You can see it on satellite imagery—a chaotic blur of wind and water shaped like a giant alien starfish over Australia, extending a tentacle diagonally across the Pacific and right into the Golden Gate. It’s called an atmospheric river, over a thousand miles of water and wind.

This weekend is going to be as wet as an entire typical April. On the official scale, the storm will probably top out at “strong”—it might have enough water vapor transport to rate as “extreme,” but probably won’t reach “exceptional,” the real freak-out level of flood danger. That means several inches of rain. This is how California gets most of its disastrous floods, but also most of its drinking water. It’s a crazy way to do business, but understanding these atmospheric rivers might be the key to making sure the human race doesn’t die of thirst.

Atmospheric rivers come ashore on nearly every west-facing coastline on Earth. A few times a year one of them does what this weekend’s storm will do: shotguns over Hawaii and into Northern California. “It tails all the way back into the tropics, almost to the date line,” says Michael Anderson, the California State climatologist. “It’s going to come through pretty quick.”

Onshore, the fast, wet air from one of these so-called Pineapple Expresses ricochets upward off the coastal mountain ranges like a tennis ball catching the top of the net. The air rises; the water falls. That’s called “orographic enhancement,” and it’s how you get massive bursts of rain and floods.

Somewhere between 30 and 60 percent of California’s entire water supply comes from atmospheric river storms. And they are intense. In one study of six years of California rainfall, monitored hour-by-hour, just 1,460 hours qualified as atmospheric river storms. But that scant 2.8 percent of time accounted for over 50 percent of the rainfall the researchers measured.

California hoards more than half of that rain as snowfall on the tops of the Sierras. The rest it tries to save in reservoirs—a dozen big ones and about 150 littler ones scattered up and down the state. In the rainy winter months, the reservoirs fill up—ideally—leaving space for the snowmelt to come in the summer. Ideally.

During the winter, all the reservoirs stay empty enough to absorb flood waves from storms. “But on April 1, that reserve space starts being relaxed so they can capture snowmelt,” Anderson says. “With a storm this time of year, you still have that flood reserve space, so there’s room for the reservoirs to catch that water. The question will be, as that reserve space is being relaxed, can that water be kept for the conservation pool?”

It’s a balance—hold onto water for a rain-free summer and possible drought, be ready to capture snowmelt, but leave enough room to store a flood wave during the storm season. Every reservoir has a rule curve that says how much water is too much. But atmospheric rivers can make it tough to know when to apply those rules. In November and December of 2012, the Lake Mendocino Reservoir absorbed water from two atmospheric river events, enough so that the water encroached into the “flood pool,” which is supposed to stay empty for emergencies. “So the reservoir operators released the water. It was the prudent thing to do,” says F. Martin Ralph, a research meteorologist at UC San Diego's Scripps Institution of Oceanography. “But nobody knew that there would be no more atmospheric rivers basically for the rest of the year, and that was the beginning of the drought. They never had a chance to refill the reservoir in the spring.”