Bomb Cyclone To Arrive With Fall, Bringing Rain To NorCal

SAN FRANCISCO — Rain will be in the cards early next week in Northern California courtesy of a weakening cold front in the wake of a bomb cyclone setting up to soak the Pacific Northwest and the northernmost regions of California.

The weather phenomenon is associated with the colder seasons and can occur during a quick drop in pressure.

"Bombogenesis, a term used by meteorologists, occurs when a midlatitude (the latitudes between the tropics and polar regions) cyclone rapidly intensifies, or strengthens, over a 24-hour period," the National Ocean Service explains. "Bombogenesis can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. It is popularly referred to as a bomb cyclone."

Early indications suggest the latest could bring up to 3 inches of rainfall to western portions of Washington and Oregon next week, and two-and-a-half inches to northern California. Crescent City is expecting an inch of rain Sunday night with up to one-and-a-half inches falling Monday. Lighter rain is expected in the Bay Area — particularly in the North Bay — as the front weakens.

While there will be a chance for some drizzle in San Francisco and other points south on Monday, the best shot for measurable rain looks confined to the North Bay for now, but forecasters note it's still a little early to know for certain.

"Looking a bit further out towards the latter half of the weekend and into early next week, a low-pressure system pushes into the region from the [Pacific Northwest], bringing rain chances primarily north of the SF Bay region," NWS Bay Area wrote Friday. "Some slight chances of rain exist southward of this zone, over the Monterey Bay area, though at this time, the most favorable chances continue to persist over the North Bay zones."

The earliest chances for rain should arrive by Sunday evening and should end by Tuesday morning. Updated forecasts Friday showed a 43 percent chance for Santa Rosa to receive at least a quarter-inch of rain, with a 39 percent chance for Petaluma.

Should the rain materialize in the North Bay, it will be welcome news. According to the weather service, the Sonoma County Airport has not seen measurable rain since June 12, and Santa Rosa has remained dry over the same period only three other times in the last 25 years.

The pattern change should make quick work of the smoke, too.

After Tuesday, the Bay Area should be in for another sunny run of weather with temperatures holding a touch below normal and feeling a little more like fall, according to the National Weather Service.

While a strong El Niño could lend to a wet and active winter, new seasonal climate outlooks favor warmer-than-usual temperatures this fall in the West, with equal chances for a wetter or drier trend in the Golden State.