SAN FRANCISCO — The word “historic” seems to precede weather stories all too often this year and the storm front brewing in the Pacific this weekend is no different.
The remnants of Typhoon Merbok will bring potentially historical coastal flooding, high surf, coastal erosion, high winds, high seas and heavy rain to western Alaska and the Bering Sea through the weekend.
A low pressure system spun out of that storm was making its way down the coast to Northern California by late Saturday night and bring a bout of rare September rain.
“The current view of midlevel water vapor imagery shows the low pressure system that will be responsible for our upcoming rain in the Bay Area is currently exiting the Gulf of Alaska and approaching the Pacific Northwest coastline,” the weather service said very early Saturday. This low will continue to move southward along the West Coast today and park itself west of Cape Mendocino by the evening.”
Any significant rainfall will likely threaten records for the month. 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 showers will offer some relief to the parched region, the gusty winds ushering it in are not for the tinder dry hills. Wildfire concerns are elevated.
“As a response to this incoming area of deepening low pressure, air is forced to flow in from the south, so southerly winds will increase across the Bay Area ahead of its arrival,” the weather service said. “Winds will especially be windy at higher elevations with gusts up to 35 mph possible.”
Forecasters predict the North Bay will be the hardest hit area in the Bay Area, but the brunt of the weather front will be felt along the Big Sur coast.
“Rain will begin late Saturday night, first reaching the North Bay then spreading to the rest of the Bay Area by Sunday morning,” the weather service said. “Sunday into Monday is still expected to be the rainiest timeframe across the area.”
“In fact, from the Sunday morning to Monday morning timeframe, the Weather Prediction Center currently has our region from the Santa Lucias/Big Sur coast northward to Cape Mendocino under a ‘Marginal Risk’ for excessive rainfall due to the potential for hourly precipitation rates to reach 0.5 inches.”