Coming Storm: A River in the Sky

by Hilltromper staff

Dec. 10, 2014—The storm about to slam into Santa Cruz apparently is an “Atmospheric River,” and apparently an ”Atmospheric River” is a thing—there’s even an acronym widely used in meteorological circles.

Dr. Marty Ralph, Research Meteorologist at NOAA's Earth System Research Laboratory in Boulder, says ARs "are technically associated with cyclones." They include hurricane-strength winds and can yield rainfall comparable to that of hurricanes. 

"The ARs in these cyclones are regions where low-altitude winds transport huge amounts of water vapor up against the mountains, which wrings out extreme rainfall."

The "Pineapple Express," which frequently brings moisture from Hawaii to our shores, is an Atmospheric River.

From the Atmospheric River page at the Earth System Research Laboratory: "A strong AR transports an amount of water vapor roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River."


The last AR to hit Central California, in October 2009, dumped 21 inches of rain in just one day in the mountains near Big Sur and more than 10 inches on the Santa Cruz Mountains, according to NOAA. It did $10 million in damage.

2010s notorious "Snowmageddon" was an AR event, although I don't remember hearing the phrase at the time.

The storm that will hit Santa Cruz tonight is expected to deliver 3 inches of rain along the coast and up to 8 inches in the mountains, with winds up to 70 mph.