Warmer storm brings rain, hastens Sierra snowmelt. But here’s what may limit flood risk
A classic Pineapple Express steaming from the subtropics has a historically heavy — and low-lying — Northern California snowpack in its sights and communities from the Sierra Nevada to the foothills and flatlands bracing for the snowmelt to come.
Weather experts in Sacramento say straight away that the powerful storm crashing into the region will not be a repeat of 1997’s destructive and deadly flooding..
“There’s going to be a lot of water coming down as rain and is expected to melt the snow into the 2,000 to 4,000 feet area,” National Weather Service Sacramento forecaster Bill Rauch said in a Thursday morning briefing. “All that said, this is not a 1997 scenario.”
UCLA climate scientist Daniel Swain in his Wednesday video briefing said much the same: “This is not an Ark scenario,” he said. “This current event doesn’t look like a disaster. There will be flooding, significant or even serious locally but probably not on a widespread basis.”
Even so, the powerful rain-on-snow event has foothill communities on edge and river and levee monitors along waterways including the Cosumnes River near Elk Grove on alert.
Heavy rain, along with the runoff from melting snow, is expected to push the largely unregulated Cosumnes River at Michigan Bar near Rancho Murieta to a crest of 13.3 feet, more than a foot above its 12-foot flood stage at 7 p.m. Friday, say state river monitors.
Spots along the Mokelumne River at Benson’s Ferry and Tuolumne River in Modesto also will exceed flood stage.
But what will the heavy, warm rainfall mean for a snowbound Sierra and the creeks and streams of the Sierra foothills?
Climate scientist Swain had answers in his Wednesday briefing.
Outlook for 4,000 feet and below
Foothill communities from around 3,000 to 4,000 feet — think Pollock Pines in El Dorado County at 3,980 feet to Colfax in Placer County at 2,425 feet to Grass Valley at 2,411 feet — and those much lower blanketed by unusually heavy snows are “quite vulnerable to melting when multiple inches of rain fall on top of them as with this atmospheric river. That’s likely to be a concern up to about 4,000 feet in Northern California,” Swain said.
“There may be some significant snowmelt on top of the heavy rainfall to about 3,000 to 4,000 feet in the north and there is a substantial amount of snow on the ground in a lot of those places,” he continued.
In the Yuba County foothills, New Bullards Bar Reservoir on the North Yuba River east of Marysville began releasing flows Thursday to make room for rain and snowmelt, Yuba County Water Agency officials said.
“The main flood threat is coming from the fact that this storm is going to bring a significant amount of rainfall in its own right,” Swain said Wednesday. “All the rain that falls will become runoff and that’s going to be a challenge.”
Swain added that the foothills could see the worst small stream and river flooding “because that’s where you will see the maximum combination of heavy precipitation and intensive snowmelt.”
By Thursday, Bay area National Weather Service forecasters had tweaked their prediction, updating the Bay area and Central Coast to “high risk” for flooding for the first time since 2010.
“Monterey County and Southern Sierra foothills are really going to be the bullseye for flood risk during this event,” Swain said in a Thursday post to Twitter.
Deep snow, freezing cold help up higher
Farther up Interstate 80, Highway 50 and the mountain passes, concerns weighheavily on communities buried under tens of feet of snow now awaiting rain at even the highest elevations. But Swain said high elevation snowmelt may be held to a minimum.
“There’s been much concern about the snow-melting potential of this warm atmospheric river at higher elevations where the snowpack is very deep (but) it does not appear likely that this storm is going to melt that much of that very deep snowpack in the high elevations,” Swain said.
Among the reasons, extremely deep, frigid snows well below freezing that should soak up this weekend’s rains.
“It’s really difficult to melt a deep snowpack and particularly a cold snowpack,” Swain explained. “This snowpack is both deep and cold. It would take a tremendous amount of energy to melt that much snow, generally much more than rain falling at 34 to 35 degrees, which is what the rain will likely be in the high elevations during this event.”
Rain on snow is likely at 5,000 to 7,000 feet and above, but the snowpack has a lot of space for additional water to enter, then refreeze, because the snowpack is well below 32 degrees.
Swain compared the process to pouring water into a freezer. Warm water will melt a lot of the ice inside, but if the amount of water is small compared to the amount of ice and is already close to freezing, the water will freeze again.
“That’s the primary effect we’re going to see at high elevations above 5,000 feet in the south and 4,000 feet in the north. We’re going to see the snowpack absorb most or all of this water,” Swain said.
This story was originally published March 10, 2023 at 6:00 AM.
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