An explanation of Sonoma County’s atmospheric river

Wet and weary Sonoma County residents may not appreciate it, but the giant system of wind and rain that blasted us this week was severe enough to capture headlines across the U.S., Europe and even China.

What just hit us?

It’s what meteorologists call an atmospheric river event, sometimes referred to as “pineapple express,” a recently discovered phenomenon that’s responsible for many of California’s wild swings in weather, from widespread flooding to record-setting drought.

California has long experienced weather extremes, but up until the mid-’90s scientists were not sure why. Most of California’s winter storms are born in the cold, far western Pacific near Russia. They ride the jet stream east toward North America, and spin out great arcing arms of moisture, hundreds of miles long.

If the storm bands reach California, they bring rain and snow. These cyclonic storms are regions of very low pressure in the atmosphere, and like a great spinning vacuum, they pull in air from far away.

Down along the Earth’s equator, conditions are very different. There, perpetual tropical sun and heated air each day evaporate vast amounts of water from the ocean.

If conditions are right, some of that warm, wet tropical air can be sucked north by the spinning deep low pressure storms off our coast.

When that happens, a “river” of warm, moisture-laden clouds carrying huge amounts of water vapor develops, barely 100 to 200 miles wide, propelled by very high winds and very close to the ocean’s surface.

Stretching up from the tropics, drawn by the spinning low pressure center, they take aim at the coast of California.

They can race across thousands of miles of ocean in a few days. And when these atmospheric rivers roar ashore and meet mountains like those on the Sonoma County coast, they’re pushed up, dropping some of their moisture in the form of dense rain as they cool. The longer they linger, the more rain they release before moving inland, up to 8 to 10 inches or more in a day.

The water these atmospheric rivers carry can be vast. Meteorologists compare them to giant fire hoses. A moderate one transports 7 to 15 times the amount of water discharged by the Mississippi River in a week, and they are unstoppable.

The resulting storms can produce widespread flooding. All the great California floods in recent decades have been caused by atmospheric rivers.

But not every atmospheric river is a flood-spawning monster. What’s important is that on average, these tropical atmospheric rivers deliver up to half of all the state’s precipitation each year. And when they don’t arrive, we experience drought.

Are there more of these rivers out there with California in the bull’s-eye? And what, if anything, can be done to reduce the unpredictability and impacts of atmospheric river extremes, drought and flood?

Those questions are on the top of the agenda for Sonoma County’s Water Agency, which is responsible for the managing the region’s water resources. According to Chief Engineer Jay Jasperse, that includes the water on the ground, and increasingly, water in the sky that has yet to fall to Earth.

Jasperse and other Water Agency staff are now spearheading advanced projects in partnership with an elite team of federal, state and university agencies, to handle atmospheric rivers and the wild variability in water resources they create.

At present, Jasperse said, the agency’s efforts are often hampered by outdated tools and policies that date to the 1950s.

For example, the radar system being used by the National Weather Service to track our weather was designed for thunderstorms in the flat Midwest, not for California’s rugged terrain. As a result, local agencies are blind to low elevation atmospheric rivers, which can lead to severe underestimates of the rainfall they may produce. Jasperse’s agency knew this week’s big storm was coming, but couldn’t accurately predict how much rain would be deposited locally.

It’s a problem, considering that up to 70 percent of flooding in the North Bay has resulted from atmospheric rivers that can’t be detected with the conventional radar.

Can anything be done to reduce the unpredictability and impacts of atmospheric river extremes, drought and flood?

Armed with a $19 million state grant, the agency is working to build a new Bay Area information system using X-Band radar and other instruments specifically designed to monitor and measure atmospheric rivers. That data will create a better picture of what these storms look like in real time so better decisions can be made about storing excess rainwater for later use and help to reduce some flooding.

Jasperse said the agency is also working on joint programs with NOAA, Scripps Marine Laboratory, National Weather Service, California State Water Resource Board and Army Corp of Engineers that would provide more flexible management of water reservoirs, based on advanced atmospheric storm tracking and computer models.

At Lake Mendocino, they are developing what could be the model for a new system that would allow better management of the water levels in statewide reservoirs.

Currently, the Army Corps of Engineers applies rigid and conservative rules about releasing water from area reservoirs when they start to fill. Their first responsibility is to protect lives and property from the ravages of flooding. For example, Lake Mendocino had 94.9 acre feet of water Wednesday, 147.9 percent of storage capacity Wednesday, the Corps made plans to release water on Thursday, when they expect the level of the Russian River at Guerneville to be safely falling. Unfortunately, in years when future storms fail to materialize after the water has been released to prevent reservoirs from overfilling, the county can be left with inadequate supplies for a long, dry summer. That happened in 2013.

If the agency could more accurately predict in real time how much moisture an approaching storm may drop, better decisions could be made about when and how much water should be released, without releasing too much, or too much all at once. Since new dam building is not in the cards, getting the most from existing water management systems is crucial.

Understanding atmospheric rivers is crucial to the managing the state’s water as well. While Sonoma County’s drought was ended last year by two atmospheric river storms, the remainder of California has not been so lucky. And, unfortunately, the warm tropical rains that ride in on the atmospheric rivers can cause snow to melt in the Sierra Nevada during the winter, rather than adding to the snow pack that melts slowly during summer months.

Since nature can’t be relied upon to provide just the right amount of rain at the right time, Sonoma County farmers, ranchers and residents are well advised to keep one eye on the sky, and take some comfort in knowing that a small team of dedicated professionals are working to smooth out the peaks and valleys of our unpredictable, extreme weather.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, snett@californiasparks.com.