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 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.

The storm unleashed at least 4 inches of rain and wind gusts over 50 mph in every Eastern Seaboard state from Florida to Maine. Along the coast, the storm’s wind shoved ashore an ocean surge of at least 2 to 4 feet, inundating low-lying roads.

Boosted by the atmospheric river, the storm generated exceptional rainfall that caused creeks, streams and rivers to overflow. The river in the sky drew record-setting warmth from the tropics northward, melting snow in the mountains of the Northeast, which made the flooding even worse.

The impacts were wide-reaching and continue in New England, where river levels are only beginning to recede. In Hartford, Conn., the Connecticut River remains at its highest level since Aug. 31, 2011, during the passage of Hurricane Irene.

Sodden soils, drenched by a general 2 to 5 inches of rain, made it easier for trees to topple in winds that reached 70 to 90 mph in eastern New England. That resulted in widespread power outages, at one point leaving half of Maine in the dark.

The East Coast is used to windy storms in the wintertime; “nor’easters” are a staple of the cool season. But the Category 5 atmospheric river – which drew moisture from the Pacific Ocean, Atlantic Ocean and the Caribbean Sea – is what set this storm apart from so many others.

Are atmospheric rivers normal on the East Coast?

An atmospheric river is a long, narrow ribbon of deep, tropical moisture that is pulled into the mid-latitudes by a storm. In the Northern Hemisphere, storms can tug these narrow moisture plumes and stretch them over thousands of miles to the north and east. An atmospheric river that hits California sometimes has extended from as far west Hawaii, and the phenomenon has thus earned the moniker the “Pineapple Express.”

Atmospheric rivers affect the East Coast just as much as the West Coast, if not more frequently. But they’re seldom talked about.

The Mid-Atlantic states are affected by about 40 per year, according to Jay Cordeira, an atmospheric scientist at the Center for Western Weather and Water Extremes at the University of California at San Diego. But he said they do not tend to have as serious an impact on the East Coast as on the West Coast, where they often lead to double-digit rainfall totals and dump 5 to 10 feet of snow in the Sierra Nevada.

“I think, pound for pound, the same atmospheric rivers will wring out more moisture on the West Coast,” Cordeira said. “On the East Coast, even though the atmospheric rivers are often stronger, the mountains just aren’t as tall, or the atmospheric rivers are pointed the wrong way and go out to sea.”

Still, atmospheric rivers are a vital source of precipitation along the East Coast. Cordeira said that more than 90% of extreme precipitation in the Northeast outside the summer months is tied to them.

What made this week’s event so strong?

While many atmospheric rivers that affect the East Coast are tame, the one this week was exceptionally intense. It drew moisture not only from the Atlantic and Caribbean but also extended across Central America into the Pacific. At its peak, each one-meter cross-section of the atmospheric river carried overhead more than 3,500 pounds of water vapor every second.

If we assume the core of the atmospheric river was about 50 miles wide, that equates to more than 140,000 tons of water sweeping by per second. That’s more than 10 times the weight of the Eiffel Tower.

The shape of the atmospheric river also increased its impact.

“This one sort of had that curl that made it come in from the south-southeast,” Cordeira said, putting it on a collision course with New Hampshire’s White Mountains and Maine. “It was a climatologically unusual direction.”

The high terrain in northern New England, where over half a foot of rain fell, helped focus that moisture. Up to 8 inches was measured in northeastern New Hampshire.

According to Cordeira, the hazards posed by atmospheric rivers can rival those of thunderstorms and tropical systems in the East.

“The common denominator is they all contain tropical moisture,” he said. “Sure, the biggest flood events are in the summertime . . . but this event produced almost the same amount of rainfall and same degree of impacts.”

A case in point: The three wettest calendar days at Washington Dulles International Airport in the past two years all occurred in the last month. They were not caused by thunderstorms or tropical storms but by atmospheric rivers embedded in East Coast storms. While the short-term bursts of rain may be less intense in atmospheric rivers than summer thunderstorms, the rivers are conveyor belts of moisture that can produce higher amounts if they remain aimed at a location long enough.

Is climate change intensifying atmospheric rivers?

Atmospheric rivers have been around as long as the atmosphere has been.

That said, data suggests they’re becoming wetter. Winter is the fastest-warming season in 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, and the air can hold 4% more water. Most atmospheric rivers happen in the cool season.

Consider New York City; it has warmed 5.2 degrees in the wintertime since the 1870s. Winter days that feature an inch or more of rainfall are now 26% more common.

It’s not just the Big Apple. Boston’s winters have warmed 4.7 degrees since the 1870s, and 1-inch days are now 15% more common. Philadelphia has warmed by 2.4 degrees, with an uptick of more than 40% in 1-inch calendar days.

In Washington, data from Reagan National Airport dates to the early 1940s; there’s been a 3.6 degree warming in winters since then, and a 38% increase in days with an inch or more of rain.

Most sites didn’t exhibit a dramatic change in how much rain was coming down overall during winter, fitting a trend of heavier, but less frequent, rain events.

Multiple studies of atmospheric rivers in the Pacific Ocean project that they will become more intense and generate more rainfall because of human-caused climate change.

 

Jason Samenow contributed to this report.

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