An unusual summer storm arriving Monday could break rainfall records in Sacramento and many other areas of Northern California.
Officials at the National Weather Service said Friday that Sacramento could see an inch of rain or more on Monday. Areas of the northern Sacramento Valley could see 2 inches, and the North Coast could get as much as 4 inches.
The wettest June day on record in downtown Sacramento was June 4, 1993, which saw 0.81 inches of rain, said Johnnie Powell, a forecaster at the National Weather Service.
"If we're going to get wet, we might as well go for the gusto," Powell said.
Temperatures are predicted to reach 92 degrees in Sacramento today. Then dramatic changes begin. By Monday, the high temperature is expected to be 20 degrees cooler, along with skies that look a lot like winter.
"We do get rain in June. But this one's unique because it's bringing in so much moisture. That's what makes it highly unusual," Powell said.
A weak storm system will move across the state on Sunday. Little rain is expected in the Sacramento region with this storm.
The second system arrives Monday. It will be much wetter, because it is tapping into the remnants of a tropical storm on the other side of the Pacific Ocean, said Kelly Redmond, a climatologist and deputy director of the Western Regional Climate Center in Reno.
This second storm is considered an "atmospheric river," Redmond said, a narrow band of water vapor that travels long distances and often targets relatively small areas.
In this case, Northern California is in the bull's-eye, although the precise target can be hard to predict.
These storms, historically, have been responsible for some of California's worst floods.
That is not expected this time, because the predicted rainfall won't exceed a typical winter storm. But it may be enough to make roads slippery and cause minor street flooding.
Snow levels will be very high, above even the highest Sierra Nevada peaks, Redmond said.
A key difference with this storm, Redmond said, is that it will feel like continual rainfall during the day on Monday.
Most typical summer storms are showery, with brief cloudbursts, not continual rain.
"It's more like a winter situation, but occurring in late June," Redmond said.
The storm is yet another instance of odd weather this year.
Winter began with a bang, producing one of the wettest Decembers on record. Then the storm track went elsewhere and the balance of winter was one of the driest ever, resulting in an overall dry season for the second year in a row. Many regions of the state are now experiencing drought problems.
In a sign of that stress, state and federal water agencies reduced their water diversions from the Sacramento-San Joaquin Delta to avoid violating a key water quality rule. Those diversions help meet the water demands of 25 million Californians and 3 million acres of farmland south of the Delta.
The water quality rule is primarily aimed at avoiding excess salinity in the Delta, which often occurs when freshwater runoff through the estuary is reduced during drought years. Part of the intent is also to ensure enough freshwater outflow to San Francisco Bay to protect threatened fish species.
The U.S. Bureau of Reclamation, starting at 10 a.m. today, planned to shut off its export pumps near Tracy completely for 24 hours.
The California Department of Water Resources on Friday reduced its own Delta diversions from 1,500 cubic feet per second to 1,300. It also increased water releases from Oroville Reservoir from 3,500 cfs to 4,500.
"That demonstrates how fragile the state's water delivery system is," said Bill Jennings, executive director of the California Sportfishing Protection Alliance. "If they are forced to release more water to comply now, what's going to happen in a couple months? This could be a critical situation."
Reclamation planned to resume water diversions on Sunday, but at low levels.
The approaching storm and the freshwater runoff it produces may help improve Delta water quality and restore diversions.
But the storm is not big enough to improve California's water supply situation, Redmond said, because the state experienced one of the driest January-through-June periods ever.
"This would slow the use of water in the reservoir systems," he said. "But we're actually so far behind, even if we got a fairly decent rain, it would still end up being at or below the driest on record."
Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.