An atmospheric river is tearing through Western Washington this week, dumping massive quantities of water and flooding some areas in the region.
And that rain will keep falling for days.
If you feel like these climatological phenomena are hitting harder when they make landfall, you’re not alone. Some scientists think so too and figure our warming atmosphere and oceans might be at least partially to blame. Whatever the cause, we need the moisture. Washington is fresh on the heels of three severe drought years in a row. But so much rain at once can produce severe flooding.
This specific atmospheric river was forecast as an AR5 when it made landfall, the top of the scale for strength. It’s among the top 1%-2% strongest and longest-lasting the region has seen since 1959, said Samuel Bartlett, meteorologist with the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, citing an atmospheric river catalog developed by the center.
The center is launching weather balloons and coordinating research flights by a C-130 Hurricane Hunter aircraft into the atmospheric river, to collect air temperature, pressure, water vapor, wind direction and wind speed observations and feed forecast models. These extra observations aim to improve precipitation forecasts, which can help residents on the ground receive early and accurate flood warnings and inform water storage decisions.
Depending on your location, the moisture brought by the atmospheric river are likely to pour anywhere from 2 to 4 inches of rain over our lower elevations and as much as 10 inches higher up in the Cascades, said Logan Howard, a meteorologist at the National Weather Service in Seattle. Those deluges will be paired with gusts of wind up to perhaps 45 mph and higher-than-average temperatures up to the mid 50s.
The rains are likely to dip down and cover portions of northern Oregon Tuesday before jumping back up toward Washington, Howard said. They’re not likely to dissipate until around Thursday.
Already rivers, streams and streets across the region are overflowing.
This flood risk can be particularly pronounced in lowlands.
Floodwaters stranded several people in their cars overnight in Snohomish County. The Skykomish River near Gold Bar peaked Monday night 5 feet above its flood line. And some forecast near-record flooding for the Snohomish River.
To illustrate how things have changed this week, look at the Snohomish's flow on Thursday, when it was running at about 5,720 cubic feet per second.
Around 10:30 a.m. Tuesday, the river crested at around 63,600 cubic feet per second. For simplicity’s sake, envision each cubic foot as a basketball.
That’s a lot of basketballs.
Many of Western Washington’s rivers are currently flowing at rates the U.S. Geological Survey considers “extremely above” normal right now and the trend will continue a while after the rain stops falling.
So keep your rain gear and galoshes handy.
Pineapple express
These atmospheric rivers aren’t new, even if it feels like you’re hearing the term more often. You probably are. They’ve undergone a sort of unofficial rebranding in recent years.
Atmospheric rivers were often called “pineapple expresses.”
Whichever title you prefer, in simple terms what these atmospheric rivers do is to pull warm air from subtropical zones (north or south of the equator) and bring with it a plume of moisture, said Washington State Climatologist Guillaume Mauger. When they arrive, here in the Pacific Northwest or wherever else, they release that moisture in low pressure areas in the form of heavily concentrated rainfall.
“A little bit like a fire hose,” Mauger said.
Most (but not all) of our big storms here come from atmospheric rivers, Mauger said. This particular one is large but it’s probably not a record-setter, he said.
This is, after all, our rainy season.
A warming atmosphere
Atmospheric rivers are often relied on for a big portion of wintertime precipitation — they can fill reservoirs, dust the mountains with ample snow to boost spring runoff. And they can be dangerous — fueling the flooding rippling across the region now.
As the planet warms, we may actually see fewer atmospheric rivers, but when they do happen, they’re likely to be more extreme, said Shel Winkley, a meteorologist with Climate Central.
There could be a decrease of about 10% of atmospheric rivers seen today by the end of the 21st century, according to a 2018 NASA study. And climate change is projected to make atmospheric rivers 25% wider and longer, effectively doubling the amount of wind and rain they deliver when they hit land.
A warmer atmosphere can hold more moisture. For every one degree Fahrenheit of warming, the atmosphere can hold onto 4% more moisture. Meanwhile, as ocean waters warm, they are releasing more water into the atmosphere via evaporation.
Taken together, these conditions can fuel heavier rains.
Ocean temperatures along the path of the atmospheric river are about 1 degree Fahrenheit warmer than normal off the coast of Washington and Oregon, and about 2 degrees warmer north of Hawaiʻi.
Those water temperatures are at least 10 times more likely to be this warm due to climate change, according to Climate Central’s Climate Shift Index.
The air that’s moving in with this atmospheric river is also warmer than normal, bringing temperatures anywhere from about 5 to 25 degrees warmer than normal across Washington.
These are temperatures that would be rare or almost impossible at this time of year without the influence of climate change, Winkley said.
“As we continue to warm the planet through pumping heat trapping pollution into the atmosphere, these very normal rain events due to atmospheric rivers have a very good potential of becoming even more inconvenient or more detrimental,” Winkley said.
Beggars can’t be choosers
Washington’s been drier than normal for years now. So dry that state officials declared drought emergencies three years in a row.
These droughts have cut into our ability to generate hydropower, our crop production, our water supply and our environment. Some of our reservoirs sank to new lows this year, state officials cut off surface water rights for massive quantities of farmland and even entire cities. Some irrigation districts ran out of water even before that.
To say we need the moisture would be an understatement.
So, by and large, this much rainfall is a welcome development. But it’s by no means a magical end to our water woes.
At the moment, nearly every square inch of Washington remains abnormally dry, according to the U.S. Drought Monitor. The central portion of the state sits in “severe” drought while the southeast corner suffers from “extreme” drought.
The state’s reservoirs are typically low this time of year but even so, they’re exceptionally low, said Caroline Mellor, the Department of Ecology’s drought coordinator. The five reservoirs within the Yakima River basin sit at less than two thirds their normal levels for mid December.
Any and all moisture is good news, Mellor said. But because it’s falling as rain we can’t capture as much as we’d like.
And as the atmosphere warms more, the Pacific Northwest is increasingly expecting to see more of its precipitation as rainfall rather than snow.
Most reservoir operators only fill their systems so much this early in the season, leaving plenty of room for water in case of extreme weather, and to ward off the potential for widespread flooding. Once the reservoirs hit their flood-control levels, any rainfall that follows must flow out to the ocean.
What this state needs now more than rain, Mellor said, is snowfall. But temperatures aren’t cold enough to freeze the precipitation across the lower elevations. If the trend continues, these atmospheric rivers will represent a giant lost opportunity as we watch much of their water flow into Puget Sound.
It’s early in the season yet but already the state sits at less than half its normal snowpack level, Mellor said.
Snowpack on some peaks in the Olympics or Cascades still sit completely barren or in the single-digit percentages. Unless that white fluffy stuff starts building up fast and remains until late in the spring, we run the risk of yet another drought in the year ahead.
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