The Bay Area rainy season runs from November through March, and December is the wettest month. The first rain of the season usually arrives in mid-October, and the first soaking storm usually comes in early November. The rain tapers off in April, and from May through October the Bay Area is almost completely dry.
In San Francisco, November through March brings 84% of the yearly rain: 19.28 of 22.89 inches. That five-month concentration, and how much it changes from year to year, shapes nearly everything about Bay Area life, from water supply to fire season.
When Is the Rainy Season in the Bay Area?
The Bay Area rainy season is November through March. Based on NOAA 1991-2020 normals, San Francisco averages 4.76 inches in December, 4.40 inches in January and 4.37 inches in February. March averages 3.15 inches and November 2.60 inches. October (0.94 inch) and April (1.60 inches) are transition months that can be dry or stormy.
June through September is the dry season, when San Francisco averages a combined 0.37 inch over those four months, so summer gray days are fog, not rain. For month-by-month totals and rainy days, see San Francisco rainfall by month.
When Does the Rainy Season Start in the Bay Area?
The Bay Area rainy season usually starts in mid-October to early November. From 1991 to 2020, the first day with at least a tenth of an inch of rain in San Francisco fell on October 14 in a typical year. The first soaking storm, with half an inch or more, came around November 3. In half of those years, the first soaking storm arrived between October 24 and November 26.
The start date swings widely from year to year. On October 24, 2021, an atmospheric river dropped 4.02 inches on downtown San Francisco, one of the wettest days in city history. In 2020, the first tenth of an inch did not fall until November 17, and the first soaking storm waited until December 13. The 2025-26 season started on October 13, 2025, with a half-inch storm.
The timing depends on when the Pacific jet stream shifts south. All summer, the North Pacific High steers storms north into Oregon and Washington. As that high weakens in fall, the storm door opens for California. For this winter's outlook, see the 2026-27 El Niño guide.
When Does the Rainy Season End in the Bay Area?
The Bay Area rainy season usually ends in April, with the last meaningful spring rain in a typical year falling in late May. From 1991 to 2020, San Francisco's last day of a tenth of an inch or more before summer came around May 24. April averages 1.60 inches, May 0.70 inch and June just 0.20 inch.
After that, the Bay Area usually goes five months or more without a soaking rain. That long dry season is why water storage, conservation and fire management are constant concerns in the region.
Why Does the Bay Area Have a Rainy Season?
The Bay Area has a rainy season because it has a Mediterranean climate, driven by the seasonal movement of the North Pacific High. In summer, this high-pressure system sits off California and blocks storms from reaching the state. In winter, it weakens and shifts south, letting the storm track reach Northern California.
The jet stream, the fast-moving air high in the atmosphere that guides storms, sits over the northern Pacific in summer and dips south in winter. When it drops to roughly 35 to 45 degrees north latitude, storms can travel straight from the Pacific into the Bay Area. Read more about the Bay Area's Mediterranean climate and seasons.
What Does a Typical Bay Area Wet Season Look Like?
A typical Bay Area wet season is a series of storms separated by dry spells, not steady rain. Storms usually last one to three days, with dry breaks of a few days to a few weeks between them. Those dry spells bring some of the Bay Area's best weather, with clear skies, cool air and long-range visibility.
December and January together bring about 40% of San Francisco's yearly rain. February is nearly as wet as January, and March still averages more than 3 inches. The heaviest storms are often atmospheric rivers, which can drop several inches in a day.
How Much Does the Wet Season Vary From Year to Year?
Bay Area wet seasons vary enormously: San Francisco's wettest water year brought six times as much rain as its driest. The record ranges from 8.14 inches in 1850-51 to 49.25 inches in 1861-62, the winter of the Great Flood of 1862. Modern seasons have ranged from 9.04 inches in 2020-21 to 46.56 inches in 1997-98. Most American cities vary far less from year to year.
