Portland is getting a ton of rain right now. We’re in the midst of a weather phenomenon called an atmospheric river.
What is an atmospheric river?
Like an actual river, the weather pattern is long and narrow — and carries a lot of water (in the form of vapor). Meteorologists define the phenomena by measuring the amount of moisture the storm is bringing — over the duration of the storm.
“The average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River,” according to the National Oceanic and Atmospheric Administration.
What’s different with this storm?
Atmospheric rivers are a relatively common weather phenomenon in Portland. But one of this magnitude? “Not that often; this is an outlier,” said meteorologist Adam Batz, who’s with the National Weather Service’s Portland office. It’s lasting longer than our usual ones.
What’s the danger?
There’s a risk of flooding. It’s likely that Johnson Creek will have some flooding (from this morning to tomorrow evening). But there’s also a 50% chance that Johnson Creek will have major flooding — which would mean “extensive inundation of structures and roads.” The National Weather Service advises that if you live in an area prone to flooding, you should monitor the situation and have a plan for leaving.
There’s also a risk of landslides, particularly in areas where there was a recent forest fire. Maybe you would never be tempted to go hiking in the Gorge in this weather. But either way, it's not the best time for that.
Is there a benefit?
We need rain, but at this time of year it's best if the rain is adding to our snowpack, storing it up for the summer. And this storm is bringing relatively warm air with it. That means there's no snow expected in the mountain passes through the weekend (though the peaks might get something). TLDR; Stay tuned, ski bums!
