall
totals of 2 - 4+" (locally higher) continue to be forecast across coastal
areas of southwestern OR and Northern CA, from Eureka to San Francisco as
well as along the Sierra Nevada mountain range, through the end of the
week. Snowmelt across higher elevations of the Coastal Ranges and Sierra
Nevada through day 2 (Sat) could exacerbate flooding concerns and ongoing
flooding. Rainfall over the past 48 hours (amounts of 2 - 8", locally
higher, MRMS) have saturated deeper soil layers (75-95%, 10 - 40 cm RSM,
NASA SPoRT) as far south as San Francisco, CA, and streamflows are much
above normal for this time of year (USGS), indicating no infiltration
capacity and lessened channel 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 major river flooding
for portions of Northern CA, as well as rises to action stage. The Eel
River at Fernbridge, CA (major) and Mark West Creek near Mirabel Heights,
CA (moderate) are forecast to crest today.

The National Water Model (NWM) Short Range Forecast (SRF) rapid onset
flooding (ROF) probabilities are generally between 25 - 50% in areas north
of San Francisco, CA with localized probabilities south of Eureka and along
the Sierra Nevadas, south of Redding, between 50 - 75%. Corresponding
annual exceedance probabilities (AEPs) within this same area, are near 50%
with some probabilities as low as 10 - 2%, suggesting confidence in small
stream flooding.

Southcentral Alaska...
Ice dam formations are causing localized flooding and water rises in
Anchorage, as well as on smaller rivers across the Mat-Su and Kenai
Peninsula, with impacts possible through the weekend.

//Kirkpatrick



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