Rare September rainstorm taking aim at North Bay; up to 1-3 inches in forecast

SAN RAFAEL -- A rare September wet weather storm front continued its march down the West Coast toward the San Francisco Bay Area Friday, packing with it cooler temperatures, intense downpours, gusty winds and the threat of lightning.

The brunt of the storm was predicted for Sunday, but the wet unstable weather could linger through Tuesday.

"A change in the weather pattern is underway as a deep, closed low is on track to move south from the Gulf of Alaska and park itself off the NorCal coast," the National Weather Service said.

"More excitingly, there continues to be high confidence that this system will bring rain to much of the Bay Area as this system will provide a continuous stream of moisture-rich air through midweek," the weather service continued in its Friday morning forecast discussion. "This beneficial rain is still expected to arrive to the North Bay as early as Saturday evening and will continue trekking south to the rest of the Bay Area and Central Coast by Sunday night into Monday morning."

Forecasters predicted that the North Bay would bear the brunt of the storm that is the remnants of Typhoon Merbok that battered Asia.

"Expected rainfall totals have not changed much, with the majority of rain is expected over the North Bay," the weather service said.  "1 to 3 inches of rain is expected total beginning Saturday evening through Tuesday evening. Coastal mountain ranges such as the Santa Cruz Mountains could see 1 to 2 inches."

Elsewhere, residents can expect to get up to half of an inch. Isolated thundershowers could also pop up across the Bay Area on Monday.

While the storm may bring record rainfall to the Bay Area, it's impact in Alaska could bring "potentially historical" flooding, with some coastal areas seeing water levels up to 11 feet higher than the normal high tide. 

"This is a dangerous storm that will produce widespread coastal flooding south of the Bering strait with water levels above those seen in nearly 50 years," the weather service said. 

Back in the Bay Area, September is usually one of the driest months of the year so the forecast is surprising for the drought parched region.

"September is climatologically the third driest month of the year, thus forecast rain amounts on the order of a few tenths of an inch to 1 inch (locally 1-2 inches hills/mtns) compared to the 30-year Sept normals may easily reach 800% of normal in a lot of areas to near 1,000% in the North Bay," the weather service said.

Unlike the storm that recently triggered mudslides and flooding in Southern California, this weather system has no connection to the humid air of the tropics. It's rolling in from the northern Pacific and the Gulf of Alaska.

"What is highly unusual about this particular pattern is the lack of connection to the eastern Tropical Pacific since if we're going to get rain/showers in September typically there's some moisture connection or origin to the eastern Tropical Pacific since it's only late astronomical summer and it's too early for the larger scale southern shift in westerlies," the weather service said. "But, this system is breaking through. "

This time of year, forecasters added: "Northwest arriving systems typically are a bit moisture starved."  

While the storm will bring some relief from the drought conditions in the Bay Area, it's not exactly being welcomed by the thousands of firefighters battling the massive Mosquito Fire in the Sierra.

The system is forecasted to bring colder temperatures and precipitation — from 1/4 inch to more than 1 inch of rainfall over several days — to the fire burning in Placer and El Dorado counties.

But stronger winds are also expected to arrive in the area beginning Saturday and the winds could throw burning embers and create spot fires despite the rain.

"That's a bit of a mixed blessing here," Fire Behavior Analyst Jonathan Pangburn said Thursday.