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Ask the Weather Guys: What is an atmospheric river?

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Q: What is an atmospheric river?

A: The extreme and persistent drought that has plagued parts of California for several years will be at least slightly remedied by the torrential rains that fell over the weekend over much of the central and northern part of the state.

These rains were associated with a phenomenon called an “atmospheric river.”

Atmospheric rivers are organized flows of deep, moist air from the subtropics and tropics that bring many locations in California a large portion of their annual precipitation.

These rivers are not really a distinct feature of the atmosphere. Rather they are organized and pushed poleward by the circulations around extratropical cyclones that are strong enough to tap the substantial moisture endemic to the subtropics and tropics.

California storms

Rocks and vegetation cover Highway 70 following a landslide Sunday in the Dixie Fire zone in Plumas County, Calif. Heavy rains blanketing Northern California created slide and flood hazards in land scorched during last summer's wildfires.

This weekend’s event is directly tied to the most intense extratropical cyclone to ever visit the waters off the Pacific Northwest. A cyclone with a central pressure in the 944 mb range was just offshore of Washington state. For perspective, the average sea-level pressure is about 1012 mb. Additional perspective on the strength of this storm arises from the fact that category 3 or 4 hurricanes are often characterized by such low central pressures.

The atmospheric river associated with this storm was really a deep flow of moist air ahead of the cold front associated with this monster cyclone. Though it is not truly a distinct meteorological animal, categorizing such moist flows as atmospheric rivers is a useful way to gauge the likely impact of these features on precipitation prospects both in the Central Valley — an enormously important agricultural region of our country — and in the Sierra Nevada, where winter snows are like money in the bank for spring agriculture in Valley.

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences. Send them your questions at stevea or jemarti1


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