California storms prompt questions about accuracy of seasonal predictions
By Dan Stillman and
January 15, 2023
Coming into this winter, California was mired in a three-year drought, with forecasts offering little hope of relief anytime soon. Fast forward to today, and the state is waterlogged with as much as 10 to 20 inches of rain and up to 200 inches of snow that have fallen in some locations in the past three weeks. The drought isn’t over, but parched farmland and declining reservoir levels have been supplanted by raging rivers and deadly flooding.
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 outlook for this winter, issued on Oct. 20, favored below-normal precipitation in Southern California and did not lean toward either drier- or wetter-than-normal conditions in Northern California.
However, after a series of intense moisture-laden storms known as atmospheric rivers, most of California has seen rainfall totals 200 to 600 percent above normal over the past month, with 24 trillion gallons of water having fallen in the state since late December.
The stark contrast between the staggering amount of precipitation in recent weeks and the CPC’s seasonal precipitation outlook issued before the winter, which leaned toward below-normal precipitation for at least half of California, has water managers lamenting the unreliability of seasonal forecasts.
“You have no idea come Dec. 1 what your winter is going to look like because our seasonal forecasts are so bad,” said Jeffrey Mount, a senior fellow with the Public Policy Institute of California’s Water Policy Center, in an interview. “They are just not reliable enough to make definitive water supply decisions.”
An evolving forecast
The CPC’s seasonal and monthly outlooks do not provide specific forecasts of precipitation amounts, 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 come in the months ahead and minimize weather’s impacts on lives and livelihoods,” NOAA stated in its winter outlook.
The precipitation forecast for California remained virtually unchanged in the CPC’s Nov. 17 update to the winter outlook. That forecast called for a 33 to 50 percent chance of below-normal precipitation in the southern half of California, and equal chances of precipitation being above or below normal 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 globe — often have an outsize effect on prevailing seasonal conditions in many parts of the world.