A warm spell forecast for Wednesday, Thursday and Friday is raising concerns that the rate of melting in California’s huge Sierra Nevada snowpack will increase, potentially boosting the likelihood of flooding in the Central Valley and other areas as April gives way to May.
“The big melt is now here. This is that week,” said Daniel Swain, a climate scientist with UCLA, on Monday.
There are no flood risks in the Bay Area, where temperatures mid-week will hit 90.
But the National Weather Service on Monday issued a flood watch for Thursday morning through next Tuesday for the Merced River in Yosemite National Park, which is forecast to exceed flood stage by 1 to 2 feet by the weekend.
If the river, which has a flood stage of 10 feet, hits 12 feet at the Pohono Bridge, that can causing moderate flooding on Northside and Southside Drive, the main roads in and out of Yosemite Valley, which can result in the park being closed for a few days.
Last week, park officials announced that Yosemite was likely to close on some days between late April and July as the state’s massive snowpack sent waterfalls raging, but also rivers and streams surging. A flood stage of 12 to 13 feet has occurred several times in recent years, in 2017 and 2018. The record is far higher, however, 23 feet, during the disastrous floods in Yosemite on New Year’s Day in 1997.
The more significant flood danger has been focused farther south, in the Southern San Joaquin Valley.
There, at least 60,000 acres of farmland in Kings County and Tulare County already is under roughly 3 feet of water, according to the Kings County Sheriff’s Department, due to heavy rains in mid-March from atmospheric river storms.
Farmers in the area, and residents of towns like Corcoran, a community of 22,000 people between Fresno and Bakersfield west of I-99, were watching the warming weather nervously.
“It definitely means continued pressure and concern that some of our levees that have been temporarily repaired and shored up may not do so well,” said Tricia Stever Blattler, executive director of the Tulare County Farm Bureau. “All of these 80 and 90 degree days are going to be very problematic.”
The area most at risk is the Tulare Lake Basin.
Up until the late 1800s, a lake there, Tulare Lake, was the largest freshwater lake west of the Mississippi River. It supported vast expanses of fish, ducks, elk and other wildlife, and was home to native tribes, particularly the Yokuts. Steam boats sailed on it.
It was last full in 1878. Farmers and ranchers began to settle the area, and diverted streams and creeks that fed the area so much that it began to shrink. Dams on the main rivers flowing from the Southern Sierra into the area also held back water.
By 1899, Tulare Lake had dried up.
Farmers planted row crops and orchards. They established dairy farms. Today Tulare County has an $8 billion agriculture industry, the third largest in California, behind Kern and Fresno counties.
But in very wet years, Tulare Lake, called the “ghost lake” by some locals, begins to reappear. The area has flooded regularly, including in 2009, 1997, 1986, 1983, 1955 and 1938. In 1983, a particularly wet year with a similar snowpack, it took nearly 2 years to dry out.
The hot weather this week — which will reach 95 degrees in the San Joaquin Valley until it begins cooling back into the 70s next week — will double the rate of flow on some Southern Sierra rivers as the snow melt picks up, said Michael Anderson, the state climatologist, with the Department of Water Resources.
Anderson said Monday, however, that there is enough room in reservoirs that have been letting out water to capture the excess flows that he doesn’t expect flooding this week.
“We’re dealing with a melt and it will accelerate this week,” he said. “But it’s not going to accelerate to such a rate that everything falls apart.”
That risk will increase into May, he said, as hot weather continues.
“Eventually there is a risk that reservoir releases will exceed channel capacity,” Anderson said.
Geography is also a challenge. Most areas of the state have places where large amounts of water can quickly drain. The San Joaquin River to the north drains into the Delta and San Francisco Bay, en route to the ocean. But the Tulare Basin, to the south, because of its history, and because of years of groundwater overpumping by farmers that has caused land to subside, sits a large bowl where a 10 million-acre lake once flourished.
Now, that area is home to miles of farmland and huge dairies between Interstate 5 and Highway 99.
“The problem is there’s nowhere else for this water to go in the Tulare Lake Basin,” Swain said. “It’s just going to fill up like a bathtub.”
On Monday, the statewide Sierra snowpack was 256% of average. The Southern Sierra, which received the most snow ever recorded this winter, was at 322%.
Officials from the State Office of Emergency Services have been working for the past month with local officials in the Tulare Lake Basin area to prepare for flooding, Brian Ferguson, a spokesman for the office.
He said officials have been mapping out the locations of wastewater treatment plants, hospitals, prisons and other sensitive areas where flooding could cause a major problem during a sustained heatwave in the weeks or months to come. They are planning with local fire departments places to set up shelters or to pre-position rescue crews.
“We are going to have a challenging few weeks to come,” he said.
But he acknowledged that state officials don’t know what areas will flood seriously in the area, when or how much. It depends on the weather.
Luckily, California was not hit with the kind of warm “Pineapple Express” storms this spring that could melt much of the snowpack in one huge surge, like happened in 1997. Added to that, temperatures in April have been cooler than normal. And next week will see relief from the temperature spike.
“Slow and steady is the name of the game,” Ferguson said. “We’ve been fairly lucky so far with the pace of the water. We hope to keep it that way.”
