MAJOR RAINSTORM INBOUND. PREPARE NOW. AR LEVEL 4 OF 5 FOR NORTH BAY COAST.

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ATMOSPHERIC RIVER FORECAST TO STALL OVER NORTH COAST

A major and potentially dangerous 3-4 day rainstorm is inbound to the North Bay and North Coast of California. This storm is now rated 4 out of 5 on the Atmospheric River scale, meaning the strength of the moisture plume and its duration will be exceptional, especially for November.

After days of uncertainty, the forecast models have come into agreement that the center of the “firehose” of moisture arriving along the California coast will be south of Eureka, with Sonoma and Mendocino Counties (and the North Bay) likely at or near the center.

You can see the AR ramping up quickly on Wednesday, with a prolonged period from early Wednesday to Friday in which the “firehose” of moisture is directed at the Sonoma and Mendocino County coasts.

Flash Flood Watches for urban and small stream flooding are up for the entire North Bay Area from early Wednesday through early Saturday. Wind Advisories have also been issued for south to southwest gusts to 45 mph at times through Thursday.

THE MOISTURE PLUME

A rapidly deeping center of low pressure off the Pacific Northwest coast, a so-called “bomb cyclone,” will draw huge amounts of moisture in from the Eastern Pacific. You can see the low pressure center and the developing moisture stream at midday Tuesday.

The center of this moisture stream is forecast to come ashore from Marin County north very late onTuesday night with rainfall ramping up through the day on Wednesday.

This plume of very moist air is forecast to “stall out” and stay pointed at the North Bay and North Coast for 2 or 3 days before pushing south.

THE RAINFALL

The North Bay Area may see up to 5 inches of rain (more in the hills) in the first 24 hours of this storm, from early Wednesday to early Thursday. Amounts will be heaviest from Healdsburg north and in the hills of Sonoma, Mendocino and Napa Counties.

After 48 hours, by early Friday, the North Bay may be topping 8 inches in the wettest spots. The second 24 hour period from early Thursday to early Friday may add 2 inches or more to the overall totals. More will fall in the north, and less to the south.