The Bay Area's wettest days do not come from ordinary storms. A typical storm in the wet season, brings a half inch to a couple of inches over a day or two. They come from strong atmospheric rivers, long, narrow bands of moist air that flow in from the warm Pacific. When one of these stalls over the region and runs straight into the coastal mountains, a single day can bring as much rain as some months. Totals like that overwhelm creeks and storm drains built for gentler storms. As a result, the wettest days are also the days most likely to bring floods and landslides.
Bay Area Single-Day Rainfall Records
San Francisco has kept rain records downtown since the Gold Rush, which makes its list of wettest days one of the longest in the West. The top spot belongs to November 5, 1994, when 5.54 inches fell. A close second came on December 31, 2022, when an atmospheric river dropped 5.46 inches and flooded streets, freeways and BART stations across the region. The October 24, 2021 storm, a powerful bomb cyclone, brought just over 4 inches and ranks among the city's wettest days.
To put those numbers in context, downtown San Francisco averages about 23 inches of rain in a full year. A 5-inch day therefore delivers more than a fifth of a normal year's rain in 24 hours. In a typical year, the city's wettest day brings around 2 inches. A day above 4 inches is a rare event even in a very wet winter, and it has happened only a handful of times since the 1800s.
The city's totals are modest next to the mountains around it. When moist air from the ocean hits steep slopes, it is forced upward, cools and squeezes out much more rain than falls on the flatlands. This effect is called orographic rain, and it gives the Santa Cruz Mountains and the slopes of Mount Tamalpais the region's highest totals. In the strongest storms, these ranges can collect 10 inches or more in about a day.
The benchmark event is the storm of January 3 to 5, 1982. In little more than a day, San Francisco received about 5 inches and Kentfield in Marin County more than 10. Parts of the Santa Cruz Mountains got around 20 inches or more. The rain set off thousands of landslides across the region. In all, 33 people died, 25 of them in landslides. The deadliest single slide hit the Love Creek neighborhood near Ben Lomond, where 10 people were killed when a hillside buried their homes.
What Extreme Rainfall Does to the Bay Area
Most of the Bay Area's creeks and storm drains are sized for typical winter storms, not for several inches falling in a few hours. When 3 to 5 inches fall in a day, small creeks that are usually ankle deep can rise many feet in a few hours. Storm drains fill and back up, and water collects on streets and freeway underpasses. Low spots on the shoreline flood faster when a high tide keeps water from draining out to the bay.
Landslides are the other major danger in a storm like this. Rain that soaks into a hillside adds weight and loosens the soil. After weeks of earlier storms, the ground is already full of water, and a final burst of heavy rain can send a slope sliding downhill. The most dangerous kind is the debris flow, a fast-moving slurry of mud, rock and trees that can travel down a canyon faster than people can run. The 1982 storm showed how quickly these flows can strike homes built at the base of steep slopes.
Where the rain falls matters as much as how much falls. Heavy rain in the hills runs downhill into creeks, and those creeks gather the water from a whole watershed before they reach the flatlands. Rain on the Santa Cruz Mountains drains toward the South Bay and the coast, and rain on the Marin hills drains toward the North Bay. Rain in the Diablo Range feeds creeks such as Alameda Creek. A neighborhood near the mouth of one of these creeks can flood even when its own rain gauge shows only a moderate total.
Warming is likely to make these days more intense. Warmer air can hold more water vapor, roughly 7 percent more for each degree Celsius of warming. That means atmospheric rivers can carry more moisture than they did a few decades ago, and more of it can fall at once. Climate scientists expect the strongest storms to bring heavier bursts of rain, even in years when the total for the season is ordinary.
Preparing for Extreme Bay Area Rainfall
Forecasters can usually see a strong atmospheric river coming several days in advance. The National Weather Service office for the Bay Area issues a Flood Watch when heavy rain could cause flooding in the next day or two. It upgrades to a Flood Warning or Flash Flood Warning when flooding is happening or about to happen. Forecasters also rank incoming atmospheric rivers on a five-step scale, from weak to exceptional, which helps people judge how serious a storm is likely to be.
If you live near a creek, in a low-lying area or below a steep slope, use the days before a big storm to get ready. Clear gutters and storm drains near your home, move cars out of spots that flood and know how you would leave if roads close. During the storm, avoid creek banks and never drive through standing water, which can hide a washed-out road. If you live below a steep slope, listen for cracking or rumbling sounds, which can mean a slide is starting.
The time between the heaviest rain and flooding downstream can be just a few hours, and landslides can strike with almost no warning. That is why the days of warning before a storm matter so much. A storm that is well forecast and taken seriously does much less harm than one that catches people off guard.
