It’s time for new weather term to enter your lexicon: atmospheric river. Since last week, we Portlanders have been living under a series of back-to-back atmospheric rivers. So many, in fact, that they’re called an “atmospheric river family.”

An atmospheric river is a 250–375 mile-wide band of water vapor in the atmosphere, like a river in the sky, that pulls moisture-saturated air from warm tropical waters either northward or southward. When an atmospheric river reaches land, it dumps rain and snow, which is normal—typically a third to half of West Coast precipitation comes from atmospheric rivers.

But this week’s atmospheric rivers are unusual in their ferocity, expected to drop near-continuous rain, which brings the possibility of urban flooding, especially in areas like underpasses, as was already seen on Sunday in Portland on I-84. Due to high rain levels that melt snowpacks, the National Weather Service has placed Northwest Oregon under a flood watch through Wednesday, December 6. The National Weather Service reminds drivers and pedestrians to avoid flooded roadways rather than attempting to cross them. Many Northwest Oregon and Southwest Washington rivers have a 20 to 40 percent chance of reaching minor flood stage, while the Wilson River in Tillamook has a 65 percent chance. The Willamette and Columbia rivers look less likely to flood, at around 5 percent.

Things look scarier at high altitude. Parts of Mount Hood are at high risk of avalanche, with greater risk at elevations above 5,000 feet, according to the Northwest Avalanche Center’s avalanche forecast. Specifically, there’s danger of wet slab avalanches, in which rain loosens a large, slab-like layer of snow that has potential to be destructive. Stay safe, Oregonians.