A warm, moisture-laden weather system drenched western Washington and Oregon. In early December 2023, a powerful atmospheric river flowed toward the Pacific Northwest of the United States. The long stream of moisture brought warm, wet weather to much of western Washington and Oregon, breaking daily rainfall and temperature records and raising the risk of flooding and mudslides.
Monitor atmospheric conditions
This map shows the total amount of precipitable water vapor in the atmosphere at 1:30 PM Pacific Time on December 4. Precipitable water vapor is the amount of water when all the water vapor in the atmospheric column condenses into liquid. Dark green areas on the map highlight bands of moist air flowing northeast from the tropical Pacific. This image comes from NASA's GEOSADAS (Goddard Earth Observing System Atmospheric Data Assimilation System), which uses satellite data and models of physical processes to calculate changes taking place in the atmosphere.
Impact on terrain and weather records
When a storm hits land, water vapor is pushed up to the surface, where it cools, condenses, and falls primarily as rain. Most of the storm's precipitation fell between the Pacific coast and the ridges of the Cascade Mountains. In the 36 hours ending in the afternoon of December 5, 5-9 inches (130-230 mm) of precipitation are expected in some mountainous areas of Washington state.
Meanwhile, in low-lying areas, Seattle saw a record daily rainfall of 1.51 inches (38.3 millimeters) on December 4, according to news reports. More rain fell in northwestern Oregon on December 5, with 2-6 inches of rain expected in the Coast Mountains that day. Flood warnings were also issued as rivers swelled.
Record-breaking temperatures and water supplies ease
Both Portland and Seattle set new heat records during the storm. On December 4, the maximum temperature in Portland reached 65 degrees Fahrenheit (18 degrees Celsius), setting a record for the highest temperature that day and tying the all-time record for the highest temperature in December. Seattle reached 59 degrees Fahrenheit (15 degrees Celsius) in the early morning hours of December 5, surpassing the previous record high of 58 degrees Fahrenheit (14 degrees Celsius) set in 2020. Warm temperatures across the region are exacerbating flooding risks and melting mountain snow that fell in previous days.
The rainfall will help relieve pressure on the region's water supply. In the Seattle area, an unusually dry summer has raised concerns about whether municipal water supplies and fish habitats will have enough water. Atmospheric rivers occur frequently during the winter and account for 50% of total rainfall and snowfall in the western United States.
NASA Earth Observatory image by Lauren Dauphin, using GEOS-5 data provided by NASA's GSFC Global Modeling and Assimilation Office.
Compiled source: ScitechDaily