That big weekend storm is still aimed at the Mother Lode, Yosemite National Park and the rest of the Central Sierra, threatening to bring warm, heavy rains that could melt mountain snows, swelling streams, creeks and rivers with potential for flooding unseen in a decade or more.

Forecasts through Thursday show the atmospheric river of concentrated moisture, set to unload on significant snowpack in the high mountains, shares similarities with the disastrous storms and flooding of December 1996 and January 1997.

But the approaching megastorm is unlikely to match the historic rains and flooding of 20 years ago, according to forecasts.

That devastating series of storms, described as a classic Pineapple Express laden with tropical moisture dumping on significant snowpack, wreaked havoc up and down the Mother Lode.

Among the disasters, just a few days before Christmas Eve 1996, a storm-toppled ponderosa crushed 130 feet of rickety flume on the Tuolumne Main Canal below Lyons Reservoir, cutting water supply to 10,000 Tuolumne County residents and washing out a section of South Fork Road in Twain Harte.

Over a three-day period centered on New Year’s Day 1997, warm moist winds from the southwest blowing over the Sierra Nevada poured more than 30 inches of rain into watersheds already saturated by one of the wettest Decembers on record, according to a state flood emergency action team’s final report.

Torrents of rain and snowmelt in the Tuolumne River exceeded flood control capacity of Don Pedro Reservoir, overtopped a spillway, tore out Bonds Flat Road and caused more flood damage downstream in West Modesto.

Record-setting runoff swelled the Merced River to burst its banks and put much of Yosemite Valley under several feet of water, stranding 2,100 visitors in the park, flooding meadows, roads, campgrounds, buildings and dwellings, including parts of Yosemite Lodge, and causing an estimated $178 million in damage and destruction. The park was closed two months for repairs and tourism in Groveland suffered.

Calculating costs

The January 1997 flood was the largest in a century of records on several Central Sierra rivers, including the Tuolumne and the Merced.

According to federal weather agencies that reviewed the 1996-97 storms and flooding statewide, more than 100,000 people had to be evacuated in Northern California as high waters threatened homes and businesses in the mountains, foothills, valleys and cities.

At least eight people died, several towns were inundated, three hundred square miles flooded, including Yosemite Valley, major roads remained impassable for weeks after rains stopped due to flood damage and mudslides, and 48 counties declared disasters, including all 46 counties in northern California.

Damage was calculated at $1.8 billion in economic losses, including 23,000 homes and 2,000 businesses damaged or destroyed.

In addition, the series of Christmas and New Year’s storms caused an estimated $1 billion in damages across the eastern Sierra Nevada and portions of western Nevada. Casinos in Reno used sandbags to protect their properties, allowing several to remain open to gamblers. Four major casinos needed to close during the peak of the flooding due to extensive damage.

Flood waters from the Walker River destroyed several miles of Highway 395 through narrow Walker River canyon. Reno-Tahoe International Airport was completely shut down to air traffic as flood waters submerged runways and portions of the terminal building.

Mike Kochasic, a National Weather Service meteorologist in Sacramento, said the approaching storm is expected to bring heavy rains and flooding potential but it’s unlikely it will match the historic storms and flooding of January 1997.

“It’s similar but it’s not quite as severe,” Kochasic said Thursday. “The last time we saw this type of flooding was back in December 2005. There was rain on snow that time, too. In 96-96 all the mainstem rivers were all running really high. Here now the mainstem rivers and reservoirs still have capacity.”

What to expect this time

Continued confidence in forecasts for this weekend’s storm system prompted National Weather Service staff in Sacramento to issue a video briefing on flood safety, urging people in communities including the Central Sierra to make flood plans and consider evacuation routes if circumstances turn nasty.

Forecasters on Thursday were still describing the approaching storm as an atmospheric river, a narrow corridor of concentrate moisture, aimed at California.

It’s expected to bring heavy precipitation and high snow levels, dumping on soils already saturated from recent storms.

“Moderate precipitation starts Saturday, but the brunt of storm will be Sunday, tapering off Monday,” National Weather Service staff said. “Majority of the precipitation will fall as rain in the mountains as snow levels rise above 8,000 feet. This is going to be some of the more significant flooding we’ve seen in the past decade.”

More stormy weather is possible Tuesday and later next week, forecasters say. They urge people to remember flooding could take several days to recede in some areas once rains cease.

Sand and sandbags are available at multiple locations in Calaveras County and Tuolumne County. They include Arnold Maintenance Yard, Glencoe Maintenance Yard, Jenny Lind Yard, Mountain Ranch Community Park, Murphys Fire Station and the Corp yard on Mountain Ranch Road in San Andreas.

In Tuolumne County, sand is available at Columbia Airport, 18870 Birch St. in Tuolumne, 18188 Seventh Ave. in Jamestown and 11240 Wards Ferry Road in Big Oak Flat. Tracie Riggs, the county Office of Emergency Services coordinator, urges people to remember to bring their own sandbags, which can be purchased at local hardware stores.

Forest closures

Stanislaus National Forest roads and day-use areas in the Groveland and Summit ranger districts are at high risk for landslides or flooding this weekend, and they will be closed, Forest Service staff announced Thursday.

In the Groveland Ranger District, Lumsden Road, the Cherry Borrow Pit Trail, and Preston Falls Trailhead will be closed Friday.

“We’re closing these areas because they are prone to landslides or flooding during rain events,” said Groveland District Ranger Jim Junette. “We want to be sure visitors don’t get caught on the wrong side of a landslide or flash flood, since the area is expecting as much as seven inches of rain next week.”

Junette said the areas will reopen as quickly as possible, but it will depend on whether they experience problems this weekend.

“We don’t have a specific reopening date and won’t have one until we can get in and assess whether they have been impacted by the weather,” Junette said.

In the Summit Ranger District, Tri-Dam Project staff will close the north gate of Beardsley Dam just before the spillway, for the rest of the winter season. Tri-Dam Project staff say they are removing the bridge across the spillway due to forecasts for extreme flow estimates in the Stanislaus River watershed. Access to China Flat Day Use Area could be limited until late May or early June, depending when Tri-Dam re-installs the bridge.

What about Yosemite?

Stream gauge data for the federal California Nevada River Forecast Center on Wednesday showed for a time that the Merced River in Yosemite Valley could rise this weekend to more than double its flood stage, and approach the historic high water mark set in early January 1997.

That scenario was downgraded Thursday, with new data showing the Merced River could still rise 5 feet or more above its 10-foot flood stage at Pohono Bridge. Flooding is still very much in the forecast but it is not expected to approach or exceed devastating flooding witnessed and recorded 20 years ago in Yosemite.

The Wednesday forecasts spurred National Park Service public information officers in Yosemite to urge potential visitors to make alternate plans in case authorities decide to close the park.

“Yosemite National Park is making preparations for visitor and employee safety in response to weather reports predicting significant precipitation, and possible flooding, over the next several days and through the weekend,” said Jamie Richards with the National Park Service. “The predictions for significant rainfall in Yosemite Valley, well above flood stage on the Merced River, could prompt the park to be closed in the next few days.”

Richards described the significant flood event of January 1997, which caused extensive damage to park roads, campgrounds, lodging, and utilities. The park was closed until March 1997 due to extensive damage to park infrastructure. During the closure, there was no running water and electricity was intermittent.

Yosemite National Park staff were monitoring forecasts Thursday. Richards distributed video of the January 1997 flooding in Yosemite Valley. No decision was made Thursday on whether the park will remain open this weekend.