Watch overhead! A river may be coming at you.
If you are in California, you will welcome the giant atmospheric river that brings rain to your parched state. In the East, you might not be so grateful. One of those rivers created Snowmageddon in 2010 and, really, who needs more snow?
When cold fronts carry rivers of tropical water vapor north, it brings rain, sometimes too much rain. Source : NOAA
An atmospheric river (AR) is a band of tropical water vapor caught on the edge of a cold front and carried toward the poles. These 250 to 350 miles-wide bands carry as much as 300,000 tons of water, 7 to 15 times as much as the mouth of the Mississippi River. When they hit land, rain and snow sometimes causes landslides, major floods and general misery. Ask Californians about the AR at the end of February. They were pleased to see the rain and snowfall (except for some of the damp visitors to the Oscars) but not with the power outages, accidents, and mudslides.
The rains and floods of California this February and March are due to atmospheric rivers. Source: FEMA
We are only beginning to understand these rivers. Discovered by Reginald Newell and Yong Zhu in the 1990s, they were originally connected with West Coast flooding, particularly the infamous “Pineapple Express” that flows through Hawaii. We now know they have a central role in the global water cycle. On any given day, these relatively narrow bands of water vapor account for over 90% of the movement of tropical moisture north and south. Most of the time, they bring warm, welcome rainfall in moderate amounts.
The good news is that scientists are increasingly able to warn people where and when the worst atmospheric river floods or snows will hit. They have also learned that these AR events can happen throughout the world. The floods that have been hitting England this winter are river storms. Some of the worst Nor’easters, like 2010’s Snowmageddon were caused by an atmospheric river flooding the chilly East Coast.
Snowmageddon, When an atmospheric river hits the East Coast in winter, it can create a Nor’easter. Source: NOAA
So look up in the skies. It’s a bird, a plane—no—an atmospheric river coming at you. You may not enjoy it, but the flowers this spring will be grateful.
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin  and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.