Cars drive on a flooded road in Guerneville in January 2017. The Russian River town is just downstream from Venado, a site in the northern Sonoma County hills that is one of the rainiest locations in California.  Justin Sullivan/Getty Images
Cars drive on a flooded road in Guerneville in January 2017. The Russian River town is just downstream from Venado, a site in the northern Sonoma County hills that is one of the rainiest locations in California.  (Justin Sullivan/Getty Images)

Weather Geek, Why Do Some Parts of the Bay Area Always Get More Rain?

Weather Geek, Why Do Some Parts of the Bay Area Always Get More Rain?

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random weather question surfaces in the KQED newsroom, to wit:

"A member of my household (OK, my husband) would like to know why do some parts of the Bay Area always get more rain than others? We get the same storm systems, so why do parts of the North Bay and the Santa Cruz Mountains get hit so much harder?"

As someone with a coffee mug that reads "Weather Geek," I've been called upon to reply.

So to begin: Yes, it's a real thing. There are parts of the North Bay, which the local office of the National Weather Service defines as stretching all the way to the Sonoma-Mendocino county line, that are well known for their high winter rain totals. Ditto for the area around Mount Tamalpais in Marin County and the Santa Cruz Mountains.

That's what we've seen in our most recent series of storms, during which nearly 10 inches of rain has fallen in the mountains south of San Jose and nearly nine inches has come down in northern Sonoma.

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The main cause here, as in regions throughout California and the world, is local topography and what meteorologists term orographic lifting.

Meteorologists point to other factors, too, especially the general tendency for rainfall to increase on the West Coast as one moves from south to north due to large-scale atmospheric factors.

But first, as to that orographic thing: That's a process in which air colliding with obstacles in the landscape — mountains, ridges and hills — is forced to rise. Rising air cools, and the cooling causes water vapor in the air to condense.

When moisture-laden air is driven across coastal terrain by vigorous winter storm winds, that process of orographic lifting leads to significantly higher amounts of rainfall over the mountains and hills.

At the same time, areas to the east of the high terrain are in "rain shadows" that receive dramatically lower amounts of precipitation than sites in or to the west of the mountains.

"What happens with orographics ... that air flow that's below two or three thousand feet, or in the case of San Francisco below a thousand feet, it can't run through the mountains, so it gets forced up over the top," says veteran Bay Area meteorologist Jan Null. "That process lifts that air up, and our better precipitation-producing clouds are those that have stronger vertical motions. So you get vertical motion, you get more air rising up, you get more precipitation out of it. On the east side, then you get sinking air, which is detrimental to the formation of precipitation."

Neil Lareau, a professor of atmospheric sciences at the University of Nevada, Reno, says the orientation of the mountains and ridges in the Bay Area — they generally face southwest — is also important in squeezing out more rainfall from storm winds blowing from that direction.

The southwest-facing mountains "are lifting that really moist layer of marine air ... and they're adding a whole bunch of water to clouds at a low level as the air runs up the mountain slopes," Lareau says. "Technically, this is called a warm-rain process — you can picture it as really, really intense drizzle that gets added to the storm system."

Since rain will start much sooner in such orographically favored areas, he says, "those places get a huge amount of rain, both in terms of intensity and duration."

T

hese effects — enhanced orographic rainfall and corresponding rain shadows — can lead to surprisingly dramatic differences in rainfall over short distances. Bay Area meteorologist Jan Null points out in his narrative summary of San Francisco's climate that the city's average annual rainfall varies from more than 22 inches in the hills in south-central neighborhoods to 18 inches in the city's northeastern corner.

Perhaps the best-known example of a local contrast between a wet site and a nearby dry site is in the South Bay.

"At San Jose airport, you get 15 inches of rain a year," Null says. "You get up around the Ben Lomond area in the Santa Cruz Mountains, you get around 60 inches. That's a 4-to-1 ratio in about 15 miles as the crow flies."

Similar, if less startling differences, can be found throughout the region. Half Moon Bay, on the San Mateo coast, gets about 29 inches of rain a year; Palo Alto, 15 miles to the southeast, and across ridges that rise to more than 2,000 feet, gets about 16 inches.

Lareau notes that a couple of other factors explain the tendency for more rain to fall in the northern half of the Bay Area.

One is called "terrain channeling and convergence," in which southerly winds blowing from the Santa Clara Valley and up the bay meet a moist flow of air flowing through the Golden Gate.

"This can lead to an enhancement of precipitation where those two air streams run together," Lareau says.

He also points to the Bay Area's location relative to the winter jet stream — a sort of high-altitude blast of air that plays a central role in steering and enhancing storms and plumes of moisture arriving from the Pacific. The jet often takes aim at our part of the coast.

"The Bay Area is remarkably at this dividing point — the South Bay versus the North Bay, there are many more hours of precipitation once you move up into Marin and locations to the north than there are to the south. And really, that dividing line is just about San Francisco," Lareau says.

But the region's rainiest spots — in the hills and ridges of the Russian River watershed in northern Sonoma County — owe their reputations mostly to orographic influences.

Cazadero, a tiny community north of the river and about 10 miles inland from the coast, was once dubbed by no less than the Los Angeles Times the "state's wettest town." It's a mess finding up-to-date climate data for the area, but according to one fragmentary record from the Western Regional Climate Center, Cazadero and environs records average rainfall in the 75- to 80-inch range.

Which is a lot.

But we hear more frequently these days about Venado, a remote ridgetop location just west of Healdsburg, which has a networked and frequently reported-on rain gauge.

According to my own crunching of Venado's rainfall numbers from the Department of Water Resources California Data Exchange Center, Venado has averaged about 59 inches of rain a year since the early 1980s.

Venado does deserve acknowledgment, though, for its ridiculous rainfall total two seasons ago, in the water year that ran from Oct. 1, 2016, through Sept. 30, 2017.

You'll remember that was a fabulously wet non-El Niño year, featuring all sorts of weather mayhem. According to California-Nevada River Forecast Center records, 48 stations in the state, from the mountains above Big Sur to the highlands above Shasta Lake and Lake Oroville to the Oregon border, recorded 100 inches of rainfall or more.

Venado received 154.2 inches of rain for that season — ranking No. 4 in the state behind three sites in the mercilessly deluged Feather River watershed above Oroville.