Weather & Natural Disasters

Warming weather in California causing early snowpack melt, raising threat of new flooding

Snow covered mountain tops rise behind downtown Los Angeles last month.
Richard Vogel | AP

A warm spell hitting parts of California has raised the risk that the state's Sierra snowpack could melt too fast and produce flooding in some areas.

Temperatures in parts of the state are expected to get into the 90s by the end of this weekend, and are coming after heavy precipitation from storms in January and February. Some of the warmest weather will be impacting portions of Southern California, including parts of L.A. County.

"There may be reservoirs where releases will have to be increased and you have to be concerned with some downstream flooding issues," said Brad Rippey, a meteorologist with the U.S. Department of Agriculture.

Indeed, there's a flood advisory already issued for below Friant Dam on the San Joaquin River due to above-normal water releases that could impact Fresno and Madera counties in Central California. The National Weather Service said Thursday that the water releases could "bring a threat of minor flooding along and adjacent to the San Joaquin River downstream to Mendota."

Northern California experienced significant flooding in January and February from several major storms, with the city of San Jose one of the hardest hit. Also, Santa Cruz, Guerneville and Petaluma were other communities with flood issues.

As temperatures rise and the Sierra snow melts, other regions of the state are at risk of flooding too.

"Above-normal snowpack and mild temperatures are melting snow that is causing snowmelt-fed streams, creeks and rivers to rise," the NWS said Friday in a hydrologic outlook for northeastern parts of the state such as Modoc County.

According to Rippey, there's likely to be a gradual increase in flood advisories over the next week as the sun continues to beat down on the snowpack in the mountains.

In Southern California, temperatures this weekend are expected to run 15 to 20 degrees above normal in some areas, according to Stuart Seto, a weather specialist with the NWS in Oxnard, California.

"The warmer weather pattern looks like at least for the next 10 days or so," said Seto.

In Northern California, there's already signs of the Sierra snowpack melting faster into Oroville Dam, one of the state's largest reservoirs. Inflows into the lake are rising despite the dam's hydropower plant operating all five of its units and releasing water.

Last month, water flowed over Oroville Dam's emergency spillway and caused significant erosion and risk of structural failure that led to massive evacuations downstream. The dam's primary spillway also has major erosion damage but there's a chance it may need to be used next week to reduce water levels as more snow melts.

The snowpack supplies about 30 percent of California's water, particularly in the spring and summer months. As of Friday morning, the Sierra snowpack was 178 percent of average for this time of year and had approximately 47 inches of liquid on average. The Sierra snowpack historically reaches its peak around April but Rippey said California may have already hit the peak earlier than normal.

"I can think of a worse situation if we had stayed stormy through March and then it all decided to melt at once," he said. "We're fortunate that we've been dry for the last several days. It is early enough in the year that if we can bleed off some of this snow early it could actually help to reduce some of the risk later on."