Wednesday, January 4, 2017

NWS warns of valley flooding from 'atmospheric river'

We're about to get absolutely dumped on. The National Weather Service is cautioning that this weekend's storm could be epic -- bringing 10- to 25-year highs in single-event rainfall -- and cause flooding in the Central Valley as soils are already saturated from previous storms.

From an agency update issued this afternoon:
An atmospheric river will take aim for California this weekend with heavy precipitation, high snow levels and soils that are already saturated from recent storms. Moderate precipitation starts Saturday (7th), but the brunt of storm will be Sunday and Monday. Majority of the precipitation will fall as rain in the mountains as snow levels rise above 8000 feet. This is going to be some of the more significant flooding we've seen in the past decade...areas may flood that haven't seen impacts for several years. Use the break in the weather late Thursday and Friday to prepare. Keep in mind that flooding will take several days to recede in some areas after the rain stops.

Widespread flooding of urban areas, roadways, small streams, and main stem rivers
Enhanced downstream river flooding from flood control releases on numerous Sierra reservoirs
Mud and debris flows on burn scars from past 4 years
Rock and mudslides on mountain roadways
Downed trees and power outages from recent storms and saturated soils
Mountain travel delays and chain controls Saturday and Monday

Forecast Confidence

Timing and Strength
Precipitation begins Saturday morning and continues through Monday
heaviest Sunday and Monday
Rivers and Streams
rising water levels Sunday - next week
some points reach flood stage by Sunday
streams and creeks that are free-flowing (no dams) will bear close watching; we could see impacts in those areas unseen for several years.
Snow levels
begin ~3000 feet northern mountains and 4000 feet in the Sierra Saturday
rise above 8000 feet Saturday night
drop back below pass levels early Monday
Valley gusts 30-40 mph
Mountain ridgetop gusts 45 to 60 mph

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