Normally, as I sit at my desk in front of a window that overlooks the San Francisco Bay, I can see much of Berkeley, all of Oakland, including the downtown area with its cluster of office buildings, and the usually busy Bay Bridge that connects the East Bay to San Francisco.
For the past five days I haven't seen any of it. That's because there have been back-to-back storms dumping rain all throughout the Bay Area (and much of the West Coast), from Oregon down into the Sierra Nevada Mountains — where they are getting massive snowfalls — through my part of town, and all the way down to Los Angeles.
An atmospheric river or AR is causing the intense storms that we're seeing, the likes of which the state hasn't seen in a decade.
It's an apt name — the Earth Systems Research Laboratory at NOAA describes them as "... relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. While ARs come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods."
While most of these rivers are weak, when they're not, they can wreak havoc — and that's exactly what's happening in California.
You can see the breadth of the problem in the video below:
And it's not just rainfall; ARs can include strong winds and water vapor in addition to rainfall. In fact, I've seen the thickest fog I've ever seen here during this series of storms.
These rivers can act like their terrestrial cousins, flowing at a regular (or reduced) rate, or getting into extreme events, like the one going on right now. A strong AR can move 7.5 to 15 times as much water as the average flow on the Mississippi.
Considering the years of drought the West Coast has been experiencing, these events can be useful: "On average, about 30-50 percent of annual precipitation in the West Coast states occurs in just a few AR events, thus contributing to water supply," according to NOAA. But too much water all at once means that some of it comes down quickly and washes off into the ocean, meaning it doesn't make its way into aquifers and other natural water storage systems. So a large AR like the current one may not alleviate drought as much as you might expect just from looking at rainfall totals.
But where the precipitation falls as snow, that contributes to the snowpack, which has been light for the last several years — an extra-thick snowpack in the Sierra Nevada mountains means more water come spring and summer as those snows melt slowly. (But if melting happens fast, that can cause flooding too.)
This current storm system has seen plenty of flooding in areas where the precipitation has come down as rain, some of it extreme, as in the Truckee River in Nevada, which overflowed into Reno. And trees are down everywhere (including a 1,000-year-old redwood.)
The term atmospheric river has only been around since 1998, when MIT researchers coined the term to help people understand the concept of vast amounts of water vapor travels on four-to-five narrow bands of the atmosphere. It's a sort of magical and terrifying vision, of rivers of water just above all our heads — but it does help me understand where all this water is coming from.