Although California may be ending its winter with extinct reservoirs and a near-record snowpack, meteorologists are warning that the state will face an increased risk of flooding in the coming months as the snow melts in the Sierra Nevada fills the rivers and streams.
The National Oceanic and Atmospheric Administration’s spring flood forecast on Thursday indicated that drought conditions will continue to improve across much of the state, but flood risk will worsen in the face of the thick snow cover and high soil moisture.
“About 44% of the United States is at risk of flooding this spring,” said Ed Clark, director of NOAA’s National Water Center. “California’s historic snowpack, coupled with spring rains, increases the potential for spring flooding.”
The severity of these floods, however, remains to be determined and depends on various meteorological factors, according to experts.
“It’s going to happen, and the question is whether it happens quickly or slowly,” said UCLA climatologist Daniel Swain. “Is it a gradual snowmelt – in which case flooding issues would mostly be minor – or is it more rapid…in which case we could be talking about something in major flood territory.”
Potential triggers for the rapid snowmelt could be an early-season heat wave or another series of warm storms, Swain said, both of which could lead to “significant snowmelt flooding, particularly in the San Joaquin Valley and some of the eastern slope watersheds of the Sierra that drain into the Great Basin and into Nevada.
The NOAA report predicts moderate flooding in the Central Valley and Sierra, and minor flooding across much of Northern California and along the coast.
For a state that has endured three years of drought, only to be hammered by 11 atmospheric rivers since the start of the year, the prospect of clear-sky flooding is a mixed blessing as some of that water is destined to flow. seep into very depleted aquifers. .
But for communities that have suffered death and displacement from flooding, the forecast is dire. Those communities include Pajaro, Monterey County, which flooded after a levee burst Friday night.
The floods have displaced hundreds of people in the mainly migrant city, with no clear timetable for the return. It has also raised significant concerns about crop yields in the heavily agricultural region this year.
A similar series of storms in January breached levees along the Cosumnes River near Sacramento, flooding low-lying fields and roads and contributing to the deaths of at least three people.
State and federal officials who faced dwindling reservoirs just months ago are now strategically releasing dams to make room for inflows.
Water managers “ensure that reservoir discharges are coordinated as best they can – given the amount of accumulated snow – to minimize downstream impacts as much as possible,” Jeremy Arrich, director of the division of the flood management from the California Department of Water Resources, said during a briefing Wednesday.
“However, there is a ton of snow above these watersheds, and we expect a significant amount of flow during the snowmelt season.”
Minor flooding is already occurring along the Sacramento, Salinas, Merced and San Joaquin rivers due to recent storms, and “many rivers and streams along the Sierra Nevada foothills, especially in the San Joaquin Valley, will continue to sink high next week. from a combination of heavy rain and snowmelt,” according to the NOAA outlook.
The latest round of storms saw a near-record crest on the San Joaquin River in Patterson, with waters rising within a foot of its high water mark starting in February 2017, according to the US Drought Monitor.
The Pajaro River at Chittenden reached its highest crest since February 1998, while the nearby Salinas River at Spreckels rose 3.89 feet above flood stage, just behind the record set in 1995. The river Nacimiento exceeded its 1969 high water mark by 1.51 feet. .
Such swells only add to the potential for flooding, as snowmelt will come to the top of rivers, streams and tributaries that may be high.
Snowfall at the UC Berkeley Central Sierra Snow Lab in Donner Pass exceeded 650 inches this season, compared to a normal total of about 360 inches for the entire season.
“If we have average or below average temperatures and no significant warm atmospheric rivers with rain, there will still be a lot of snowmelt, but it will be more gradual,” said Andrew Schwartz, senior scientist at the lab. of snow. “If we see long periods of abnormally high temperatures, flooding can become a significant risk because of this heavy snowpack.”
Although the storms and associated flooding have proven dangerous and deadly, they have also been a boon to water supplies and other conditions in the state. The latest Drought Monitor update shows that nearly 64% of California is no longer classified in any drought category, including 45% completely drought-free and 19% considered “abnormally dry.” It’s a remarkable turnaround from just three months ago, when nearly the entire state was deeply mired in a record-breaking drought.
Areas out of drought completely include the Northwest Coast, much of central California and the Sierra, and parts of southern California, including swathes of Kern and Los Angeles counties.
Additionally, no part of California remains in the two worst categories — extreme or exceptional drought — down from 36% just three months ago.
The Department of Water Resources increased its supply allocation for state agencies to 35% last month, and officials said this week that number could rise.
The bounty also allowed the state to divert more water to groundwater basins, including diverting more than 600,000 acre-feet of the San Joaquin River to areas where it can spill and seep. in the aquifer below the San Joaquin Valley.
On Wednesday, Southern California’s Metropolitan Water District — the wholesaler that supplies 26 agencies and 19 million people in the region — canceled its unprecedented water conservation mandate. The mandate, which came into effect last June, saw nearly 7 million people subject to strict water use limitations, including one- and two-day-a-week outdoor watering limits.
However, officials warned that while the state has seen improved drought and water conditions, Southern California’s other major source, the Colorado River, remains in jeopardy.
“One of the big challenges for California is outside of California in the Colorado River Basin,” said state climatologist Mike Anderson. “The basin has been in drought for 23 years and continues, facing some pretty significant challenges.”
He added that spring will see a “very dynamic condition as we try to deal with all the water that has come up, both in January and March here, as well as in February when there was a lot of snow”.
Indeed, California’s 154 major intrastate reservoirs gained 9.9 million acre-feet of water between Nov. 30 and Feb. 28, according to the Drought Monitor, bringing their total storage – 23.2 million acre-feet – at 96% of the historical average for the time of year.
But in the Colorado River system, storage as of Feb. 28 was 15.1 million acre-feet, just 46% of average and 29% of capacity.
The rest of March appears to be potentially wetter, with yet another atmospheric river set to hit California early next week and possibly another after that. Both can be colder storms that will add to the state’s snowpack.
The forecast comes just days after climatologists declared the end of La Niña, a climate pattern in the tropical Pacific associated with drier than normal conditions in the state, particularly in southern California.
The El Niño-La Niña Southern Oscillation, sometimes called ENSO, has a major influence on temperature and precipitation in different parts of the world. Neutral ENSO conditions are expected to continue into early summer, with increasing chances of a wetter El Niño pattern developing shortly thereafter.
Surveying the damage in Pajaro on Wednesday, Governor Gavin Newsom said winter’s wild run was indicative of extreme weather fluctuations brought on by climate change.
“You look back at the last few years in this state – it was ice fire, and no hot tub in between,” he said.