Scientists this weekend are dispatching a research plane off the coast of California to study what’s known as an atmospheric river.
It's a body of rushing water vapor traveling about a mile above the earth.
These rivers are relatively narrow in global terms, only a couple hundred miles across, but they can stretch for thousands of miles making them look like a river.
"They transport massive amounts of water vapor," says Marty Ralph, a researcher with the Scripps Institution of Oceanography and part of the team going out this weekend.
He says imagine all the water flowing from the Mississippi river into the Gulf of Mexico at any given moment. Atmospheric rivers are believed to carry 10 times as much moisture.
Ralph adds that half of all the rain and snow in California comes from these events.
They're a byproduct of global wind patterns created as warm air from the equator interacts with cool air from the poles, as seen in the animation below.
Atmospheric rivers have become a topic of interest to scientists over the last decade, but there is still much to learn.
That's why Scripps and the National Oceanic and Atmospheric Administration are flying a Gulfstream IV aircraft a thousand miles off the coast of San Francisco where a river is forming.
They plan to drop dozens of small censors with parachutes.
"They will measure temperature, pressure, wind and moisture as they descend," Ralph explained.
He says this experiment will give researchers valuable details about atmospheric rivers that would be impossible to get any other way.
For instance, they hope to learn more about where these flows get their moisture. Does it come from the ocean where it forms or does it flow up from the tropics?
Forecasters hope to use this data to better predict where and when an atmospheric river is likely to dump rain and snow on land.
Researchers expect this weekend's atmospheric river to hit the Bay Area on Friday, Saturday and Sunday. Still, it won't drop enough precipitation to end California's ongoing drought.