Atmospheric flooding intensifies as California braces for storm impacts

Overnight, a low pressure system off the northwest Pacific rapidly intensified, bottoming out with a central pressure near 940 mb, approaching the record for lowest central pressure ever recorded in the northeast Pacific.

The intensifying system reached into a deep atmospheric plume of river water headed directly for the California coastline, with the most severe impacts expected Wednesday.

Here’s a detailed breakdown of Wednesday’s forecast across the state:

The most extreme impacts will be felt across Mendocino, Humboldt, Trinity and Siskiyou counties, where torrential rain, heavy snow and hurricane-force winds will hit the region from the coast to the interior.

Gusts of 65 to 75 mph are expected along the coast, peaking Wednesday morning. Meanwhile, several rounds of moderate to heavy rain will move through the area throughout the day, dropping 2 to 5 inches of rain in places like Healdsburg, Ukiah, Point Arena and Eureka. Higher rainfall amounts are likely in the high ground surrounding Highway 101 and extending to the Oregon border, with 6 to 8 inches forecast from the Mendocino National Forest through the Trinity and Klamath national forests. Rivers and streams in these areas will begin to rise to flood stage late Wednesday.

In the interior Cascade Range near Shasta Lake and Mount Shasta, precipitation will take the form of snow, with totals of several feet. Cold air trapped in the Shasta Valley will lead to snowfall accumulations of 1 to 3 feet on Wednesday alone. Places like Mount Shasta City will see 15-20 inches, with local snowfall rates up to 3 inches/hour at times on Wednesday. In the Siskiyou Mountains above 3,500 feet, 20 to 36 inches of snow will fall, leading to blizzard conditions at times. Travel along the stretch of I-5 from Mount Shasta to the Oregon border will be significantly affected, with some road closures possible at times.

The deep plume of moisture will bring the heaviest precipitation to the Central Valley, foothills and Sierra during the first half of Wednesday before moving further north later in the day.

Flood watches are in effect across the Central Valley, with 2 to 4 inches of rain expected Wednesday in areas such as Clearlake, Red Bluff and Redding. Along the Central Valley foothills, 3 to 5 inches of rain will fall around Chico and Paradise, increasing the risk of debris flow flooding near the massive Park Fire burn scar.

At elevations above 3,500 feet, 5 to 10 inches of precipitation is likely, mostly Wednesday morning. As warm air moves in, snow levels will rise to 6,000 feet by the end of the day, with 10 to 20 inches of snow forecast. In the Tahoe area, light snow may fall early Wednesday morning before quickly changing to rain.

Winds will gust from 30 to 45 mph in the Central Valley, with stronger gusts up to 55 mph in the mountains.

It will be a mix of feast or famine rainfall all around the Bay Area on Wednesday. The rain will begin in the North Bay during the early morning hours and spread to the rest of the region by late morning. Pockets of moderate to heavy rain will move through the North Bay and eventually reach San Francisco and the East Bay in the late morning or early afternoon.

Rainfall totals on Wednesday will range from 2 to 4 inches in areas such as Petaluma and Santa Rosa, with 4 to 6 inches in the high ground of Sonoma, Napa and Marin counties. A flash flood watch is in effect from Wednesday morning through Saturday for urban areas, small creeks and streams in the North Bay.

Rainfall in San Francisco, Oakland and the interior East Bay will be between 0.5 and 1.5 inches, with slightly higher totals in the East Bay hills. In the South Bay, including San Jose and Santa Cruz, rainfall will generally range from 0.25 to 0.5 inches, except in the Santa Cruz Mountains where 1 to 2 inches is expected.

Winds will be strong over the North Bay, San Francisco and along the Pacific coast, with gusts of 45 to 50 mph at times. A wind advisory is in effect until early Thursday for the North Bay, San Francisco Bayshore and Pacific Coast with gusts up to 50 mph.

Thursday is likely to be mostly dry around the Bay Area as the atmospheric river moves further north, targeting Northern California and giving the rest of the region a brief respite.

A strong cold front will sweep across the state early Friday morning, delivering a quick 0.5 to 1 inch of rain. The atmospheric river is expected to recharge over the weekend, bringing more rain to the Bay Area and additional snow to the Sierra.

Reach Greg Porter: [email protected]