This story appeared on Calmatters
The immense amounts of rain and snow California is experiencing this winter could morph into devastating floods under certain conditions.
The winter of 2022-23 will go into the meteorological record books for one of the heaviest – if not, the heaviest – precipitation ever experienced in California.
California has been buffeted for more than two months by a seemingly nonstop series of storms rolling in from the Pacific, soaking virtually every corner of the state. Their most unusual aspect has been the huge snowfall in Southern California mountains, where hundreds of people remain trapped in resort communities.
Overall, California’s snowpack is approaching 200% its average for this point of the season.
“This snowpack actually rivals 1982-83, which is the largest snowpack on record,” Sean de Guzman, who manages snowpack measurement for the state Department of Water Resources, said Friday after his crew conducted a survey near Echo Summit (7,377 feet elevation) in El Dorado County.
The record will likely be broken because meteorologists are forecasting storms for weeks to come.
While the wet and snowy winter has provided welcome relief from several years of drought, there’s a potential downside: major flooding if warm tropical storms drop huge amounts of water and suddenly melt the snowpack. Such juxtapositions of meteorological conditions can overwhelm the carrying capacity of the rivers and the dams that are supposed to control river flows.
“I hope I’m wrong, but I’m afraid people celebrating the great snows in the Sierra Nevada are seriously underestimating the risk of spring flooding in California, including reservoir operators and state and federal water managers,” Peter Gleick, one of the state’s foremost experts on water, tweeted on Saturday.
Not only could it happen this year, but it could happen in the San Joaquin Valley this week, the National Weather Service warned in a bulletin issued early Monday morning, citing a warm storm that will hit the region beginning Thursday.
“An abundance of subtropical moisture will move inland over Central California along the southern periphery of this storm system Thursday night through Friday night,” the NWS said, adding that “heavy rainfall and the mild air mass will result in rapid snow melt in areas that have received several feet of snow in the past few weeks.
“The combination of heavy rainfall and rapid snow melt will cause water levels to rise on area rivers and streams Thursday night through Saturday. Persons living near rivers and streams should closely monitor water levels and be ready to move to higher ground if the threat of flooding becomes imminent.”
Northern California is not likely to feel the impacts of this week’s subtropical storm, although it will see additional rain and snow. Whether the immense snowpack evolves from friend to foe depends not only on the pattern of precipitation in the next few months, but on the ability of dams and reservoirs to absorb its runoff and the integrity of river levees.
Sacramento, the state capital that lies at the juncture of two major rivers, the Sacramento and the American, is especially threatened if the “killer flood” scenario becomes reality.
An immense bypass channel protects the city from the Sacramento River – as long as its levees are not breached – but the American River is problematic. Its major control facility, Folsom Dam/Folsom Lake, is generally regarded as too small to handle a major surge of melting snow.
The American’s levees are being strengthened, but the work is not complete. Folsom Dam’s outflow capacity has been expanded by a new spillway. The new works will likely be tested when the massive American River watershed releases its huge snowpack as winter turns to spring and then summer.