The weather whiplash Northern California saw the past two years — dramatic swings from severe drought to extreme rains — will only become more common in coming decades, a team of UCLA climate scientists said in a new study.
While average precipitation totals won’t change much, the state’s already brief rainy season will become even shorter, with the autumn and spring shoulder seasons bringing less rainfall than they currently do. The majority of precipitation instead will hold off, storing up for monumental releases during late winter atmospheric rivers like the ones Northern California saw during the 2016-17 winter, according the report published Monday in the journal Nature Climate Change.
The research is based on atmospheric warming rates, calculated from the current trajectory of greenhouse gas emissions.
It predicts a pattern that’s already being seen on the North Coast, where water managers have been preparing for such weather extremes in earnest since 2012 when a series of intense atmospheric rivers pushed the water levels at Lake Mendocino into the flood control pool. The sudden surplus called for the release of 30,000 acre-feet of water from the reservoir, but then the North Coast saw little rainfall for 14 months, said Jay Jasperse, chief engineer and director of groundwater management for the Sonoma County Water Agency.
“It got pretty dicey,” Jasperse said. “It was in the height of the drought, and in hindsight people were like, ‘Why did you release all that water?’”
The experience changed the way the water agency and its partners store water, pivoting to a model that incorporates storm predictions, he said.
“That certainly catalyzed our focus on trying to figure out new approaches to reservoir management and working with the (Army) Corps of Engineers,” Jasperse said. “We had been working on climate and weather and things like that before that, but I think that really got us moving in the realm of reservoir management and adaptation.”
In addition to the weather whiplash, UCLA researchers said the state will see more freak serial storms, like the plague of atmospheric rivers that brought 40 days of rain to California in December 1861 and January 1862. The storm caused disastrous flooding across the then-new state, forcing it to temporarily relocate its capital from Sacramento to San Francisco.
If emissions levels don’t change, such a flood could hit the state by 2060, the report stated.
“I will say that this part of California, the Bay Area region, does typically see some of those bigger swings anyway, but evidence is showing that the swings between drought conditions and flooding conditions could become more extreme as we go forward,” said Brian Garcia, a National Weather Service meteorologist.
California winters with average rainfall will become less common, with a two-fold increase in the frequency of very wet winters and an increased likelihood between 50-150 percent of very dry winters, the report said.
“With these changes, we could see more patterns like this in the future, where we go through a few years of very intense drought, followed by a year or two of significant rainfall,” Garcia said. “And with that brings the concern of possible wildfire.”
Periods of drought followed by sudden bursts of moisture can result in super blooms of vegetation — long-dormant grasses that then dry out and turn into wildfire fuel — like the one Northern California saw last spring, he said.