An “atmospheric river” phenomena is forecast to bring another round of heavy precipitation to the U.S. West, with California likely to receive the brunt of it in the form of heavy rain and snow. Rainfall measured in inches, snowfall measured in feet, and potentially damaging winds in excess of 50 mph are expected to blast into California again not long after other storms have hit the state over an exceptionally active winter weather pattern.
An atmospheric river refers to a narrow stream of concentrated moisture in the atmosphere. The most common atmospheric river event is nick-named “Pineapple Express”; in this type of situation, a robust stream of moisture near the Hawaiian islands flows north and east into western North America, dropping copious amounts of rain and snow as it interacts with the terrain there.
These long, narrow atmospheric rivers help transport moisture from the tropics, in water vapor form, to areas far from the tropics. The liquid equivalent of these moisture plumes could be comparable to the water flowing through the mouth of the Mississippi River, according to NOAA.
On average, 30-50% of annual precipitation on the U.S. West Coast occurs from a few atmospheric river events. In addition to replenishing water supplies, they can also create problems associated with heavy precipitation: flooding, rock/mud slides, and avalanche dangers.
As the stream of water vapor hits the mountain ranges over the U.S. west coast, the air rises, cools, and condenses the moisture out of it. This condensed moisture falls out of the sky as heavy rain, heavy snow, or both.
A significant atmospheric river event is expected to unfold in the coming days, with the worst arriving later Thursday into Friday. With bad weather arriving, the National Weather Service has issued hundreds of watches and advisories across California, Nevada, Oregon, Idaho, Utah, and Arizona. In addition to heavy snow and rain, freezing conditions harmful to agriculture, damaging winds, and rough surf are also possible across large portions of California.
The heavy precipitation is also falling over areas that had previous burn scars from earlier wildfires. When heavy rain hits those scars, mudslides and debris flows may also happen, creating additional hazards on area roads and in regional communities.