The Pacific Northwest is experiencing a sequence of atmospheric rivers that are expected to bring heavy rain to Oregon and Washington. These weather systems coincide with king tides—annual high tides—and rising sea levels, creating conditions that meteorologists say increase the risk of flash flooding, urban flooding, and landslides.
Atmospheric rivers are narrow bands of water vapor, often reaching 2,000 miles in length and 600 miles in width, that transport moisture through the atmosphere. F. Martin Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography, describes them as "a river in the sky." Approximately 50% to 60% of California's annual rainfall is generated by these systems, according to Josh Weiss of the National Oceanic and Atmospheric Administration (NOAA).
Two atmospheric rivers already moved through the region earlier in the week, dropping multiple inches of rain. Forecasters expect two additional systems to move across Oregon and Washington on Friday and Saturday. National Weather Service meteorologist Jeff Michalski stated that the Washington coast could see 20-foot swells during the weekend.
While atmospheric rivers are a natural climate pattern, their impact can be intensified when they occur alongside El Niño conditions and king tides. This combination can lead to higher water levels in waterways and along shorelines. A previous "Pineapple Express" system in late October caused flooding and multiple deaths in the Four Corners region, illustrating the potential severity of these weather events.
Climate scientists report that while a warming atmosphere holds more water vapor, there is currently insufficient data to directly link the weather pattern to human-caused global warming. Research by F. Martin Ralph also suggests these systems do not necessarily become stronger during El Niño years. Following the systems expected this Friday and Saturday, meteorologists will continue to monitor moisture levels throughout the winter season, which is predicted to be warmer and wetter.