NewsNovember 29, 2012

Picturing the West Coast 'Atmospheric River' Event

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Andrew Freedman

By Andrew Freedman

A series of powerful Pacific storm systems is rolling into parts of the West Coast, bringing a slew of weather hazards, from heavy rain to obscene amounts of higher elevation snows (how does 100 inches sound?). The storms are tapping into a ribbon of tropical moisture that extends like a branch from the tropics, a phenomenon known as an “atmospheric river.” Atmospheric river events are responsible for bringing beneficial rains to the West Coast, but they can also deliver so much rain in such a short period of time that they cause major flooding.

Latest loop of Integrated Water Vapor, used to detect atmospheric rivers.
Credit: NOAA.

Satellite view of atmospheric water vapor in the Eastern Pacific Ocean, as a plume of moisture (arrow) heads into Central California.
Click to enlarge the image. Credit: NOAA.

Fortunately, the West has largely been dry—too dry, in fact—so the flooding danger is somewhat lower now than it might otherwise have been. Still, with 10-20 inches of rain possible through the weekend in northern and central California, as well as parts of Washington and Oregon, the National Weather Service has issued flood watches and warnings. In flood-prone Sacramento, Calif., about 5 inches of rain is expected through Sunday, with more falling in higher elevations nearby.

Computer model forecast showing an atmospheric river (narrow area of yellow/orange/red shading) moving into Western California on Sunday, Dec. 2.
Click to enlarge the image. Credit: NOAA ESRL.

Areas that saw forest fires this past summer could be at risk for mudslides, too. Here’s the technical explanation for this from the National Weather Service’s Sacramento Office:

“Fresh burn scars are the most susceptible to these mud and debris flows because wildfires clear out most of the vegetation that helps consolidate soils and absorb water. The charred remains of wildfires typically consist of loose hydrophobic soils that resist absorbing water leading to further risk of debris flow. During periods of heavy rainfall, the water and these loose soils combine to form a viscous, muddy debris mass. Gravity pulls the viscous debris mass downslope which can crush and overwhelm anything caught in front of it.”

7-day rainfall forecast issued on Thursday morning.
Credit: NOAA/HPC.

The parade of storms should come to an end, at least for the time being, early next week. Perhaps then the rest of the country can start seeing some precipitation, since the West will be receiving almost all of it during the next seven days.

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