Atmospheric Rivers, Famous On U.S. West Coast, Are A Peril To East, Including Vermont, Too

The atmospheric river that caused the big New England
flood of December 18-19 turned the tiniest stream
in front of my house into a raging river too.

 The December flooding that befell New England, especially Maine, New Hampshire and Vermont, was largely the handiwork of an atmospheric river.  

If you've heard of atmospheric rivers, you probably associate them with storms that sometimes slam into the West Coast during the winter. 

But these so-called atmospheric rivers can come off of the Atlantic Ocean or Gulf of Mexico, too. As we've seen lately, these sometimes have disastrous results. 

As the Washington Post describes it in an article published Thursday: 

"Atmospheric rivers - jets of intense precipitation that curl around powerful ocean storms - are generally considered a West Coast phenomenon. But it turns out a top tier atmospheric river - rated Category 5 on a scale of 1 to 5 - was part of the East Coast storm that killed at least five people and cut power to more than 800,000 customers this week."

Every state along the eastern seaboard from Florida to Maine - including Vermont - saw at least four inches of rain and wind gusts of over 50 mph. 

The December storm will almost certainly join the long list of weather disasters this year costing $1 billion or more. 

Atmospheric rivers are long narrow corridors of deep tropical moisture that are pulled into the mid-latitudes by storms. These atmospheric rivers usually aren't that wide in the grand scheme of things. The one that just hit us in New England was probably about 400 miles wide.

But they can be thousands of miles long. Some of the atmospheric rivers that strike the West Coast extend as far west as Hawaii, which is why those storms are often dubbed a "Pineapple Express."

The distance between Los Angeles and Honolulu by the way is about is about 2,500 miles.

The reason we hear so much more about atmospheric rivers on the West Coast than elsewhere is because of geography.

The West Coast atmospheric rivers that come generally west to east across the Pacific encounter California, Oregon and Washington with land that rises from sea level to mountains in perpendicular to the flow of this wet air.  

That atmospheric rivers collision with mountain ranges such as the Sierra Nevada forces the air to rise.   The rising air forces all that moisture in the atmospheric river to condense in rain and snow, which dumps big time on the Pacific coast. 

One reason why the December flooding was actually worse than the already bad inundation in Vermont was for the same reason California gets slammed by those atmospheric rivers,

The one last week that came off the Atlantic approached New England from the southeast. The air flow was only partly perpendicular to the north/south oriented Green Mountains, so the rising air releasing moisture was impressive, but not as efficient as it was in New Hampshire and Maine.  

Map shows strong atmospheric river hitting the Northeast
and a weaker one hitting California on December 18.
Map and data from the Center for Western Weather
and Water Extremes. 

The White Mountains of New Hampshire and the mountains of western Maine run generally southwest to northeast. That was a perfect arrangement to wring out the most rain possible.

Other historic storms in New England - including Vermont  - were products of atmospheric rivers.   The Great Flood of 1927 in Vermont also stemmed from an atmospheric river off the Atlantic Ocean. 

In some unlucky years, you can have multiple atmospheric rivers. In October 1996, an episode caused severe flooding in eastern New England, and another one in November the year triggered serious flooding in eastern New York.

Atmospheric rivers can also cause flood disasters far from a coastline. One such event in May, 2010, caused what was probably the worst flood in Tennessee history. 

These atmospheric rivers are surprisingly common along the East Coast, actually. One estimate pegs the number per year to about 40 in the Mid-Atlantic states, though they are seldom severe enough to cause significant flooding. 

Atmospheric rivers are categorized in a one to five ranking system, similar to tornadoes. Not surprisingly, the one we had on December 18 was a Category 5, the most severe you can get. 

But here's something to worry about. As the Washington Post points out, atmospheric rivers have been around ever since there was an actual atmosphere on Earth.

Climate change, though, is tending to make them worse, in general. Per WaPo:

"....data suggests they're becoming wetter. Winter is the fastest-warming season run the Northeast and Mid-Atlantic, and warmer air can hold and transport more water and thus release more of it. In fact, for every degree Fahrenheit the air temperature rises, the air can hold 4 percent more water. Most atmospheric rivers happen in the cool season."

The bottom line for us in Vermont is: Most atmospheric rivers will remain tame and not cause too many problems. But a growing percentage of them will cause trouble, expense and danger like the one a week ago did.