Meteorologists say warm water and air and unusual weather conditions traced to tropical cyclone flooding in Indonesia have increased the flow of stubborn atmospheric rivers, dumping nearly 5 trillion gallons (19 trillion liters) of rain on Washington state over the past seven days, threatening record flood levels.
The heaviest, most persistent heavy rain will continue into late Thursday and early Friday morning, watering the same spots, before the moisture weakens and moves in slightly. But Matt Jeglum, acting science chief for the National Weather Service’s western region, said the moisture “fire hose” on the West Coast likely won’t end until Christmas week.
Atmospheric rivers are long, narrow strips of water vapor that form over the oceans and flow through the sky, transporting moisture from the tropics to northern latitudes. Meteorologists say the Pacific Northwest sees dozens of these types of weather events each year, more than the rest of the West Coast. But they’re usually not this big.
Washington state climatologist Guillaume Mauger said Wednesday’s downpour combined with Monday’s rain is expected to result in record flooding, particularly on the Skagit River, which flows through northern Washington state and flows into Puget Sound.
“The atmospheric river, the AR, is constantly reloading,” said Ryan Maue, a former chief scientist at the National Oceanic and Atmospheric Administration who is now a private meteorologist. “You know, you could get 20 to 30 inches (51 to 76 centimeters) of rain in three weeks. That’s really extreme.”
Mauer added: “I don’t want to live there. Not now.”
Based on rain gauge observations, Moir estimated that nearly 5 trillion gallons of rain fell in the region over the past week, enough to fill Oregon’s Crater Lake or more than 18,000 Empire State Buildings. A weather station on Mount Rainier measured 21 inches (53 centimeters) of rain since Thursday, Jeglum said.
“These are large numbers, but they are not unheard of,” Mogg said.
The moisture originates hundreds of miles north of Hawaii, where Pacific Ocean temperatures are several degrees warmer than normal. Meteorologist Jeff Masters said this further exacerbates the flow of atmospheric rivers, which is further exacerbated by warmer air. He said because the weather was so warm, much more moisture fell in the form of rain than snow.
Mauer said the storms were “intensified by a series of events that began two weeks ago” and were farther away than Hawaii.
He pointed to an area near Indonesia that suffered deadly flooding from tropical cyclones. This coincides with a natural seasonal weather pattern (the Madden Julian Oscillation) that changes every 30 days or so, and Moir said it was the strongest at this time of year in decades. The waves it sends out help carry “unbroken lines of moisture” and energy from Indonesian events to the Americas. A ridge of high pressure off the coast of California pushes atmospheric river systems northward, and unusually warm skies over Russia and cold skies over Alaska further push atmospheric river systems northward.
Washington became the bullseye.
Atmospheric river storms will become larger and wetter as the world warms from the burning of coal, oil and natural gas, according to studies, computer models and meteorologists. A study earlier this year found that this may already be happening. By looking at events since 1980, the researchers calculated that the region where storms are located has seen 6 to 9 percent more storms, is 2 to 6 percent more frequent, and is slightly wetter than before.
A rapid analysis of heavy rains by Climate Central found that ocean temperatures beneath atmospheric rivers were ten times warmer than normal due to human-caused climate change. Temperatures in the Pacific Northwest are much warmer than normal, and that scenario is four to five times more likely due to climate change, said Climate Central meteorologist Shel Winkley.
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