![]() On the same day that the AR was slated to hit San Francisco, I was scheduled to present new insights into these storms at the 2016 American Geophysical Union meeting there. As big as they are, they can be fickle because numerous forces can affect their progression, from ocean-surface temperatures to pockets of cold air aloft. It's hard to know more than a few days ahead where exactly a storm will make landfall, however. They can beneficially boost snowpack and help fill natural and human-made water reservoirs. They can also reach far inland: the raging terrestrial rivers that tore apart roads in Yellowstone National Park and forced the park to close this past June were fueled in large part by a remarkably strong AR.ĪRs are not always destructive sometimes they bring welcome rain to parched regions. Several times a year they pummel the western coasts of the U.S., Canada, Europe, Africa, South America and New Zealand. ![]() They can occur in families-a series of storms, as if rolling in on a treadmill. These storms can produce disastrous flooding, including the biggest floods that some areas may see in a century. An average AR brings far greater rainfall than a typical rain or thunderstorm in those parts of the world, transporting enough vapor to equal 25 times the flow rate of the Mississippi River where it pours into the Gulf of Mexico. They can grow to 2,000 miles long, 500 miles wide and two miles deep by the time they strike the western coasts of continents. The meteorological community formally defined them only in the early 2010s, after improved satellite imaging and science revealed how these storms can form far out over the remote ocean. The symbol was completely inadequate for communicating the threat of the approaching storm.ĪRs are essentially rivers of water vapor in the sky that are pushed along by strong, low-altitude winds, sometimes at hurricane speeds. I had been studying detailed satellite data and weather models, and they indicated that a major atmospheric river (AR) was likely to hit the city. I knew Thursday's conditions would be much rougher than the symbol conveyed. Wednesday had a friendly-looking cloud and a few raindrops, and Thursday had a dark, threatening cloud with heavier drops. A little symbol along the bottom showed a happy-looking sun for the rest of the day. I was eating breakfast on a Monday morning at Sears Fine Food in downtown San Francisco, casually watching the local five-day weather forecast on a television screen behind the counter. They have been well predicted and accurately forecast. Scientific American recently explained how meteorologists have become so much better at doing this work so quickly. ![]() Editor’s Note (3/13/23): Atmospheric rivers have been inundating California with record rainfall and snow depth, causing flooding. ![]()
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