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Earlier this winter, California was mired in a three-year drought, and forecasts gave little hope of recovery anytime soon. Fast forward to today, and the state has been soaked with 10 to 20 inches of rain and up to 200 inches of snow in some places over the past three weeks. The drought isn’t over yet, but parched farmland and falling reservoir levels have been replaced by raging rivers and deadly floods.
The National Oceanic and Atmospheric Administration’s Climate Prediction Center (CPC) releases seasonal forecasts of precipitation and temperature for one to 13 months in the future. The CPC’s first outlook for this winter, released Oct. 20, favored below-normal precipitation in Southern California and trended toward neither drier nor wetter conditions than normal in Northern California.
A series of atmospheric flows hit California the week of January 9, causing flooding, landslides, power outages and more across the state. (Video: John Farrell/Washington Post)
After a series of intense, moisture-rich storms known as atmospheric flows, most of California has rained 200 to 600 percent above normal for the past month, with 24 trillion gallons of water falling into the state since late December.
Floods, landslides, sinkholes: see the devastation caused by heavy rain in California
The stark contrast between the astonishing amount of rainfall in recent weeks and the CPC’s pre-winter seasonal rainfall forecasts, which trended toward below-average rainfall for at least half of California, has water managers bemoaning the reliability of seasonal forecasts.
“You have no idea what your winter is going to be like on December 1st because our seasonal forecasts are so bad,” Jeffrey Mount, a senior fellow at the Public Policy Institute of California’s Water Policy Center, said in an interview. “They just aren’t reliable enough to make definitive water supply decisions.”
CPC’s seasonal and monthly forecasts do not provide specific predictions of precipitation amounts, but rather the likelihood that precipitation will be above or below average. Such information is intended to “help communities prepare for what is likely to come in the coming months and minimize the impact of weather on lives and livelihoods,” NOAA said in its Winter Outlook.
The California precipitation forecast remained virtually unchanged in the Nov. 17 CPC update on the winter outlook. That forecast called for a 33 to 50 percent chance of below-average precipitation in the southern half of California and equal odds of above- and below-normal precipitation in the northern half of the state.
CPC Director David DeWitt said the outlook has been heavily impacted by the expected continuation of La Nina conditions. El Niño and La Niña—the cyclical warming and cooling of the eastern tropical Pacific that affects weather patterns around the globe—often have an outsized impact on prevailing seasonal conditions in many parts of the world.
“Seasonal timescale forecasts are dominated by the El Niño/La Niña cycle,” DeWitt said in an interview. “La Niña conditions are generally characterized by or associated with below-average rainfall for central and southern California. Northern California is kind of like rolling the dice.”
The flood watch covers most of California amid severe storms
As recently as mid-November, the odds were high that La Niña would continue for the third straight winter, which it has done so far, although it appears to be slowing down. During the previous two La Niña “three-peat” winters since 1950, much of California experienced below-average rainfall.
Despite their usually strong influence on seasonal conditions, El Niño and La Niña aren’t the only games in town. They can be counteracted by other large-scale atmospheric phenomena evolving on shorter timescales. One of these factors is a group of storms in the tropics known as the Madden-Julian Oscillation, which travels around the globe approximately every 30 to 60 days.
While such factors “can make a big impression on average winter conditions…they are very difficult to predict more than a few weeks in advance,” wrote Nat Johnson, a researcher and meteorologist at Princeton’s Geophysical Fluid Dynamics Laboratory, in a blog post about the winter outlook the NOAA.
As these additional factors began to come into focus in mid-December, the CPC began changing its forecast for California. For example, the monthly precipitation forecast for January released on December 15 showed that a smaller portion of the state is expected to experience precipitation below normal.
The first signs of above-average rainfall for California didn’t appear until December 19, when CPC released its rainfall forecast for the next eight to 14 days. That forecast, which covered the period December 27 through January 2, called for a 33 to 70 percent chance of above-average rainfall across California, with the highest probabilities in the northern part of the state.
“These eight to 14 year products generally have a much higher qualification than a monthly or seasonal outlook because of this shorter timescale,” DeWitt said.
On December 31, with a week-long wet soak already underway, CPC issued a monthly rainfall forecast that suggested wet weather could continue into January.
“Can’t rely on long-term forecasts.”
Experts say seasonal precipitation forecasts should be viewed with caution and should not be interpreted as weather forecasts.
“They are designed to show end users what the odds are one way or the other for wet, dry, or normal conditions based on all relevant information available at the beginning of the water year,” Michael DeFlorio, research analyst at the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography in San Diego, wrote in an email.
Such prospects are particularly difficult for California, which experiences wild swings between wet and dry conditions from year to year.
“California receives a large portion of its annual precipitation from a small number of intense storms, often in the form of atmospheric fluxes,” Johnson wrote in an email. “This means that California’s seasonal to annual precipitation totals can be significantly impacted by chaotic weather variability that only occurs within a few days.”
The winter guessing game has long been a challenge for state officials and water managers as they make decisions about how much water to allocate to farms and cities, plan reservoir and dam approvals, and prepare for impacts on agricultural production and hydroelectric generation.
Climate change has made the task even more complicated, 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’re seeing in long-term trends are drier dry spells and wetter wet spells.”
How climate change will make atmospheric fluxes even worse
At the local level, agencies can use seasonal forecasts as background information, but not necessarily for critical decisions.
“We plan to be able to handle anything that comes our way,” said Willie Whittlesey, general manager of the Yuba Water Agency, which manages flood risk and water supplies on the Yuba River northeast of Sacramento, in an interview . “Even during La Niña there can be significant storms at the watershed level — you really can’t rely on the general longer-term forecast for watershed management.”
Ways to get better precipitation forecasts
Ongoing research at the Scripps Institution of Oceanography aims to improve shorter-range predictions for atmospheric fluxes. This winter, data from reconnaissance flights into these spreading storms was fed into forecast models in real time, helping increase their accuracy in the five to 10 day range and possibly beyond, Whittlesey said. Researchers are also tackling the problem of predicting extreme rainfall with new tools like artificial intelligence.
However, the well-known gap in season-to-season prediction remains.
“Precipitation forecasts beyond two weeks are inherently valuable to society,” DeWitt said. “They have inherently low abilities because of the state of science.”
To improve precipitation predictions, DeWitt points to the importance of programs that expand research through operations like the NOAA Precipitation Prediction Grand Challenge. This program’s strategy aims to provide more accurate precipitation forecasts – on time scales from one day to a decade – by closing large gaps in atmospheric observations, reducing model errors and developing products that communicate the forecast more effectively.
“We remain committed to funding this program at an adequate and sustainable level because that is what it will need. … That will accelerate our ability to improve precipitation forecasts for stakeholders,” DeWitt said.
As evidence of what the Precipitation Prediction Grand Challenge could achieve, DeWitt cites the success of NOAA’s Hurricane Forecast Improvement Program, a program that began in 2009 from research to operations. The program met its original goal of reducing hurricane track and intensity errors by 20 percent in five years and continues to seek further increases in hurricane forecast accuracy.
“We would like to do the same for precipitation forecasts across timescales, but particularly on the subseasonal to seasonal timescales,” Dewitt said.