A warm and wet atmospheric river started rolling into Northern California late Thursday, kicking off three days of unusually heavy springtime rain and a gorging of rivers as far away as Reno.
A flood watch was issued for nearly all of Northern and Central California, and the National Weather Service said some localized flooding was likely. Some of greater Sacramento‘s most flood-prone areas could take on water as the weekend approached, including Discovery Park north of downtown and the Michigan Bar area in the southeastern end of the county.
Sacramento could get 2 to 3 inches of rain, while the foothills were in line to receive 3 to 7 inches. By the time the storm tapers off Sunday, the city could get twice as much rain as it normally gets in the entire month of April, said Tom Dang, a meteorologist with the weather service.
“This kind of storm normally comes once a year, so for it to happen in April, that’s pretty uncommon,” Dang said. “Normally we would be drying out this time of year.”
Much of the winter has been unusually dry, conjuring up fears of another drought. But March brought more rain and snow than usual, and April is bringing a late jolt of wintry weather as well.
A big difference between this storm and the year’s previous weather systems: the warmth. Temperatures were expected to remain in the 50s at elevations as high as Blue Canyon during the most intense part of the storm. That means the Sierra Nevada could expect a lot more rain than snow. Portions of the snowpack will melt, and Northern California's rivers will fill up more quickly than they would in a cold storm.
“The most immediate concern is in the mountain communities,” said Dave Rizzardo, snow-surveys chief at the California Department of Water Resources. “You're going to see very high stream levels, the rivers are going to be running fast. ... It could be dangerous. This is not one to mess around with.”
The federal Bureau of Reclamation is expected to increase flows over the weekend from Folsom Dam into the lower American River through Sacramento.
Sacramento County parks director Jeff Leatherman said the county will close access to the American River parkway at two key points in expectation of flooding as Folsom outflows increase.
The Discovery Park and Howe Avenue vehicle access roads and parking lots will be closed, starting Friday morning and likely through the weekend, Leatherman said. Both lots are low-lying and susceptible to flooding. Discovery Park is susceptible to high flows on both the American and Sacramento rivers.
“We are advising bike commuters and recreational bicyclists to just use caution,” Leatherman said. “We will post updates on our website, and signs on the trail if we get localized flooding on the trail.”
The rivers around Sacramento that tend to flood, notably the Cosumnes River in the southeast end of the county, are expected to rise to within a few feet of flood stage. The Truckee River is expected to exceed flood stage at Truckee and Reno, according to the federal government’s California Nevada River Forecast Center.
Alan Haynes, of the River Forecast Center, said the Merced River might get high enough to disrupt operations at Yosemite National Park.
“They're probably going to have to close some roads and evacuate some campgrounds,” he said.
He said the Sacramento River “should behave itself” but it could over-top the Fremont and Tisdale weirs, pouring water into the Yolo and Sutter bypasses Sunday or Monday. The bypasses are agricultural areas that serve as flood-control safety valves to protect urbanized Sacramento.
"People will probably see some water over the (Yolo) Causeway," Haynes said.
The storm began with light showers in the northern Sacramento Valley on Thursday afternoon. Considerably heavier rainfall is expected to soak much of the valley Friday night and Saturday morning, before lessening in the afternoon.
Snow elevations were expected to stay above 9,000 feet during the atmospheric river’s fiercest window, but forecasters said temperatures will turn colder Saturday. Snow levels will drop to 5,000 feet, offering a chance to bolster the Sierra Nevada’s weak snowpack. Snow remains possible through Sunday morning.
The warm storm also has triggered repeated warnings this week from the Department of Water Resources that the largely rebuilt flood-control spillway at Oroville Dam could get its first test over the next few days. DWR has been releasing water from the dam's other outlets to keep Lake Oroville down. The reservoir Thursday was still 36 feet below the point where DWR would start pouring water out of the spillway, but that could change quickly as the storm gathers ferocity.
The spillway fractured during a heavy storm last winter, prompting an emergency evacuation of 188,000 residents as lake levels rose. While DWR officials say the rebuilt structure is strong enough to handle substantial water releases, final reconstruction isn’t expected to begin later this spring.
The unusually wet April won’t end this weekend. Two weather systems originating in the Gulf of Alaska are expected to bring more rain and snow to Northern California next Tuesday and Thursday, although rainfall totals are expected to be small compared to this week's storm.
Sacramento saw 5.37 inches of rain in March, 2.35 inches above the historical average, but it wasn’t enough to fully make up for a slow start to the rainy season and bone-dry February.
“When you think about how dry the winter's been ... in some way for the reservoirs this is kind of a little bonus,” Rizzardo said.