California’s ‘big melt’ of snow is about to kick into high gear. When to expect flooding

Afternoon high temperatures well above normal for this time of year are hitting Fresno and the central San Joaquin Valley, setting the stage for speeding up the melting of a record Sierra Nevada snowpack and potentially adding to flood concerns in the Tulare Lake basin.

The forecast for Fresno called for the temperature to tickle the 100-degree mark on Sunday — a significant lurch higher after Friday’s 82 degrees.

“Confidence is overall high that we will be in a summer-like pattern for an extended period of time” into much of this week, the National Weather Service reported in a forecast discussion Friday.

A heat advisory was issued Friday for the period from noon Saturday through Monday evening, with forecasters urging people to stay out of the sun and remain in air-conditioned rooms, to drink plenty of fluids, to check on vulnerable relatives and neighbors, and not to leave children or pets unattended in vehicles.

While the Valley will see temperatures soar, “the mountains are going to warm up more than the Valley, in terms of the amount of warming,” meteorologist David Spector, from the National Weather Service office in Hanford, told The Fresno Bee on Friday. “We’re going to see increased snowmelt especially by early (this) week.”

The increased melt of the massive volume of snow in the southern Sierra Nevada on the Valley’s eastern flank may push rivers including the Kings, Kaweah and Tule rivers past the capacity of their channels in the upper reaches above the foothill dams, Spector said, “so we might see some flooding up there.”

In the Valley, however, “only the Kings River has flooding below (Pine Flat) Dam,” he added. Whatever water that’s released from Pine Flat Dam to make room for more melting snow and cannot be diverted for groundwater recharge or agricultural use flows into Tulare Lake, the normally dry vast lakebed in Kings and Tulare counties that has been rejuvenated this spring by a series of atmospheric river storms as well as by snowmelt.

Daniel Swain, a climate scientist at UCLA, said this week that cool weather in the first week of May slowed the pace of the snowmelt somewhat after a late April spurt of hot weather.

‘Huge amount’ of snow remains high in Sierra Nevada mountains

“Snow is still melting at a pretty rapid rate, but at a less rapid rate than it was during the heat wave which did end up breaking a few records for heat in late April,” Swain said in a video blog Monday.

But this weekend, he added, “snowelt will once again accelerate as temperatures warm considerably.”

“A vast majority of that water that was stored up in the Sierra as snow is still there. It has not melted yet,” Swain said. “It is definitely melting fast and a lot of the snow at lower elevations is already gone. But of course that’s not where most of the snow water actually resided.”

“The majority of that snow water was up above 6,000 or 7,000 feet anyway, and that’s where a huge amount of snow remains,” he explained. “So that Central Valley flood risk, particularly Tulare Lake and Tulare Basin flood risk, is going to persist for weeks.”

“Best estimates are still that in a lot of places, the peak snowmelt potential isn’t until late May and early June. So we still have a long way to go,” Swain added.

That prospect of a prolonged snowmelt and continued Kings River excess flows into Tulare Lake have officials scrambling to raise the level of a 14.5-mile levee protecting the city of Corcoran from the resurgent lake. The state of California put up more than $17 million for the Cross Creek Flood Control district to raise the levee — which has been compromised over the years by ground subsidence from overpumping the water table in the area — by four feet, from its most recent height of 188 feet above sea level to 192 feet.

Gov. Gavin Newsom said earlier last week that this is the third time in recent decades that either state or federal officials have taken steps to raise the Corcoran Levee because of subsidence. Newsom and officials from his administration got a firsthand look at the Tulare Lake flooding threatening Corcoran and two nearby state prisons when they visited the area on April 25.

“Raising the Corcoran levee provides greater certainty that we won’t need to evacuate critical facilities and will ensure public safety,” Newsom said Thursday. “However, the state and federal government cannot continue stepping in to raise this levee. I look forward to a conversation on what (Kings County) is going to do differently so that we don’t find ourselves in this situation again.”