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Prepare for a soggy Rose Parade.
An unusually strong “atmospheric river” approaching from Washington state will descend into Oregon today and bring Portland an inch of rain over the weekend, the height of the Rose Festival and Fleet Week.
Atmospheric rivers, systems that transport water vapor from the tropics through narrow strips of the atmosphere, brought heavy rain to the Pacific Northwest this past November and February. The timing of this weekend’s atmospheric river strikes meteorologists as surprising.
“As far as atmospheric rivers are categorized by strength, this one is categorized as much stronger than the ones we have usually seen, and we are not used to seeing them in June,” National Weather Service meteorologist Treena Jensen tells WW.
An #AtmosphericRiver has begun pushing into WA and start spreading south into OR later today, to begin a long period of rainy weather expected through the weekend. Two disturbances will result in bouts of heavier rain tonight as well as Friday night. #ORwx#WAwx pic.twitter.com/y5AI0XdWyD
— NWS Portland (@NWSPortland) June 9, 2022
The average precipitable water—or the amount of rainfall that would be generated by the release of all water vapor in a column of air—in Oregon on a typical day in early June is 0.7 inches. Forecasts for this evening indicate an estimated precipitable water value of 1.6 to 1.8 inches, comparable to the highest amount in one day ever documented by the NWS from its data collection site in Salem: 1.73 inches.
However, the historically high average precipitable water value forecasted for this weekend’s atmospheric river will not deliver as much rainfall as previous atmospheric rivers did.
“The dynamical part of this system isn’t as strong as the previous ones,” NWS meteorologist Lisa Kriederman tells WW. “In terms of amount of moisture that’s contained in the system, there’s quite a bit more, but you have to look at the whole picture. Is the atmosphere able to wring out all the moisture?”
To explain why the historic strength of this weekend’s atmospheric river does not necessarily equate to historic amounts of rainfall, Kriederman compared the dynamics of an atmospheric river system to a sponge.
“If you move it over the countertop without squeezing, it might just release a couple drips,” Kriederman tells WW. “But if you squeeze it while moving it across, you’re likely to wring out a lot of moisture.”
Through Saturday night, the NWS projects 1.5 to 3 inches of rain across Willapa Hills and the Coast Range, 2 to 4 inches across the Cascades, and 0.5 to 1 inch across the Willamette Valley.
This weekend’s atmospheric river follows a historically wet month of May for Portland. With 3.78 inches of rain, this past month ranked as the 10th-wettest month of May for Portland, according to NWS data. The average rainfall in May for Portland is 2.51 inches.