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Atmospheric Rivers Drench the Pacific Northwest

December 8, 2023
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NOAA satellites are closely monitoring a strong atmospheric river that has been bringing excessive rainfall, flooding, and high elevation snow to the Pacific Northwest, a region already soaked by previous atmospheric rivers. Earlier this week, the National Weather Service issued flood alerts for Washington, Oregon, and Idaho for river and stream flooding, as well as Northern California for heavy rainfall and snowmelt. 

Atmospheric rivers get their names from their long, narrow shape and the prodigious amount of water vapor they carry through the atmosphere–like a river in the sky. They form when winds over the Pacific draw up moisture from the band of warm, moist air over the tropics and channel it toward the West Coast of North America. Atmospheric rivers are the largest transport mechanisms for freshwater on Earth, and when they make landfall, they often release this water vapor in the form of rain or snow over relatively short periods of time.

The heavy rain has caused rivers to rise and reach major flood levels. In Washington, portions of the Skagit, Grays River and Snoqualmie rivers surged to major flood levels by late Tuesday morning, while water levels on portions of the Skokomish River climbed to a moderate flood stage. Major flooding is also occurring along the Stillaguamish River at Arlington, reaching 20.90 feet. That's also well above the major flood stage of 19 feet, and just below its record crest at 21.2 feet.

The sequence of back-to-back atmospheric rivers, called an AR family, began Saturday and did not leave much of a break period before Monday night’s system began. This lack of recovery time is a major factor in the increased risk of flooding. Additionally, the warm air accompanying the rain is melting snow, leading to excessive runoff and raising creek and stream levels. 

This warm, wet air mass also broke rainfall and temperature records in the region. Seattle recorded its wettest day of the year, while Portland experienced its warmest December 4th since 1970. 

Amtrak suspended service between Seattle and Portland, Oregon, due to the extreme weather and a landslide. The heavy downpours also increase the risk of debris flows in areas burned by wildfires, because the ground is less able to absorb the moisture effectively. The flooding has even turned deadly, after rescuers in Portland, Oregon said a man fell into a swollen creek on Monday and couldn't swim out of the swift current.

Not all atmospheric river events are harmful however; lower-level events are beneficial for water supply. However, higher-level events (Level 4 and 5) pose flood and travel risks. The current weather pattern is predicted to persist, with multiple atmospheric rivers expected throughout December. 

To gain a sense of how moisture-laden the atmosphere is, a weather balloon sent from Quillayute, Washington on Monday afternoon measured 1.41 inches of precipitable water (or the measure of water vapor in a vertical column of the atmosphere). A number that high has not been recorded in December in 75 years of record-keeping.

This atmospheric river stems from a storm churning in the Gulf of Alaska, which is drawing subtropical moisture directly from Hawaii to Oregon and the rest of the Pacific Northwest. This weather phenomenon is commonly referred to as the “Pineapple Express,” a specific atmospheric river setup where the jet stream dips south into the tropical Pacific near Hawaii before carrying warm, moist air north and east across the ocean. 

Similar weather phenomena occur frequently around the globe, but atmospheric rivers cause 80% of flood damage at an average estimated cost of $1.1 billion annually on the West Coast of the United States.

NOAA satellites are vital in providing important information about airborne moisture so we can better understand atmospheric rivers to not only make better weather forecasts, but manage water resources, and predict flood risks.