In California, the drought turned into flooding. Forecasters didn’t see it coming.


Heading into this winter, California was mired in a three-year drought, with forecasts showing little hope of any short-term relief. Fast forward to today, and some parts of the state have seen up to 10 to 20 inches of rain and up to 200 inches of snow over the past three weeks. The drought is not over, but parched farmland and falling reservoir levels have given way to raging rivers and deadly flooding.

NOAA’s Climate Prediction Center (CPC) issues seasonal forecasts of precipitation and temperature for the next 1 to 13 months. The Chinese Communist Party’s preliminary outlook for this winter was released on October 10. 20, with below-normal precipitation favoring Southern California, while conditions in Northern California were neither dry nor wet.

During the week of January 1, a series of atmospheric rivers hit California. 9. Cause floods, mudslides, power outages, etc. across the state. (Video: John Farrell/The Washington Post)

However, much of California has seen a total of 200% to 600% more rainfall than normal over the past month after a series of intense moisture storms known as atmospheric rivers, with the state seeing heavy rainfall since late December. Rainfall has reached 24 trillion gallons.

Floods, landslides, sinkholes: A look at the devastation wrought by California’s torrential rains

The stunning precipitation in recent weeks stands in stark contrast to the seasonal precipitation outlook issued by the CPC ahead of winter, which saw at least half of California receive below-normal precipitation, water managers said on the reliability of seasonal forecasts. Pity.

“Don’t you know December is coming? 1 What will your winter look like because our seasonal forecasts are terrible,” Jeffrey Mount, senior fellow at the California Water Policy Center’s Public Policy Institute, said in an interview said in the interview. “They’re just not reliable enough to make definitive water supply decisions.”

The CPC’s seasonal and monthly outlooks do not provide specific forecasts of precipitation, but rather probabilities of above or below average precipitation. In its winter outlook, NOAA said the information is intended to “help communities prepare for what might happen in the coming months and minimize the impact of the weather on lives and livelihoods.”

The CPC’s November 8 precipitation forecast for California is little changed. 17 Updated winter outlook. The forecast puts a 33 percent to 50 percent chance of below-normal precipitation in the southern half of California, with an equal chance of above- or below-normal precipitation in the northern half of the state.

CPC Director David DeWitt said the outlook was heavily weighed by the expected continuation of La Niña. El Niño and La Niña events—the periodic warming and cooling of the eastern tropical Pacific Ocean that affect global weather patterns—often have a large impact on prevailing seasonal conditions in many parts of the world.

“Predictions on seasonal timescales are dominated by the El Niño/La Niña cycle,” DeWitt said in an interview. “La Niña is often characterized by or associated with below-normal precipitation in central and southern California. Northern California is a bit of a roll of the dice.”

Flood watch covers nearly all of California amid storm

Back in mid-November, the odds were high that La Niña would persist for a third winter in a row, although it appears to be waning so far. The first two “triple” La Niña winters since 1950 saw below-normal precipitation across much of California.

While El Niño and La Niña often have a big impact on seasonal conditions, they aren’t the only game in town. They can be offset by other large-scale atmospheric phenomena that evolve on shorter timescales. One factor is a swarm of tropical storms, known as the Madden-Julian Oscillation, which orbits the Earth roughly every 30 to 60 days.

Nat Johnson, a researcher and meteorologist at the Geophysical Fluid Dynamics Laboratory at Princeton University, in a blog post on NOAA’s winter outlook.

As these additional factors began to come into focus in mid-December, the CPC began to change its forecast for California. For example, its monthly precipitation outlook for January, published on December 12. On the 15th, there was little precipitation in a small number of areas.

The first signs of above-normal precipitation in California did not appear until December. On the 19th, the CPC released the precipitation forecast for the next 8 to 14 days. The outlook covers the period beginning December 17. 27 to Jan. 2, a 33% to 70% chance of above-normal rainfall is forecast across California, with the highest chance in the northern part of the state.

“Those 8- to 14-day products are actually usually higher skill than monthly or seasonal outlooks because of the shorter time scale,” DeWitt said.

On December 31, with several weeks of rain already underway, the CPC issued its monthly precipitation outlook, indicating that the wet weather could continue into January.

‘Cannot be relied upon’ for long-term forecasts

Experts say the outlook for seasonal precipitation should be viewed with caution, rather than interpreted as a weather forecast.

“They’re designed to show the end user how the odds stack up one way or the other for wet, dry or normal conditions based on all the relevant information available at the start of the water year,” said Michael Dever, a research analyst at West Research Center. Michael DeFlorio, director of weather and water extremes at the Scripps Institution of Oceanography in San Diego, wrote in an email.

Such a prospect is especially difficult for California, which experiences wild swings between wet and dry conditions each year.

“A significant portion of California’s annual precipitation comes from a small number of severe storms, often in the form of atmospheric rivers,” Johnson wrote in an email. Significant effects of chaotic weather changes that occur within days.”

The winter guessing game has been a perennial challenge for state officials and water managers, who need to decide how much water to allocate to farms and cities, plan for the release of reservoirs and dams, and prepare for the impact on agricultural production and hydropower .

Climate change complicates this task, as historical experience may no longer be a useful guide for estimating the severity of droughts and floods.

“Things are changing,” Mount said. “What we’re seeing in the long-term trend is drier periods of drought and wetter periods of wetter.”

How climate change will make atmospheric rivers worse

At the local level, agencies may use the seasonal outlook as background guidance, but not necessarily for key decision-making.

“We plan to be able to deal with whatever situation we come across,” Willie Whittlesey, general manager of the Yuba Water Agency, which manages flood risk and water supply on the Yuba River northeast of Sacramento, said in an interview. . “Even during La Niña, you can have severe storms at the watershed level — you really can’t rely on general long-range forecasts for watershed management.”

Ways to Improve Precipitation Forecasts

Ongoing research at the Scripps Institution of Oceanography aims to improve short-term forecasts for atmospheric rivers. This winter, data from reconnaissance flights of these spreading storms have been fed into forecast models in real time, helping to improve their accuracy over timescales of five to 10 days and beyond, Whittlesey said. Researchers are also tackling the problem of forecasting extreme rainfall using new tools such as artificial intelligence.

However, known gaps in subseasonal-to-seasonal forecasts remain.

“Precipitation forecasts beyond two weeks have intrinsic value to society,” DeWitt said. “Because of the state of science, they are inherently unskilled.”

To improve precipitation forecasts, DeWitt noted the importance of programs that expand research through initiatives such as NOAA’s Precipitation Forecast Grand Challenge. The program’s strategy aims to provide more accurate precipitation forecasts—from one-day to decadal timescales—by addressing major gaps in atmospheric observations, reducing model errors, and developing products that communicate forecasts more effectively.

“We continue to work hard to provide adequate and sustained funding for the program because that’s what it’s going to do. … This will accelerate our ability to improve our stakeholders’ precipitation forecasts,” DeWitt said.

As evidence of what the Precipitation Prediction Grand Challenge can achieve, DeWitt cites the success of NOAA’s Hurricane Forecast Improvement Program, a research-to-operations initiative that began in 2009. The program achieved its original goal of reducing hurricane track and intensity errors by 20 percent over five years and continues efforts to further improve the accuracy of hurricane forecasts.

“We want to do the same for precipitation forecasts across timescales, especially on subseasonal-to-seasonal timescales,” DeWitt said.

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