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April 18, 2025 This article has been reviewed according to Science X's editorial process and policies . Editors have highlightedthe following attributes while ensuring the content's credibility: fact-checked proofread by David Hosansky, NCAR & UCAR As atmospheric rivers pounded the U.S.
West Coast last winter, scientists deployed increasingly advanced observing tools over the Pacific Ocean to improve forecasts of the powerful storms. Using airborne observations and ocean buoys, scientists targeted remote regions in the eastern Pacific that most influence the development and path of atmospheric rivers . Those observations were fed into leading forecast models, enhancing one- to five-day predictions in ways that helped safeguard not only vulnerable West Coast communities but the entire nation.
"These targeted observations provide a huge payoff," said U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) scientist Christopher Davis.
"By taking these observations, we improve forecasts of atmospheric rivers for the West Coast and, because the storms keep moving east, we also improve winter forecasts for the entire continental United States." Davis is one of the mission directors of Atmospheric Rivers Reconnaissance (AR Recon), a major effort involving multiple federal agencies, universities, and other partners. It's led by the Center for Western Weather and Water Extremes, which is based at the University of California San Diego's Scripps Institution of Oceanography.
The goal of AR Recon is to enable forecasters to better anticipate the track and intensity of the ferocious storms, helping emergency managers to protect lives and property and reservoir operators to save millions of dollars through the more efficient management of water levels. Studies indicate that, since launching its first observations in 2016, AR Recon has generated significant improvements in forecasts. The data from the observations, for example, improved 3-day forecasts of heavy precipitation over California in 2022 by up to 12%.
During this past season, AR Recon conducted 50 flights over the Pacific between November and March, collecting key measurements as atmospheric rivers formed and gained strength. "The AR Recon program fills a big information gap over the Pacific Ocean where these atmospheric rivers form," said Marty Ralph, director of the Center for Western Weather and Water Extremes and principal investigator of AR Recon. "If you want to predict where a car is going to be five minutes from now, you need to know where it's starting from and how fast it's moving.
Similarly, if there is an atmospheric river out near Hawaii, and we want to forecast where it will hit the California coast a few days later and how strong it is, we need to get out there and take direct measurements." The primary sponsors of AR Recon are the US Army Corps of Engineers, California Department of Water Resources, US Air Force Reserve Command 53rd Weather Reconnaissance Squadron, and NOAA Office of Marine and Aviation Operations. Significant cost savings Atmospheric rivers are long and narrow "rivers in the sky" that transport enormous amounts of water—so much so that the average atmospheric river carries an amount of water vapor roughly equivalent to 25 times the average flow at the mouth of the Mississippi River.
The most powerful atmospheric rivers unleash torrential rains and cause widespread flooding, especially when they stall over vulnerable watersheds. But they're also a vital feature of the West Coast's hydrological cycle, delivering needed moisture during the winter and replenishing snowpacks needed by farmers and other water users during the dry summer months. Accurately predicting these powerful storms is a priority for water managers and other West Coast officials.
In addition to protecting life and property, forecasts are critical for the cost-effective management of reservoirs, enabling operators to keep reservoirs as full as possible and release only enough water to prevent incoming rains from overtopping a reservoir and causing catastrophic damage. Since every acre foot of water in California is valued at hundreds of dollars, this precise approach to operating reservoirs can potentially save the state many millions of dollars every winter, Davis said. For example, forecasts enable reservoir operators at Lake Mendocino—one of three reservoirs in California that currently make decisions based on the forecasts—to use a flexible water management approach and reduce their annual costs by an estimated $9.
4 million to $9.9 million, according to a 2021 report . "Setting aside all the other economic value of this work, just that one specific piece of being able to store added water is worth tens of millions of dollars a year," Davis said.
"The economic benefits of this program have been remarkable." Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights.
Sign up for our free newsletter and get updates on breakthroughs, innovations, and research that matter— daily or weekly . Targeted observations AR Recon relies in part on flights by the U.S.
Air Force and NOAA "hurricane hunter" aircraft. For each flight, an aircraft releases approximately 30 dropsondes, which are cylindrical instruments developed at NSF NCAR that descend slowly to the ocean on parachutes. They measure air temperature, pressure, water vapor, and wind speed and transmit the data back to the aircraft and to the Global Telecommunications System to be fed into weather forecast models at operational centers.
To augment the measurements from dropsondes, the aircraft also use an innovative technology called airborne radio occultation. This measures delays in signals from GPS satellites to infer atmospheric properties such as moisture and temperature. In addition to these flights, AR Recon adds atmospheric pressure sensors to dozens of NOAA ocean buoys that collect water temperature and wave measurements.
When atmospheric rivers approach the coast, AR Recon's land-based stations launch weather balloons that ascend up to 115,000 feet, carrying radiosondes that, similar to dropsondes, take measurements of key atmospheric properties. One of the challenges for the scientists is to home in on those regions of the Pacific where observations make the biggest difference for forecasts. They have found that sending aircraft and buoys into overcast regions, often within an atmospheric river, produces especially significant benefits because satellites cannot easily see through clouds.
Targeting weather fronts and portions of the jet stream is also useful because measurements in those areas help pinpoint the locations of clashing warm and cold air masses. To determine which observations generate the greatest benefit, the scientists engage in "data denial" experiments. These consist of running parallel systems, one of which incorporates the observations and the other excludes them.
By comparing the forecasts generated by the two systems, they can quantify how much the observations improved the forecasts and help scientists target specific regions with ever-increasing precision. "It's not practical to take observations for the entire eastern Pacific, so our strategy is to be as efficient as possible with the use of our resources and place observing instruments where they will improve the forecasts the most," Davis said. Davis and his colleagues are most focused on those atmospheric rivers that are especially intense and may plow into a vulnerable West Coast community or watershed.
However, their observations also benefit communities across the entire nation because the same weather systems can fuel a blizzard in the Great Plains or a flood in the Southeast. "The weather systems that produce the atmospheric rivers keep going all the way across the continent," Davis said. "That's one thing about the atmosphere: what starts local doesn't stay local.
When you add buoy observations at the ocean surface and you add observations in the atmosphere, you're improving the whole forecasting system." Provided byNCAR & UCAR.
