A storm fed by an airborne fire hose of water – known as an atmospheric river – is set to bombard California this weekend, dropping up to 5 feet of snow in the mountains, with rain falling everywhere else.

At the same time, it will present another opportunity for scientists to better understand these phenomena – earlier in California’s wet season than in years past – by flying planes into them.

Since 2016, Hurricane Hunter aircraft have been flying into atmospheric rivers from January to mid-March. But this year, such flights began in November – extending the window to collect vital data, at a time when these events can be particularly intense.

“Climatologically, November and December can bring some of the worst floods” to the western United States, said Marty Ralph, director of the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography.

During the last three winters, landfalling atmospheric rivers have triggered presidentially-declared flood disasters in the Pacific Northwest.

That’s what motivated the decision to make early-season flights part of the federal National Winter Season Operations Plan. As a result, the Air Force Weather Reconnaissance Squadron is available on an as-needed basis to fly through atmospheric rivers headed toward the West Coast, in addition to coverage from the Gulfstream IV jet from the National Oceanic and Atmospheric Administration.

Under this expanded framework, the first two missions were flown last month. On Nov. 5, the season’s inaugural flight collected data on an atmospheric river poised to strike the Northwest; the day after, a second flight headed toward California.

The early November storm got the West’s water year off to a much-welcome soggy start following three years of drought, and the good fortune has continued into December.

According to Anna Wilson, the field research manager for the Center for Western Weather and Water Extremes, the Hurricane Hunters are looking for clues on atmospheric river intensity, such as the strength of their water vapor transport. Atmospheric rivers can carry 25 times the water equivalent of the Mississippi River in a stream up to 500 miles wide and 2,000 miles long.

To learn this information, the planes deploy dropsondes, instruments measuring 16 inches long and less than three inches in diameter that drop by parachute. During their 20-minute fall to the ocean surface, they transmit data about temperature, pressure, water vapor and winds. The resulting data is relayed from the plane to a central repository that feeds global weather forecasts.

The aircraft data improved the three- to five-day forecasts for where precipitation would fall in the West by up to 15%, Wilson told the Post. For the Pacific Northwest and Northern California, they jumped up 20 percent.

Atmospheric rivers are tricky to study because they’re hard to see with conventional observations. With atmospheric rivers found at altitudes around 10,000 feet, it wasn’t until relatively recently that satellites could distinguish them from clouds.

“It’s difficult as you get closer and closer to the surface,” Wilson said. “And since that’s where the majority of the water vapor and wind is in the AR (atmospheric river), that’s why it’s so important to have these vertical profiles that we can get from the dropsondes.”

In addition to improving forecasts, Wilson said, the information collected on these missions is used for research purposes. The real-world data is used to check the accuracy of what the weather models predicted the conditions would be. Then the proofing is worked the other way.

“The most simple and straightforward technique is running the model again without those observations and seeing what the difference is with the forecast,” Wilson said. “Did we improve what we thought we would improve?”

Wilson said the knowledge gained from Hurricane Hunter missions already has led to findings such as:

- Observations of water vapor transport enables splitting atmospheric rivers into sectors, revealing distinct characteristics across them.

- Errors in atmospheric river forecasts increase with strength.

- Model errors in atmospheric rivers can be associated with low-level winds.

“After the fact, we can better understand the fundamental processes involved with atmospheric rivers and their evolution, and such that we can eventually better parameterize [represent] them in the models themselves, too,” Wilson said.