Is San Francisco windy in October?

Dry winds from the Diablo Range normally act like a blow dryer this time of year, clearing off summer fog and warming up the Bay Area to comfortable highs in the 70s along the coast.

Instead, our mornings have been dominated by gloomy skies and cold air branching into the coastal valleys and plains that surround San Francisco and San Pablo bays — even leaving a few bouts of drizzles in their wake.

Multiple weather ingredients — both on land and sea — are coming together to pour this bizarre concoction of October weather into the Bay Area, and they’re expected to continue for the next few days.

How atypical is this October fog?

The summer months — especially Fogust — are known for their consistent gray skies along the coast and bays. But once September and October roll around, winds shift away from the misty coast and filter in drier air from land. So, it’s unusual for summer’s fog to spill over into early October.

However, this year, the Bay Area and much of the California coast has been spared from the worst of these dry winds. The Diablo and Santa Ana wind season normally runs from September to November, with this month typically seeing the tail-end of the intense Diablo winds that dry out the region and fan wildfires, like the infamous October 1991 Oakland Hills firestorm.

But, this year, September and October’s Diablo winds have been missing from the equation.

Instead of those dry, fire-enabling winds, moisture from September’s storms has been seeping into the roots of trees and other vegetation, and it’s had some far-reaching effects on this year’s fire season.

Moisture levels on large vegetation that borders San Francisco Bay, where the blue line represents current levels, the gray represents the average for this time of the year and the reds represent minimum levels from 2021.

All the excess rains in September resulted in an early-season spike in moisture levels in vegetation in the Bay Area, surpassing the average for the month by a large margin.

But even after the rains cleared up and this moisture began to evaporate by the end of the month, another spike is visible starting around October 1, according to data from the Northern California Geographic Coordination Center.

The culprit?

The fog that has been creeping over the Bay Area since the start of October.

So much moisture from this fog is seeping into plants in the region that these levels outpaced the average for this time of year at one point last week.

This October fog spell — and the cool weather over the past few weeks — has been nothing short of benign. Fire risk is running low across the Bay Area and much of the Sierra Nevada and Southern California’s mountain ranges. And it’s all thanks to the widespread fog.

So, where is all this fog coming from?

This odd October fog has been steered toward the Bay Area and much of the California coast by round-after-round of low-pressure systems rolling into the West Coast since September.

Each one — from the torrential rains in Northern and Central California last month to October’s drizzle fests — has consistently brought cool, misty air from the heart of the cold California ocean current into the Bay Area.

The latest sea-surface temperature readings from buoys just off the coast of California show temperatures hovering in the 13-15-degree Celsius range (55-59 degrees Fahrenheit) from the California-Oregon border to Point Conception as of October 9, 2022.

Sea-surface temperatures — SSTs — off the coast of Northern and Central California are kept cool by the California ocean current, which channels cold waters from the Gulf of Alaska all the way down to Baja California. Once these cold waters make it to the California coast, they undergo a process known as upwelling, sending more water from the depths of the ocean up to the surface.

These SSTs tend to run in the mid- to upper 50s Fahrenheit, and they’ve been hovering around those numbers since this past Sunday.

The Pacific Ocean heavily regulates the temperature of the surrounding air between Eureka and Point Conception just outside of Santa Barbara, and the Bay Area happens to be right in the middle.

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The difference in temperature between land and sea creates a cool sea breeze that carries fog and low clouds from the Pacific Ocean to the Bay Area from May to August. This is why San Francisco stays so cool and foggy during the summer compared to inland cities like Walnut Creek.

By September, a high-pressure system — that sits over the middle of the Pacific Ocean for most of the summer — inches closer to the coast. This high-pressure system then brings clockwise flow to the state that churns out the dry Diablo and Santa Ana winds.

These dry winds rush into California’s coast and replace the cool, misty air, increasing fire risk through the end of October.

But that transition hasn’t happened so far this year.

The ebb and flow of this fog has lasted well into October this year, even though the driver of the summer pattern dropped off at the end of August. So, something else has to be bringing in all this fog.

That’s where all the recent low-pressure systems come into play.

These low-pressure systems have been undercutting the high-pressure system off the coast since September, bringing round-after-round of cool, misty air from the Pacific to California. They’ve essentially delayed the usual shift toward dry, Diablo and Santa Ana winds that the state should have seen by now.

This pattern of low-pressure systems coming ashore is largely due to the dance between La Niña and the Pacific North American oscillation. So far, the Pacific North American oscillation — which usually signals wet, unsettled weather for the Bay Area — has been winning out against the dry signature that La Niña tends to bring to the table.

These weather features will keep churning out moisture and producing a delay in fire season for a couple more weeks.

For now, firefighters and residents in some of the most fire-prone areas in the Bay Area and California are breathing a sigh of relief. But it’s important to remember: The season doesn’t officially end until October 31.

We will continue to keep an eye out for any sudden changes to this pattern between now and then.

Gerry Díaz (he/they) is a San Francisco Chronicle newsroom meteorologist. Email: [email protected] Twitter @geravitywave

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