National Weather Service meteorologists are monitoring a phenomenon developing over the Pacific Ocean that will slam the West Coast by the weekend. Portions of California and the Pacific Northwest are expected to be pounded by an atmospheric river that could bring flooding rain and heavy mountain snow to the Western United States as early as late this week, and it could continue into early next week.
"We've had systems that have characteristics of atmospheric rivers, but this one and potentially some after — it'll be an active pattern — will have the true qualities of an atmospheric river," NWS meteorologist Joshua Wood explained to Newsweek.
"Travel is likely to be impacted by snow, rain, and wind," the Weather Prediction Center cautioned. "In addition to the potential for river flooding, small stream and urban flooding are possible. Interests should continue to monitor forecasts as the situation develops."
Rain and mountain snow will impact Western Washington to Northern California starting late Thursday. The WPC U.S. Hazards Outlook for Thursday through Monday highlights not only the heavy precipitation but also the threat of significant waves and winds along the coast of Washington, Oregon, and Northern California.
Atmospheric rivers are narrow but powerful plumes of moisture that carry much of the world's water vapor outside the tropics. They are like an atmospheric firehose, fueling storm systems that unleash copious amounts of moisture.
"While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River," the National Oceanic and Atmospheric Administration notes. The most intense atmospheric river events can move up to 15 times that amount of moisture.
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This moisture can supercharge atmospheric rivers, making them even more prolific precipitation producers, raising the risk of mudslides and power outages.
"Warmer air holds more moisture, increasing the potential for heavy rainfall in a warming climate," according to researchers at the nonprofit Climate Central. "For every 1 degree Fahrenheit of warming, the air can hold an extra 4% of moisture. The U.S. has already warmed by 2.6 F since 1970 — meaning our atmosphere can hold about 10% more moisture on average."
More moisture in warmer air can boost total atmospheric river precipitation. Research suggests that this thermodynamic relationship could drive future increases in total precipitation during atmospheric river storms in California.
"As human-caused climate change continues to warm the planet, the number of days that the Western U.S. will experience atmospheric rivers is projected to increase," the Department of Agriculture warns. "Some research shows that they are expected to be 25% longer and wider, meaning more rain over more area for longer. Therefore, heavy rainfall and extreme winds caused by these rivers will increase. This also means that the number of atmospheric rivers that cause damage will increase."
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