What the 2022–2023 Atmospheric Rivers Taught Us About Urban Flooding

Between late December 2022 and mid-January 2023, California was hit by nine consecutive atmospheric rivers in the span of three weeks. It was the longest stretch of continuous atmospheric river conditions in the seventy years that records have been collected.

The sequence delivered roughly 32 trillion gallons of water to the state, triggered more than 700 landslides, killed at least 22 people, and produced an estimated $5 to $7 billion in damage. The San Francisco Bay Area recorded its wettest three-week period in 161 years.

For those of us working restoration in the Gateway Cities, it was a concentrated lesson in how urban flooding actually behaves.

What an atmospheric river is

A narrow corridor of concentrated water vapor in the atmosphere, transporting moisture from the tropics toward higher latitudes. The strong ones carry more water than the Mississippi River, and when one makes landfall and stalls, it delivers that moisture as sustained, high-intensity rain over a small area.

They are not unusual — California depends on them for a large share of its annual precipitation. What made the 2022–23 sequence remarkable was that they arrived back to back, with no drying interval between them.

Lesson 1: intensity beats total

The single most useful thing that sequence demonstrated is that rainfall total is nearly irrelevant to flood damage. Rate is everything.

Storm drains are sized for a design storm. Once rate exceeds that capacity, the excess has nowhere to go — it backs up at the inlets, sheets along the gutter line, and finds the lowest opening it can reach. That opening is very often a garage door or a threshold on a street with no flooding history.

Lesson 2: back-to-back storms are the real danger

One intense storm on dry ground is manageable. The soil absorbs some of it, drainage systems clear between bands, and the system recovers.

Nine in three weeks removes all of that. The soil reaches saturation and stops absorbing anything, so essentially 100% of subsequent rainfall becomes runoff. Groundwater rises, which pushes moisture up through slabs and infiltrates cracked sewer laterals. Drainage systems never get a chance to clear, so debris accumulates at inlets. And any roof or envelope failure created by storm one gets tested by storms two through nine.

The third storm in a sequence causes more damage than the first, from less rain.

Lesson 3: the delayed sewage wave

This is the pattern that most surprises homeowners, and we saw it clearly through that period.

Our sewage calls did not peak during the heaviest rain. They peaked one to two days after it stopped. Groundwater takes time to rise to its maximum after rainfall, and when it does, it infiltrates the cracked joints in old clay-tile sewer laterals. That extra volume tips a marginal line into a full backup.

So a household would get through the storm with no problem at all, then have sewage coming up through a shower drain on the first clear day. In Rio San Gabriel and the low ground of Pico Rivera, that sequence is close to reliable.

Lesson 4: wind damage is a delayed fuse

A large share of the roof leaks we attended during that sequence were not caused by those storms. They were caused by earlier Santa Ana wind events that had lifted tiles or torn flashing months before — damage that leaked nothing at all until sustained heavy rain arrived with the right wind direction.

The homeowners had no idea anything was wrong with their roofs. Nothing had ever leaked.

What to actually do before the next one

1. Get the roof inspected after any significant Santa Ana wind event, not after the ceiling stains — this is the highest-value item on the list
2. Clear every yard, area, and driveway drain before November; they silt up over a dry summer and nobody checks them
3. Confirm downspouts discharge well away from the foundation rather than pooling against it
4. On a flat or low-slope roof, clear the drains and scuppers — ponding water 48 hours after rain means the drainage is not working
5. Keep sandbags or a threshold barrier if your garage has taken water before
6. Get anything valuable off the garage floor and onto shelving
7. If your drains have been slow or gurgling, scope the sewer lateral before the wet season, not after
8. Check whether you have flood coverage and a sewer backup endorsement — both are commonly missing

And when it does get in

Storm water that entered from outside is Category 3 contaminated, however clear it looks — it crossed soil, roadway, and storm drain infrastructure.

Keep people and pets out of it. Do not run box fans on it, which aerosolizes contamination into the air the household is breathing and is the single most harmful mistake we see. And expect that porous materials which absorbed it, carpet pad above all, will need removal rather than drying.

During a major sequence, call volume across the whole basin spikes at once. We will give you an honest arrival window rather than a number we cannot hit — which we think is more useful than a promise that slips.