Every winter, narrow, fast-flowing plumes of atmospheric moisture stream across the Pacific ocean and bombard the West Coast with heavy rain.  Known as “atmospheric rivers,” they are critical for West Coast water supply and have enormous economic implications for the region.

For the first time ever, “Hurricane Hunter” aircraft are collecting data from these rivers above the Pacific to attempt to improve forecasts — just like they sample data from tropical storms in the Atlantic.

Those involved in the project say the mission is important because as much as half of the rain and snow that falls in West Coast states comes from these rivers.

“It’s just the few wettest days every year which determines whether we’re in drought or flood,” said F. Martin Ralph of the Scripps Institution of Oceanography, who is coordinating the mission. “And those wettest days come from these atmospheric river events.”

The rivers sometimes lead to costly flooding and mudslides and unleash damaging winds.  During a strong El Nino event, such as this year’s, warm ocean waters energize these rivers and add even more moisture to their torrent.

Ralph, who directs the Center for Western Weather and Water Extremes at Scripps,  said the typical atmospheric river transports a quantity of water — in the form of vapor — equivalent to 26 Mississippi Rivers.

“Research has shown that most of the extreme precipitation and flooding events in California (and other areas of the west), and much of the snowpack and water supply are associated with atmospheric rivers,” notes the Center’s website.

In the current research mission, two of the same Air Force C130 aircraft used to investigate tropical storms in the Atlantic and Gulf of Mexico are dropping sensors into the rivers above the Pacific to gain valuable information about their water vapor distribution and their winds, data that are not available from satellites.

Each flight, originating from Hickam Air Force Base in Hawaii, includes two 800-mile transects through an existing atmospheric river. Two flights have occurred over the past week, and two more are planned in the coming week.

“We’re sending in those data to be assimilated by global forecast models to reduce uncertainties in the atmospheric river water vapor content, which is the greatest source of uncertainty in terms of predictions,” Ralph said.

Ralph explained that computer models can see these rivers coming five days ahead of time but, because they are often extremely narrow, it is difficult to pinpoint exactly where they will make landfall.  “Where it hits is off by an average of plus or minus 500 kilometers,” Ralph said.  “An atmospheric river is 500 kilometers wide, so the error can mean the difference of whether you’re hit or not.”

Ralph also hopes the data obtained from the aircraft will help enhance forecasts  of where the heaviest rain within a river occurs.  The zone of heavy rain is considerably smaller than the size of the river itself and occurs within waves embedded within the river that sometimes stall. Once ingested by forecasts models, the data from the aircraft may lead to improvements in the predictions of  the location and timing of the heaviest rainfall, Ralph said, which is so important for identifying flooding potential.

In the coming months and year, Ralph said he and his team will evaluate and quantify the impact of the additional data.  If forecasts improve, he believes they could serve as impetus for future flights into atmospheric rivers ahead of threatening West Coast storms.

“As it proves out to be effective, it will be a tool to be available for consideration for future winters,” Ralph said. “In a way, it complements air reconnaissance efforts [to collect tropical storm data] in the Southeast and Gulf of Mexico.”

The effort came together through a partnership between NOAA, the U.S. Air Force and Scripps Institution of Oceanography.

“I can’t think of a campaign that was put together so quickly with so many different parties working together to rapidly respond and address a feature of this El Nino,” said Scott Rayder, senior adviser to the president at the University Center for Atmospheric Research. “This rapid reaction capability is a great example of the atmospheric research and forecast communities working together to address big societal issues.”

This atmospheric river research mission has been coordinated amidst a broader NOAA-led campaign to study this year’s El Nino from the land, ocean and air.