Bay Area winters are defined by storms that arrive from the Pacific with varying strength and frequency. Summer runs on one steady pattern, the marine layer held in place by the North Pacific High. Winter instead brings a series of separate storms, and some are weak and drizzly, while others flood roads, knock out power and bring snow to the highest peaks. The Bay Area sits near the southern edge of the main Pacific storm track. Small shifts in the jet stream can mean the difference between a drought year and a flood year.
The Bay Area Storm Season
Almost all of the Bay Area's rain falls in the wet season, from November through March. October and April bring occasional rain at the edges, but most of the year's total falls in those five months. That pattern is typical of Mediterranean climates, and it means the region gets its whole year's water supply in a short window.
Average yearly rainfall varies widely from one part of the region to another. San Francisco gets about 23 inches, Oakland about 24 and San Jose about 14. In the North Bay hills, Kentfield averages more than 40 inches, and the upper slopes of Mount Tamalpais more than 50. The Santa Cruz Mountains and the higher East Bay hills also get far more rain than the valley floors. Hills force moist air upward and squeeze out extra rain, a process called orographic enhancement. A storm that drops an inch of rain in Oakland can drop two or three inches on the ridge above it.
Types of Bay Area Winter Storms
Bay Area winter storms come in a few main types. The most common is the ordinary low-pressure system, which brings a warm front, a period of steady rain and then a cold front. After the cold front passes, showers taper off and the sky clears. These storms are usually manageable, typically dropping an inch or two in the lowlands.
Atmospheric rivers are a different and more dangerous class of storm. They are long, narrow bands of concentrated water vapor that stream across the Pacific. When the moisture comes from near Hawaii, the storm is often called the Pineapple Express. A strong atmospheric river can drop 5 to 10 inches of rain on the coastal mountains in two days. That much rain swells creeks to flood stage and overwhelms storm drains in low areas. It can also set off landslides on the steep slopes of Marin and the Santa Cruz Mountains. The winters of 2016-17 and 2022-23, and the huge storm of October 2021, were all driven by atmospheric rivers that caused serious flooding.
At the other extreme, a stubborn ridge of high pressure can block the storm track and bring weeks of dry, sunny winter weather. Many California droughts come from these blocking ridges, which steer storms north or south of the state. In the driest years, the Bay Area can go much of the winter with little rain.
El Niño, La Niña, and the Bay Area Winter
El Niño and La Niña, the warm and cool phases of the tropical Pacific, can tilt the odds for a Bay Area winter. During El Niño, unusually warm water in the central and eastern tropical Pacific can shift the jet stream south. Strong El Niño events have often brought wetter winters to California. La Niña tends to push the storm track north and raises the odds of a dry winter. For the Bay Area, though, these are tendencies rather than guarantees, and many winters break the pattern.
The winter of 2022-23 shows how loose that link can be. It was a La Niña winter, yet a long series of atmospheric rivers made it one of California's wettest in recent memory. Those storms pulled the state out of drought. The two La Niña winters before it had been very dry. Swings like these are why Bay Area water managers plan for the full range of possible winters rather than for an average one.
Winter Weather by Location
Winter makes Bay Area weather more alike across the region than any other season, because every community gets the wet season. Important differences remain, though, and hills that face incoming storms, near the coast and inland, get the most rain. The valleys get less rain but can flood when rivers and creeks run high. Only the highest peaks see snow with any regularity.
Mount Hamilton, at about 4,200 feet in the Diablo Range east of San Jose, gets snow several times in most winters. Mount Diablo's summit and the higher ridges of the Santa Cruz Mountains see snow less often, and Mount Tamalpais gets only an occasional dusting. A white peak on a clear morning after a cold storm always draws attention across the region. The Sierra Nevada, about 150 miles east, matters even more, because a good Sierra snowpack fills the reservoirs that supply much of the Bay Area's water.
