From tracking landslides to tracking storms: What atmospheric rivers mean for you

What is an atmospheric river?

Picture a river floating a mile above your head — not liquid water, but invisible water vapor rushing through the sky, stretching 1,000 miles long (San Francisco to Seattle) and 250 to 400 miles wide (San Francisco to Los Angeles). These “rivers in the sky” are a normal part of how Earth moves water around — at any moment, four or five are flowing somewhere across the planet.

When these moisture-rich air masses hit our mountains, the vapor turns into rain and snow. Atmospheric rivers provide 30% to 50% of California’s annual rainfall and most of our Sierra snowpack, supplying about a third of state drinking water. Without them, California would be in permanent drought.

How often do they hit?

California sees 25 to 50 atmospheric rivers each year. Most are mild — welcome rain that fills reservoirs and moves on. But our region typically gets about five strong atmospheric rivers each winter, and those few storms deliver half our annual precipitation. Five storms. Half our water.

The season runs October through March, peaking December through February. In winter 2022-23, nine back-to-back atmospheric rivers hit California over three weeks — the longest stretch in 70 years. Some brought desperately needed water. Others caused devastating floods. Same storm type, very different outcomes depending on strength and timing.

Where geology meets meteorology

Here’s something most people never consider: The same forces that make the Bay Area beautiful make it vulnerable. The Pacific Plate grinding against the North American Plate created our famous faults — but also folded and fractured our bedrock into steep hillsides of weak, erodible materials. Layer on our Mediterranean climate — bone dry for six months, then suddenly drenched — and you have a recipe for disaster.

Soil is not just dirt. It has structure, layers and history. The Bay Area has ancient marine sediments, volcanic rocks, serpentine, sandstone — each absorbing and channeling water differently. Some act like sponges. Others act like slides. When an atmospheric river dumps inches of rain in hours, the geology determines whether that water soaks in harmlessly or triggers a catastrophe.

I have walked hillsides after major storms and seen where failures begin — often at the boundary between soil types, where water hits a clay layer and has nowhere to go but sideways, lubricating the slope above. The same storms that bring manageable rain to flat terrain become killers when they hit our complex landscape.

Why emergency management obsesses over weather

When I moved into emergency management, colleagues asked why focus so heavily on meteorology. The answer: Weather is the only major hazard we can see coming. We cannot predict earthquakes. Wildfires explode with little warning. But atmospheric rivers? Scientists can forecast them seven to 10 days out — and improving every year.

That warning window changes everything. We can pre-position sandbags, alert flood-prone residents, coordinate utility staffing, have search-and-rescue ready. But that only works if people heed the forecasts. Every year, some residents ignore warnings. Every year, some need rescue — or worse.

The bottom line

Atmospheric rivers are a natural part of California life. Most are harmless — even essential. Some are dangerous. The difference between weathering storm season and becoming a statistic comes down to a few decisions: Pay attention to forecasts. Sign up for SMC Alert. Respect warnings and evacuation orders. And never, ever drive through floodwater. It is not worth your life to avoid a detour.

San Mateo County’s Office of Emergency Management prepares year-round. But we need you to do your part. Take five minutes right now — before Saturday — to sign up for emergency alerts at smcalert.info. With 120 hours of rain heading our way, those five minutes could matter more than ever.

When we prepare together, we weather the storms together.

Ryan Reynolds is the assistant director, San Mateo County Office of Emergency Management.