"Atmospheric River" To Flood Pacific Northwest In Coming Days

This map shows the atmospheric river potetial
later this weekend in the Pacific Northwest. The blue
and green hues hitting the region in this map
Sunday represent a corridor of moisture from the
tropics headed to British Columbia and Washington

It's inevitable that this would happen, as it occurs almost every winter along the West Coast.

An "atmospheric river" is taking aim at Washington State and neighboring coastal British Columbia later this weekend and maybe on into early next week. That almost always means at least some flooding.

I'll let NOAA define what an atmospheric river is for us:

"Atmospheric rivers are relatively long, narrow regions in the atmosphere - like rivers in the sky - that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow."

As you can imagine, there's an absolute huge shitload of water that comes through the lower Mississippi River, so you can imagine that these atmospheric rivers can unleash an incredible amount of rain or snow.

The upcoming atmospheric river in Washington and British Columbia won't be at all extreme, certainly in comparison to some epic ones in the past.

Some of the nation's worst floods both in historic times and in recent years have been caused by atmospheric rivers.

In the winter of 1861-62, probably the worst and most persistent atmospheric rivers struck California. The firehose of water shifted a bit during the winter from southern to northern California and back again, but it just wouldn't quit.

Los Angeles got 66 inches of rain, four times its annual normal. Sacrameto remained submerged in ten feet or more of water for months. Telegraph poles in some areas were completely under water.  Nowadays, California has nearly 40 million people, so you can imagine what a mega-disaster we'd see if 1862 happened again.

Atmospheric rivers don't just hit the West Coast. They've caused immense flooding in Great Britain, for example. The Gulf of Mexico, Caribbean and Atlantic Ocean can trigger mean atmospheric rivers. An extreme Tennessee flood in 2010 was caused mostly by such a set up, for example. A catastrophic South Carolina flood in 2015 was in large part caused largely by at atmospheric river

I don't mean to be alarmist. Most atmospheric rivers aren't as destructive as the ones I just outlined. Not all atmospheric rivers are historic, and the one about to hit Washington State and British Columbia won't be the Worst Ever.

One saving grace is the atmospheric river will move, first hitting British Columbia then sinking south into Washington, so any one place won't have days of torrents.

Still, flooding is very likely in the western half of Washington State. The atmospheric rivers that come into the West Coast do have their origins in the tropics, as most of them do, so snow levels in the Cascades will rise quite a bit.

That means rain will fall at high elevations. There won't be that much snow to capture the water, and the atmospheric river will melt some of the snow in the high elevations. The combination of heavy rains and snowmelt will make the risk of flooding that much worse.

Sometimes, atmospheric rivers, once they're done with the West Coast, help spin up pretty intense storms in the central and eastern United States. This time, the atmospheric river in the Pacific Northwest might end up being one ingredient that could spin up a fairly strong storm in the eastern United States in about a week.

That scenario, though, is still quite uncertain.