The National Oceanic and Atmospheric Administration’s Climate Prediction Center (CPC) issues seasonal forecasts of precipitation and temperature for one to 13 months into the future. The CPC’s initial forecast for this winter, released on Oct. 20, favored below-normal precipitation in Southern California and did not lean toward drier or wetter-than-normal conditions in Northern California.
However, following a series of intense, moisture-laden storms known as atmospheric rivers, most of California saw rainfall totals 200 to 600% above normal last month, with 24 trillion gallons of water falling on the state since the end of the year. from December.
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The stark contrast between the staggering amount of precipitation in recent weeks and the CPC’s seasonal precipitation forecast issued ahead of winter, which skewed toward below-normal precipitation over at least half of California, has water managers lamenting the lack of reliability of seasonal forecasts.
“You have no idea what your winter will be like on December 1st, because our seasonal forecasts are so bad,” Jeffrey Mount, senior fellow at the Public Policy Institute of California’s Water Policy Center, said in an interview. “They are simply not reliable enough to make definitive decisions about water supply.”
The CPC seasonal and monthly perspectives do not provide specific predictions of precipitation volumes, but rather the probability that precipitation will be above or below average. This information is intended to “help communities prepare for what is likely to happen in the coming months and minimize climate impacts on lives and livelihoods,” NOAA said in its winter outlook.
The precipitation forecast for California was largely unchanged in the November 17 CPC update to the winter outlook. That forecast predicted a 33% to 50% 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 influences weather patterns around the world – often have an outsized effect on the 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 generally characterized by or associated with below normal precipitation for central and southern California. Northern California is something of a dice roll.
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By mid-November, chances were high that La Niña would continue for the third consecutive winter, which it has so far, although it appears to be weakening. In the previous two “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 aren’t the only games in town. They can be counteracted by other large-scale atmospheric phenomena that evolve on shorter timescales. One such factor is a cluster 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 leave a big mark 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 Laboratory of Geophysical Fluid Dynamics, in a blog post. . 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 forecast for January, published on Dec. 15, showed that a smaller portion of the state expected to see below-normal precipitation.
The first signs of above-normal precipitation for California didn’t appear until December 19, when the CPC released its precipitation forecast for the next eight to 14 days. That outlook, which covered the period December 27 to January 2, predicted a 33% to 70% chance of above-normal precipitation across all of California, with the highest chances in the northern part of the state.
“These 8-14 day products will generally have much greater skill than a monthly or seasonal perspective because of that shorter timescale,” DeWitt said.
On December 31, with what would become a weeks-long shower already underway, the CPC issued a monthly precipitation forecast suggesting the wet weather could continue into January.
‘I can’t trust’ long-term forecasts
Experts say that seasonal precipitation prospects should be viewed with caution and not interpreted as weather forecasts.
“They are intended to show end users how probabilities stack one way or another for wet, dry or normal conditions based on all relevant information available at the start of the water year,” Michael DeFlorio, research analyst at the Center for Western Climate and Water Extremes at Scripps Institution of Oceanography in San Diego, wrote in an email.
These prospects are particularly difficult for California, which experiences strong year-to-year swings between wet and dry conditions.
“California receives a large fraction 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 influenced by chaotic climate variability that occurs on just a few days.”
The winter guessing game has been a longstanding challenge for state officials and water resource managers who must make decisions about how much water to allocate to farms and cities, plan reservoir and dam releases, and prepare for effects on agricultural production and generation. of hydroelectric power.
Climate change has made the task even more complicated, because historical experience may no longer be a useful guide for estimating the severity of droughts and floods.
“Conditions are changing,” Mount said. “What we’re seeing in the long-term trends are drier dry spells and wetter wet spells.”
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At the local level, agencies can use seasonal perspectives for background 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 supplies on the Yuba River northeast of Sacramento, said in an interview. “Even during La Niña, you can have significant storms at the watershed level – you really can’t rely on the overall long-range forecast for watershed management.”
Paths to better precipitation forecasts
Ongoing research at the Scripps Institution of Oceanography aims to improve short-range predictions for atmospheric rivers. This winter, data from reconnaissance flights of these expanding storms has been fed into real-time forecast models, helping to increase their accuracy in the five- to 10-day range and possibly beyond, Whittlesey said. Researchers are also grappling with the problem of predicting extreme rainfall with new tools like artificial intelligence.
However, the known gap in subseasonal to seasonal forecasting remains.
“Precipitation forecasts beyond two weeks are inherently valuable to society,” DeWitt said. “They have inherently low skill because of the state of the science.”
To improve precipitation forecasts, DeWitt points to the importance of programs ranging from research to operations, such as NOAA’s Grand Precipitation Prediction Challenge. This program’s strategy aims to provide more accurate precipitation forecasts – on time scales from one day to one decade – by addressing key gaps in atmospheric observations, reducing model errors, and developing products that communicate the forecast more effectively.
“We continue to seek funding for this program at a sufficient and sustained level, because that is what will be needed. … This will accelerate our ability to improve precipitation forecasts for stakeholders,” said DeWitt.
As proof of what the Precipitation Prediction Grand Challenge could accomplish, DeWitt cites the success of NOAA’s Hurricane Prediction Improvement Program, a research-to-operations program that began in 2009. The program achieved its original goal of reducing forecasting errors. hurricane track 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 for precipitation predictions on time scales, but especially on the subseasonal to seasonal time scales,” Dewitt said.
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