The U.S. West Will See Its First Atmospheric River of the Season

The U.S. West Will See Its First Atmospheric River of the Season
A pedestrian carries an umbrella while walking on a path in front of the Golden Gate Bridge in San Francisco on Wednesday, Oct. 20, 2021. (Photo: Jeff Chiu, AP)

The West in the U.S. desperately needs rain — and it’s set to get some, although it might be too much of a needed thing. An atmospheric river will bring heavy rain and snow to California and parts of the Pacific Northwest this weekend, in what could be the most precipitation the region has seen in nine months.

Atmospheric rivers are long streams of moisture in the sky. These fast-flowing bands of air bring precipitation from the tropical Pacific to the West Coast, dumping copious amounts of rain at lower elevations and snow at higher elevations. Larger atmospheric river events can carry a lot of water vapour; in some cases, its equivalent to the average flow of the Mississippi River, according to the National Oceanic and Atmospheric Administration. The atmospheric river currently building could be extremely intense.

A scale created by the Scripps Institution of Oceanography to categorise atmospheric rivers like the Saffir Simpson scale does for hurricanes puts this one at a Category 5. Unlike the hurricane scale, which is based on wind, the Scripps one factors in location, moisture content, and duration. And the one coming this weekend will be a doozy on those fronts.

This atmospheric river will bring back-to-back storms to the West Coast that start Thursday and will last through the weekend. The pulses of moisture will ramp up intensity over the coming days, with the heaviest blast arriving Sunday night and lasting through Monday morning. Northern California might see up to a foot (30 centimeters) of rainfall through Monday, while the Sierra Nevada mountain range could see up to 3 feet (1 metre) of snow. Outside of California, western Washington could see up to 2 inches (5 centimeters) of rainfall through Monday, and some areas of Washington and Oregon along the coasts could see between 3 and 10 inches (8 and 25 centimeters).

Much of the West has been in drought for months, and less intense rainfall and snow over a more prolonged period would be great news for parched states. Data released Thursday from the Drought Monitor shows that almost 60% of the West is in extreme or exceptional drought, the two worst categories. California’s map is particularly stark, with every inch of the state in some form of drought. That includes a jaw-dropping 87% of the state in either extreme or exceptional drought. In Oregon, 72% of the state is in extreme or exceptional drought, while in Washington the number is 41%. Some places in the region haven’t seen any rain for months: Sacramento just experienced its first rainfall since St. Patricks’ Day, with a hundredth of an inch (a quarter of a millimetre) of rain ending a 212-day dry streak, the longest on record.

A little rain and snow could do wonders, but the more intense precipitation that’s forecast for this weekend is a huge concern. The severe fire season that scorched the West has left fire scars across the region. The unstable topsoil, few trees to hold what soil there is in place, and mounds of debris can combine to form dangerous mudslides and landslides during heavy downpours. That was on display this summer when storms unleashed debris flows on I-70 in Colorado that closed the highway. The heavy rain fell the burn scar left by last year’s Grizzly Creek Fire, underscoring the risk not just from this fire season but also other recent ones.

The National Weather Service has already issued flash flood alerts for areas decimated by this summer’s fires, as well as warnings to be on the lookout for debris flows, landslides comprised of mud, rocks, trees, and, occasionally, manmade objects like homes or cars. In El Dorado County, California, where the Caldor Fire burned more than 221,800 acres this summer, officials warned that homes or cars near steep slopes and running water were at particular risk.

“The upcoming weather event could result in flooding, causing currently unstable trees and other vegetation to fall onto roadways, create landslides, and impact our watersheds,” Brendan Ferry, El Dorado County deputy director of the Tahoe Planning and Stormwater Division, said in a press release. “Ash from the Caldor Fire and soil movement will undoubtedly occur and we are asking that all residents and travellers in these areas and along US Highway 50 be on high alert for these hazards and take appropriate precautions to mitigate the storm’s impact.”

This weather whiplash is nothing new for the region. In January, an atmospheric river caused storms to pound California, as officials issued mandatory evacuation orders for 5,000 people living near burn areas out of caution. In 2019, California was hit with another heavy atmospheric river, which wreaked havoc, causing mudslides, flooding, and severe erosion on highways. In 2018, mudslides ravaged the state after heavy rains fell on parched and wildfire-stricken areas, forcing tens of thousands to evacuate.

This boomerang-style weather alternating between extremely dry and extremely wet may, in fact, become the new normal. Researchers in 2018 found that California is due to increasingly experiencing “precipitation whiplash” due to climate change, with the state swinging wildly between increasingly dry winters and increasingly wet ones. Natural climate patterns also play a role in the state’s weather, such as the just-formed La Niña, which increases the odds of dry weather in Southern California and wet weather in the far northern portion of the state. Buckle up!