The El Niño-Southern Oscillation explains part of that swing. El Niño years raise the odds of a wet winter, especially strong El Niños. The Bay Area sits between the northern and southern storm tracks, though, so results are mixed. La Niña years tend to be dry in Southern California and mixed in the Bay Area.
Droughts that last several years can be severe. The 2012-2016 drought was one of the most intense in California's record. The 2016-17 winter ended it with a long series of atmospheric rivers that refilled reservoirs across the state.
Deep Dive: Why Does the Jet Stream Move South in Winter?
The jet stream forms where cold polar air meets warmer air to the south. Its strength depends on how sharp that temperature contrast is, and its position follows where the contrast is sharpest. In winter, the contrast grows stronger and shifts toward the equator, so the jet and its storms move south toward California.
What Sets the Strength and Position of the Jet?
Cold air is denser than warm air, so pressure falls faster with height in a cold column than in a warm one. High above the ground, that leaves lower pressure on the cold side and higher pressure on the warm side. The wind that results, called the thermal wind, grows stronger with height and blows from west to east in the Northern Hemisphere.
The sharper the temperature difference between north and south, the faster that upper wind blows. In winter, the Arctic loses the sun almost entirely while the tropics stay warm. The contrast across the middle latitudes grows, and the zone where it is sharpest moves south over the North Pacific. The jet strengthens and follows it.
The North Pacific High moves with the season too. In summer it sits far north, off the California coast, and blocks storms. In winter it weakens and shifts south, opening the way for the jet to carry storms across the Pacific toward Northern California.
Why Does So Much Rain Come in So Few Storms?
Even within the rainy season, rain is concentrated in a handful of big events. A 2011 study in the journal Water, by Michael Dettinger, Marty Ralph and colleagues, found that atmospheric rivers provide 20 to 50 percent of California's precipitation and streamflow.
In a 2016 paper in San Francisco Estuary and Watershed Science, Dettinger looked at the wettest days in more detail. He found that the wettest 5 percent of wet days supply about a third of California's precipitation. They also account for about two thirds of the year-to-year swings in how wet a water year turns out.
That is why one winter can feel so different from the next. A few extra atmospheric rivers can turn an average season into a wet one. Missing them can bring drought, even when the number of ordinary rainy days barely changes.
When Will It Rain Again in the Bay Area?
The live forecast below shows the next hour rain is likely in San Francisco. It uses the same forecast that powers our destination pages and updates through the day.
When will it rain next?
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Beyond our 7-day forecast, the National Weather Service Bay Area forecast is the best guide to when rain is coming. It is usually reliable for rain timing four to five days ahead. For longer-range trends, NOAA's Climate Prediction Center publishes 6-10 day and 8-14 day outlooks showing whether the pattern leans wetter or drier than normal.
The strongest early signal is an atmospheric river forming in the eastern Pacific. When satellite images show a plume of tropical moisture stretching toward California, rain is usually two to four days away. For the latest local forecast, check San Francisco weather.
What Does the Wet Season Mean for Daily Life?
The wet season turns the Bay Area green. Hills that are brown by August green up within weeks of the first soaking rains and stay green into May. The first rain also clears smoke and dust from the air, cuts fire danger, and makes roads slick, because oil builds up on pavement over the dry months.
Hiking is excellent in the days after a storm, when the air is clear and waterfalls are running. During storms, though, trails in Marin and the Santa Cruz Mountains can turn muddy and eroded. Give most ridge trails a day to dry out after heavy rain.
Driving is most dangerous in the first rain after a long dry spell, when oil on the pavement makes roads slickest. During heavy storms, freeway underpasses and low roads near the Bay can flood, and coastal Highway 1 can close for rockfall or slides. Check Caltrans QuickMap for closures before a storm-day drive.
For visitors, the wet season is not a reason to stay away. Storms usually last one to three days, followed by dry, clear weather. Many residents consider February one of the best outdoor months, with green hills, early wildflowers and crisp, clear days between storms. Plan outdoor activities for those windows and keep your plans flexible.
