A new study by NASA and several partners found that in California's Sierra Nevada, atmospheric river storms are two-and-a-half times more likely than other types of winter storms to result in destructive rain-on-snow events, where rain falls on existing snowpack, causing it to melt. Those events increase flood risks in winter and reduce water availability the following summer.
Atmospheric rivers are narrow jets of very humid air that normally originate thousands of miles off the West Coast in the warm subtropical Pacific Ocean. When the warm, moist air hits the Sierra Nevada and other high mountains, it drops much of its moisture as precipitation. Only 17 percent of West Coast storms are caused by atmospheric rivers, but those storms have been blamed for more than 80 percent of the state's major floods.
The researchers also quantified the difference between atmospheric river storms that cause rain-on-snow and those that do not, using data from NASA's Atmospheric infrared Sounder (AIRS) instrument on NASA's Aqua satellite. The rain-on-snow-producing atmospheric river storms were, on average, 4 degrees Fahrenheit (2 degrees Celsius) warmer than the others.
That small difference in temperature often determines whether we gain snow or lose snow from a storm, said Bin Guan, lead author of the study.