What are atmospheric rivers and why do they keep drenching Sonoma County?

Atmospheric rivers are relatively long, narrow regions in the atmosphere — like rivers in the sky — that transport most of the water vapor outside of the tropics, according to the National Oceanic and Atmospheric Administration.

These columns of vapor can carry an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When they make landfall, they often release this water vapor in the form of rain or snow.

Atmospheric rivers can travel thousands of miles and are responsible for 30% to 50% of the wet season precipitation along the West Coast, according to NOAA.

While not all atmospheric rivers lead to calamity on the ground, those with the largest amounts of water vapor and strongest winds can create extreme rainfall and floods by stalling over areas vulnerable to flooding.

Most, however, are weak systems that often provide beneficial rain or snow crucial to the water supply. And, as such, they are considered a key feature in the global water cycle.

A well-known example of an atmospheric river is the “Pineapple Express,” capable of bringing moisture from the tropics near Hawaii over to the West Coast.

The major storms that caused 18 deaths and widespread damage across much of California in early January 2023 were the result of atmospheric rivers.

So, too, were they responsible for several notable periods of flooding rains and damaging winds since the start of 2023 in Sonoma County.