Atmospheric River deluges California and the Southwest

An “Atmospheric River” of extremely moist air continues to affect the Southwest U.S. today, and flooding rains of 1 – 2” with isolated amounts of 2 – 4” can be expected across Southern California, western and central Arizona, southeastern Nevada, and southwestern Utah. The heaviest rains will fall over the coastal mountains south of Los Angeles. The storm began on Friday night, and has brought some rather remarkable rains and snows to the Southwest. Crestline, California, in the mountains just east of Los Angeles, has received 20.05” of rain since Friday. The Sierra Mountains near Sequoia National Park received an astonishing 17 feet of snow at Pescoes since Friday night. Other heavy snow amounts from the storm include 6.4' at Heavenly near Lake Tahoe, 6.5' in Crested Butte, Colorado, and 9 – 13.5' in Mammoth in the Sierras. Some other rainfall amounts from NOAA's latest Storm Summary:

...ARIZONA...
Iron Spring 11.14"
Black Rock 9.23"
Kingman 3.86"
Grand Canyon West 3.15"

...CALIFORNIA...
Crestline 20.05"
Nature Point 18.60"
Tanback 16.94"
Mammoth Lakes 12.82"
Santa Barbara 12.39"
Los Angeles-USC 6.66"
San Diego 4.22"
San Francisco Airport 2.44"

...NEVADA...
Mount Charleston 12.66"
Las Vegas, 24 miles WNW 8.99"
Reno 2.59"

...UTAH...
Gutz Peak 15.90"
Little Grassy 15.10"
Zion National Park 6.76"


Figure 1. The total amount of rainfall one could get if all the moisture in the air were condensed and fell out as rain is called the Total Precipitable Water (TPW). Here, TPW values from microwave satellite measurements are plotted, and show a plume of very moist air connecting the subtropics near Hawaii with Southern California. TPW vales in excess of 20 mm (about 0.8 inches, blue and warmer colors) are often associated with heavy rainfall events capable of causing flooding. Image credit: University of Wisconsin CIMSS.

“Atmospheric Rivers” was a term coined in the 1990s to describe plumes of moisture that ride up out of the subtropics into the mid-latitudes along the axis of a cold front. Traditional water vapor satellite imagery does not show these plumes very well, and it was only when microwave satellite imagery from polar orbiting satellites became available in the late 1990s that the full importance of these Atmospheric Rivers came to be revealed. Atmospheric Rivers account for a significant portion of California's cold season rainfall and snowfall, and an entire session was devoted to them at last week's American Geophysical Union (AGU) meeting in San Francisco, the world's largest Earth Science meeting.

Jeff Masters