Atmospheric rivers are a normal part of the West Coast's weather pattern, and they're often the solution to months of warm-weather drought, bringing sorely needed rain and snowfall that packs water away high in the mountains.
"It's just a narrow area of high moisture that gets transported away from the tropics towards the higher latitudes," often before a cold front arrives, as NWS senior forecaster Bob Oravec recently told NPR.
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The precipitation can be extreme: a single atmospheric river "can carry more water than the Mississippi River at its mouth," as NPR has reported. And forecasters have long warned that the systems' winds are very dangerous.
