Rivers in the Sky Carry 15 Mississippis — And Forecasting Them Will Save Cities by 2035

The Question

You cannot see it from your window, but on a stormy winter day a river may be flowing directly over your head. Not a river of water you could touch — a river of vapor, a ribbon of moist air hundreds of miles wide and thousands of miles long, streaming out of the warm tropics toward colder land. Scientists call these atmospheric rivers. When one slams into a coastline and rises over mountains, all that vapor squeezes out as rain and snow, sometimes for days.

Here is the number that makes people blink. A strong atmospheric river can transport up to 15 times the amount of water that flows out of the mouth of the Mississippi River — through the sky, in a single band of air. The Pacific version that soaks California has a nickname: the "Pineapple Express," because it draws moisture from near Hawaii. And here is the twist that makes forecasting them so urgent: the very same storms that drown towns also fill the reservoirs that keep those towns alive. The question is whether we can learn to read these rivers precisely enough to catch the water without drowning in it.

What the Evidence Shows

Atmospheric rivers are both the West Coast's greatest water gift and its greatest flood threat, and that dual nature is now measured, not guessed. Researchers at Scripps Institution of Oceanography estimate these systems deliver a large share — often cited as up to half — of California's total water supply in a typical year. Yet the same studies show atmospheric rivers are responsible for the majority of the West Coast's flood damage, because when several arrive back to back, the ground saturates, rivers overtop, and hillsides slide.

To make the danger legible, scientists built a scale. In 2019, a team led by Marty Ralph at Scripps introduced a five-step rating — AR1 to AR5 — that works much like the categories used for hurricanes. A low-end AR1 is "mostly beneficial," a welcome drink of rain. A high-end AR5 is "mostly hazardous," a days-long deluge. The winter of 2023 showed the top of the scale in action: a relentless parade of atmospheric rivers struck California, and by some estimates dumped on the order of 30 trillion gallons of water on the state in a matter of weeks, causing deadly flooding even as it ended a punishing drought.

The forecasting is improving fast because scientists now fly straight into these storms. Under a program called AR Recon, aircraft cross the Pacific during atmospheric river events and release dropsondes — small sensor packages that parachute down through the storm, radioing back readings of temperature, moisture, and wind on the way. Those measurements, fed into weather models, have measurably sharpened predictions of where and how hard an atmospheric river will hit. That new precision is already changing how a single dam is run at Lake Mendocino in California, under an approach called Forecast-Informed Reservoir Operations, where managers release or hold back water based on what the incoming storms are forecast to do.

Why This Is Happening

The stakes are simply enormous, on both sides of the ledger. Water scarcity and flood disaster are two of the costliest problems the American West faces, and atmospheric rivers sit at the heart of both. Any tool that shifts the odds — capturing more supply, dodging more damage — pays for itself many times over. That economic gravity pulls research money, aircraft, and political attention toward the problem year after year.

The science matured just as the observing tools arrived. The AR category scale gave forecasters and the public a shared language, the same way hurricane categories did decades ago. At the same time, reconnaissance flights and better satellites started filling the ocean's data gaps. When a clear framework meets fresh measurements, forecast skill tends to climb quickly — that is exactly the path hurricane tracking followed.

It is not only a Californian story, which spreads the effort. Atmospheric rivers strike the United Kingdom and Western Europe too, driving major floods there, and they matter for the Pacific Northwest, Chile, and beyond. Because so many regions share the threat, research collaborates across borders, and improvements in one place quickly benefit others. A global problem attracts a global scientific push.

What Could Happen

What Can We Do

You cannot steer a river in the sky, but you can learn to live with one — and the coming forecasts only help people who know to use them.

Learn the AR scale before it appears in your forecast. Just as you know a Category 4 hurricane is serious, get familiar with what an AR4 or AR5 means: a prolonged, dangerous soaking. When these ratings enter mainstream weather reports, you will already understand the warning, and you will act sooner than a neighbor who is hearing the term for the first time.

Know your local flood and landslide risk now. Atmospheric rivers do their worst damage on saturated ground and steep slopes. Find out whether your home sits in a floodplain or below an unstable hillside, and make a plan for evacuation and documents. The forecast buys you time only if you already know what to do with it.

Support forecast-informed water management. The Lake Mendocino model — using storm forecasts to decide when to store and when to release — turns a flood risk into a water supply. Back its expansion to other reservoirs in your region. It is one of the rare policies that fights drought and flooding at the same time, using nothing but better information.

Value the flights and satellites that make it possible. Reconnaissance missions and Earth-observing satellites are the unglamorous backbone of this progress. When science budgets are debated, remember that a single well-forecast atmospheric river can save more than the program costs in a year. Public support keeps the planes flying into the storm.