National Weather Service meteorologists are monitoring a phenomenon developing over the Pacific Ocean that will hit the West Coast this weekend. Parts of California and the Pacific Northwest are expected to be hit by an atmospheric river that could cause flooding and heavy snow in the mountains of the western United States later this week, and could continue into early next week.
“We’ve had systems that have characteristics of atmospheric rivers, but this one and potentially some later ones (it will be an active pattern) will have the true qualities of an atmospheric river,” NWS meteorologist Joshua Wood told Newsweek.
“Travel is likely to be affected by snow, rain and wind,” the Weather Prediction Center warned. “In addition to the possibility of river flooding, urban and small stream flooding is also possible. Stakeholders should continue to monitor the forecast as the situation develops.”
Rain and snow in the mountains will affect western Washington to northern California beginning Thursday night. The WPC US Hazards Outlook for Thursday through Monday highlights not only heavy precipitation but also the threat of significant waves and winds along the Washington, Oregon and Northern California coasts.
Atmospheric rivers are narrow but powerful columns of moisture that transport much of the world’s water vapor away from the tropics. They are like an atmospheric fire hose, feeding storm systems that release large amounts of moisture.
“While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor approximately equivalent to the average flow of water at the mouth of the Mississippi River,” notes the National Oceanic and Atmospheric Administration. The most intense atmospheric river events can move up to 15 times that amount of moisture.
This moisture can overload atmospheric rivers, making them even more prolific precipitation producers, increasing the risk of landslides and power outages.
“Warmer air holds more moisture, increasing the chance of heavy rainfall in a warmer climate,” according to researchers at the nonprofit Climate Central. “For every degree Fahrenheit of warming, the air can hold about 4% more moisture. The United States has already warmed 2.6°F since 1970, which means our atmosphere can hold about 10% more moisture on average.”
Higher humidity in warmer air can increase total atmospheric precipitation from rivers. The 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 United States 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, which means more rain in more areas for longer. Therefore, the heavy rainfall and extreme winds caused by these rivers will increase. This also means that the number of atmospheric rivers causing damage will increase.”
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