ATMOSPHERIC RIVER. TIMING. AMOUNTS. WINDS. WHAT YOU NEED TO KNOW.

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Beginning at about 10PM Saturday, the first moisture plume from the strong October atmospheric river will reach the Mendocino and Sonoma coasts. Although initial landfall will likely be the Mendocino coast, the AR will point at the Mendocino and Sonoma County coasts for hours overnight Sunday before drifting south about midday. This is when the highest dangers of flooding, mud and debris flows and downed trees and power lines will be.

THE ATMOSPHERIC RIVER

By midwinter standards, this is a strong event. By October standards it is near record-setting. Scripps Institute at the UC San Diego rates it a 5 out of 5, the strongest category. This is due to the amount of water it will transport to our coast. (Image 1)

At its peak, the Integrated Vapor Transport will reach CAT 5 status along the Mendocino and Sonoma coasts down to the Bay Area early to midday Sunday. (Images 2 and 3)

HAZARDS AND WARNINGS

The National Weather Service has issued a Flash Flood Watch and Wind Advisory for the North Bay. (Image 4) The greatest threat in the North Bay will be in the early morning hours (approx 2AM) to about noon on Sunday, but the Watch continues to 11PM Sunday. A Low Level Jet (often associated with strong storms) will bring winds of 40-50 MPH to areas above 2K feet, but strong winds of up to 35-40 MPH could be felt in the valley areas, especially as the AR is over the region for hours early Sunday morning. The greatest hazards for flooding runoff and debris flows will be over recent burn scars, and the NWS has underscored this danger in their Watch.

RAINFALL AMOUNTS

This storm will drop large amounts of rain. The high resolution models show it arriving at the coast after 10PM Saturday (Image 5), and intensifying by 3AM (Image 6) and hovering over the North Bay with a “hose” of moisture through the mid-morning. (Image 7)

Most models agree the hose will be drift south over the Bay Area beginning around noon Sunday. But, showers will continue in the North Bay and the Flash Flood Watch continues until 11PM Sunday. By Monday, skies will mostly clear, with one last small storm possible on Tuesday.

Rainfall amounts by Monday will range from 5-10 inches across the North Bay hills and valleys, however as we know due to micro climates and rain shadowing this can vary widely. But here are a few estimates from the high resolution models. (Image 8, 9 and 10)

DEBRIS FLOWS, FLOODING NEAR BURN SCARS

Although there will likely be plenty of urban street and small stream flooding, this will be blunted somewhat by the dryness of the landscape which will initially soak up a lot of moisture (even through we’d had some recent rains). The greatest threat is obvious…the 2020 burn scars which are still largely denuded of vegetation. Special hazards will exist for mud, debris and possibly even ash flows in the Glass, Walbridge, Meyers and Napa County fire areas.

Sonoma County has created maps to help residents determine where the areas of greatest threats are in burn areas. For the Glass Fire that information is here Glass Fire Hazard Map