A category ‘AR 3’ atmospheric river is about to hit California. Here’s what that means

The winter storm that could dump several feet of snow in the Sierra and soak the Sacramento Valley with rain is listed as a “strong” category “AR 3” system by the scientists who study the powerful storms that supply California with most of its water.

In 2019, the scientists who study the storms known as “atmospheric rivers” agreed to a ranking scale similar to the “Cat” system used to describe a hurricane.

An “AR 1” is the weakest system. An “AR 5” is the most destructive. The five categories the Center for Western Weather and Water Extremes at Scripps Oceanography list are “weak,” “moderate,” “strong,” “extreme,” and “exceptional.”

One notorious example of an AR 5 was the New Year’s Day floods of 1997, which killed at least nine people and flooded 300 square miles of California.

The incoming “strong” AR 3 may be in the middle of the scale, but it’s still going to bring impressive amounts of precipitation when it hits California this weekend.

The weather service warns that as much as 6 to 9 feet could fall at elevations above 6,000 feet. Additionally, 1 to 3 feet of snow is expected farther north, near Mount Shasta, with 5 to 6 feet possible above 5,000 feet.

Strong wind gusts will accompany the snowfall, and the weather service warns mountain travel will be “difficult to impossible.”

The Sacramento Valley could see as much as 5 inches of rain between Sunday and Tuesday.

Atmospheric rivers form over the Pacific Ocean when high-powered winds drag a fire hose of tropical moisture across the ocean’s surface.

The 500-mile wide conveyor belts of water can last for days and can hit California in wave after wave. They provide up to half the state’s rain and snowfall each year. The largest storms can produce as much rain as a major hurricane.

Notoriously difficult to predict, the U.S. Air Force Reserve regularly sends its “Hurricane Hunter” aircraft above the Pacific to conduct scientific measurements on atmospheric rivers in the hopes of better understanding the storms for more accurate long-range forecasts.

This story was originally published December 10, 2021 at 10:19 AM with the headline "A category ‘AR 3’ atmospheric river is about to hit California. Here’s what that means."