What makes California flooding so different from floods in Florida?

Multiple intense storms have caused widespread flooding, dozens of deaths, and triggered a presidential state of emergency.

No, it’s not Florida getting hit this time, but rather the drought-plagued west coast with northern California square in the center of the washout.

Outside of a hurricane, Florida rarely sees the type of impacts from heavy rain California has had in recent weeks.

The flooding stems from a narrow band of wind below a mile in the sky that’s pointed directly at California. The flow is sending a plume of water vapor to the west coast like a conveyer belt. This feature called an AR or Atmospheric River, cause most flooding events along the west coast of North America in winter.

More than a foot of rain has drenched San Francisco in the past couple of weeks following Christmas according to the National Weather Service.

AR’s accomplish the same goal as hurricanes — to bring warm moist air from the tropics northward.

The amount of rain in a hurricane is much heavier than an AR but the unusual duration of multiple AR events in California has resulted in freshwater flooding worse than Hurricane Ian.

Beyond the devastation Hurricane Ian left along the Ft. Myers coastline from storm surge, Ian’s slow forward motion produced record inland freshwater flooding across portions of central and eastern Florida reaching 10-20 inches.

Orlando reported 13.20 inches of rainfall in 4 days compared to San Francisco’s 13.59-inch sixteen-day running total.

Flooding worse than an average hurricane

What makes this flooding worse in California are the successive series of storms and geography, wildfires, and ironically drought; all have compounded the flooding.

Unlike Florida, at least 40 percent of the United States has been in drought for the past 119 weeks — much of it targeting California.

This drought has been longer than at any other time in the 22 year U.S. Drought Monitor track history.

The hardened parched ground is less permeable to runoff increasing the flooding.

Years of thriving wildfires left the scorched ground covered like plastic wrap in a layer of impermeable charred organic debris. Water cannot penetrate into the soil sending a rush of surface runoff from rainstorms into homes and unleashing mudslides of ash, sediment and other pollutants.

Atmospheric rivers slam into mountain ranges with an uplifting rising motion that makes the rain heavier than flat topography.

Maps like the one below make atmospheric rivers stand out for meteorologists by highlighting, how much water vapor is being pushed around by the wind. The red colors show very high levels of integrated vapor transport or IVT, which mark the core of the atmospheric river and help classify the severity of the storm.

Several of the parading AR’s that rocked through California during this fall and winter have been ranked atop an AR Storm Scale as Exceptional which is a Cat 5: Primarily hazardous event.

Is it a drought buster?

Some droughts are ended by the arrival of atmospheric river storms.  Between 1950 and 2010, 33%–74% of droughts on the West Coast were ended.

Many Central and Northern California areas have received over 200% of their regular yearly average precipitation since October.

But even this is not enough to end the persistent drought with half of the water storage sites still remaining below the historical average.

One of California’s largest reservoirs, Lake Oroville, is now at 1.3 million acre-feet and climbing, but over 2 million more acre-feet of water is still needed to fill the lake due to the extreme drought conditions over the last few years.

“These storms have not ended the drought,” said Molly White, Water Operations Manager for the State Water Project. “Major reservoir storage remains below average, and conditions could turn dry again this winter, offsetting recent rain and snow.”