It’s official – the rainy season is back. After a bone-dry summer and an unusually long stretch of blue skies in October, Corvallis got its first real drenching of the season this weekend. The source of all that rain—and even a little snow—was a pipeline of tropical moisture driven thousands of miles right to our front door – also known as an atmospheric river.“An atmospheric river is a very moist storm system, in a very narrow concentrated band of moisture and rain,” explains Oregon State University associate professor and state climatologist Dr. Larry O’Neill. “It’s on the order of about 300-500 miles wide. They’re basically rivers in the sky, and they provide a conduit of moisture from the tropics to higher latitudes.”
Here in the Northwest, we generally know them by one of two other nicknames: Pineapple Express, because the ones that hit us tend to originate near Hawaii; and Droughtbuster, due to the ability of these giant influxes of water to dig us out of moderate droughts.
The term “atmospheric river” is a relatively new one, first proposed by two MIT professors in 1992, although these events have been an important part of the west coast’s water supply for as long as we have records of it. According to O’Neill, they provide between 30 and 50 percent of our rain in any given year, and our recent droughts have been due in large part to not getting enough of them.
“In a warming climate, atmospheric rivers might contain more rain,” O’Neill said, but along with increased intensity, climate change may be changing where these storms hit. “There’s some indication that the jet stream might move north a little bit, and with that it would take those atmospheric rivers and all that really beneficial moisture and move it north as well.”
These storms can carry a truly massive amount of water. In 2017, a research team from the University of California San Diego and the National Oceanographic and Atmospheric Administration (NOAA) estimated that an average atmospheric river moved almost 40 billion cubic meters of water per day. That’s more than twice the flow of the Amazon or 27 times the flow of the Mississippi River into the Gulf of Mexico. The biggest events can be almost twice this average.
The Good News & Bad News
Like any big storm, an atmospheric river can be good news or bad—or a mix of the two. We need them to fill reservoirs and build snowpack, but they have also caused some of the most devastating floods in the Northwest’s history. In 1861 and 1862, a series of atmospheric river events destroyed whole communities in the Willamette Valley, including Orleans, across the river from Corvallis, which was never rebuilt. We had another taste of the dangerous side of these storms in April 2019, when the Willamette River rose by 16 feet in less than a week, closing Highway 34 and causing extensive flooding. As is often the case in disasters like this, the unhoused population of Corvallis was especially hard-hit.
In Oregon, both water supply and flood control come down primarily to two things: reservoirs and snowpack. Unlike other floodplains like the Mississippi River delta that rely on levees to control flood water, we lean on reservoirs as our main sponge for soaking up storm rain for our use, and on snowpack to gradually release stored water in the dryer months. Dam releases are another tool for flood control, but as we saw in the 2019 Willamette flood, they can be a factor making floods worse, since dams close to overflow often have to be released at the worst possible time.
“An atmospheric river that’s warm is pretty hazardous because it can rain on our snowpack and all that water comes down all at once,” said O’Neill. “Instead of storing that water up there for summer when we need it, they can flush it down the rivers much earlier than anticipated.”
The difference between disaster and drought relief can be as specific as which side of the storm hits you. On the west coast, atmospheric rivers often form a north-south ridge, with the east side significantly warmer than the west. We saw this in the storm that hit Corvallis this week. We were hit with both sides of the storm—first, the warmer east side, which dumped relatively warm rain on Friday, and then the cooler west edge of the storm, bringing Sunday’s short dusting of snow.
But this early in the season, storms like the one we got this weekend carry much more reward than risk. With our soils in the valley still relatively dry, and reservoirs and snowpack low, this week’s storm poses very little danger of widespread flooding. It’s the perfect time for a good drenching rain.
The water year, a sort of fiscal year for climatologists and water managers, only started on October 1. This water year started out dry with those three weeks of clear weather in October, but according to O’Neill, 2022-23 could be a big year for atmospheric rivers.
“It is a weak to moderate La Nina year, and typically during those years, we average a few more in those years than we do in an El Nino or neutral ENSO year, so the odds are in our favor, hopefully.”
By Ian Rose
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