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Going into this winter, California was mired in a three-year drought, with little hope for any immediate relief in the forecast. Fast forward to today and the state has been inundated with up to 10 to 20 inches of rain and up to 200 inches of snow in some places over the past three weeks. The drought is not over, but parched farmland and declining reservoir levels have been displaced by raging rivers and deadly floods.
The National Oceanic and Atmospheric Administration’s Climate Prediction Center (CPC) issues seasonal forecasts of precipitation and temperature one to 13 months into the future. The CPC’s initial forecast for this winter, released Oct. 20, favors below-normal precipitation in Southern California and does not lean toward drier or wetter than normal conditions in Northern California.
A series of storm surges hit California the week of January 9, causing flooding, mudslides, power outages and more across the state. (Video: John Farrell/The Washington Post)
However, following a series of intense moisture-laden storms known as atmospheric rivers, much of California has seen rainfall totals of 200 to 600 percent above normal over the past month, with 24 trillion gallons of water falling on the state from the end of December.
Floods, landslides, sinkholes: See the devastation from heavy rain in California
The stark contrast between the staggering amount of rainfall in recent weeks and the CPC’s seasonal precipitation forecasts released before winter, which leaned toward below-normal precipitation for at least half of California, has water managers complaining about the reliability of seasonal forecasts.
“You have no idea on Dec. 1 what your winter is going to look like because our seasonal forecasts are so bad,” Jeffrey Mount, a senior fellow at the Public Policy Institute at the California Water Policy Center, said in an interview. “They’re just not reliable enough to make definitive water supply decisions.”
CPC’s seasonal and monthly forecasts do not provide specific predictions of precipitation amount, but rather the probability that precipitation will be above or below average. Such information is intended to “help communities prepare for what is likely to occur in the coming months and minimize the impact of weather on lives and livelihoods,” NOAA said in its winter outlook.
The precipitation forecast for California remained virtually unchanged in the CPC’s Nov. 17 winter outlook update. That forecast called for a 33 to 50 percent chance of below-normal precipitation in the southern half of California and 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 influenced by the expected continuation of La Niña conditions. El Niño and La Niña—the cyclical warming and cooling of the eastern tropical Pacific Ocean that affects weather patterns around the world—often have a huge effect on prevailing seasonal conditions in many parts of the world.
“Forecasting on a seasonal time scale is dominated by the El Niño/La Niña cycle,” DeWitt said in an interview. “La Niña conditions are typically characterized or associated with below normal precipitation for central and southern California. Northern California is kind of like a roll of the dice.
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As recently as mid-November, the chances of La Niña continuing for a third consecutive winter, as it has so far, were considered high, although it appears to be weakening. In both of the previous three-peat La Niña winters since 1950, much of California has seen below-normal precipitation.
Despite their typically strong influence on seasonal conditions, El Niño and La Niña are not the only game in town. They can be counteracted by other large-scale atmospheric phenomena that develop on shorter time scales. One such factor is a cluster of storms in the tropics known as the Madden-Julian Oscillation, which circles the globe approximately every 30 to 60 days.
While such factors “can leave a large imprint on average winter conditions … they are very difficult to predict more than a few weeks in advance,” Nat Johnson, a researcher and meteorologist at Princeton’s Geophysical Laboratory for Fluid Dynamics, wrote in a blog post about the winter NOAA perspective.
As these additional factors came into focus in mid-December, the CPC began to modify its forecast for California. For example, the monthly precipitation forecast for January, released Dec. 15, shows a smaller portion of the state expected to see below-normal precipitation.
The first signs of above-normal precipitation for California did not appear until Dec. 19, when the CPC issued its precipitation forecast for the next eight to 14 days. That outlook, which spanned the period from Dec. 27 to Jan. 2, called for a 33 to 70 percent chance of above-normal precipitation across California, with the highest chances in the northern part of the state.
“Those eight-to-14 daily products are really going to generally have much higher skills than the monthly or seasonal perspective because of that shorter time scale,” DeWitt said.
On December 31, with what will become a week-long irrigation program already underway, the CPC issued a monthly rainfall forecast suggesting wet weather could continue into January.
“I can’t rely” on a long-term forecast
Experts say seasonal rainfall forecasts should be viewed with caution and not interpreted as weather forecasts.
“They are intended to show end users how the odds are stacked 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,” Michael DeFlorio, research analyst at the Center for Western Extremes weather and water at the Scripps Institution of Oceanography in San Diego, wrote in an email.
Such prospects are especially difficult for California, which experiences wild swings from year to year between wet and dry conditions.
“California receives much of its annual precipitation from a small number of intense storms, often in the form of atmospheric rivers,” Johnson wrote in an email. “This means that California’s seasonal to annual precipitation totals can be significantly affected by chaotic weather variability that occurs within just a few days.”
The winter guessing game has long been a challenge for government officials and water managers, who must make decisions about how much water to allocate to farms and cities, plan reservoir and dam releases, and prepare for impacts on agricultural production and manufacturing of hydroelectric power.
Climate change has made the task even more complex, as historical experience may no longer be a useful guide to assessing the severity of droughts and floods.
“Conditions are changing,” Mount said. “What we see in the long-term trends is drier periods and wetter wet periods.”
How climate change will make atmospheric rivers even worse
At the local level, agencies may use seasonal outlooks for basic guidance, but not necessarily for critical decisions.
“We plan to be able to manage anything that comes our way,” 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 a La Niña, you can have significant storms at the watershed level — you really can’t rely on the overall long-term forecast to manage the watershed.”
Pathways to Better Precipitation Forecasts
Ongoing research at the Scripps Institution of Oceanography aims to improve shorter-term forecasts of atmospheric rivers. This winter, data from reconnaissance flights into these scattered storms was fed into real-time forecast models, helping to increase their accuracy to within five to 10 days, and possibly beyond, Whittlesey said. Researchers are also tackling the problem of predicting extreme rainfall with new tools, such as artificial intelligence.
However, the known gap in subseasonal to seasonal forecasts remains.
“Precipitation forecasts two weeks out are extremely valuable to the public,” DeWitt said. “They are inherently low-skilled because of the state of the science.”
To improve precipitation forecasts, DeWitt points to the importance of programs that extend research through operations like NOAA’s Precipitation Prediction Grand Challenge. The strategy of this program aims to provide more accurate precipitation forecasts – on time scales from day to decade – by addressing large gaps in atmospheric observations, reducing model errors and developing products that more effectively communicate the forecast .
“We continue to strive to get this program funded at a sufficient and sustainable level because it will be necessary.” … This will accelerate our ability to improve precipitation forecasts for stakeholders,” DeWitt said.
As evidence of what the Precipitation Forecasting Grand Challenge can accomplish, DeWitt cited the success of NOAA’s Hurricane Forecast Improvement Program, a research-to-operations program that began in 2009. The program achieved its original goal of reducing errors in hurricane tracking and intensity by 20 percent over five years and continues to strive to further increase the accuracy of hurricane forecasts.
“We would like to do the same thing for precipitation forecasts on time scales, but especially on subseasonal to seasonal time scales,” DeWitt said.
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