Atmospheric rivers, which can be hundred kilometers wide and stretch across thousands of kilometers from the tropical oceans to the poles, have been known to carry twenty times more water than the Mississippi River. These skyward rivers can release enormous amounts of water given the right conditions. Apart from discharging large volumes of water, the rivers are responsible for some serious weather, including heavy winds, which have been known to cause even more damage than the rainfall itself.
These “rivers in the air” can be huge, thousands of miles long and 250 to 350 miles wide. They bring rain from the tropic to the rest of the world, which is good, but if they are too strong, they cause floods. If one hits the West Coast it is called a Pineapple Express; while one that hits Louisiana and Texas is called a Mayan Express because you can trace the moisture back to Central America.
In February of 2017 the Oroville Dam in Northern California threatened to fail after heavy rainfall due in part to an atmospheric river overflowed Lake Oroville causing officials to open up the emergency spillway. More than 180,000 people downstream from the Oroville Dam fled after fears of an imminent collapse of the spillway prompted an evacuation order.
Atmospheric rivers are not restricted to the Pacific Ocean, according to the National Oceanic and Atmospheric Administration. The fact, the rivers move with the weather and are present somewhere on the planet at any given time.
Because atmospheric rivers play such an integral role for water-starved regions, scientists are hoping to lengthen the forecast time and predict the exact areas where the rivers will strike.