Heavy rain to hit snowy California, fueling flood worries
A powerful atmospheric river will send a surge of subtropical moisture into California on Friday, delivering heavy rain and even more mountain snow. It will also bring a renewed risk of serious flooding as warmer rain combines with widespread snow and saturated soil.
The storm will target Northern and central California, including the Bay Area, where flood watches blanket much of the region lasting into Sunday. Heavy rain and melting snow could cause significant flooding.
The National Weather Service made the rare move of upgrading the risk of excessive rainfall from “moderate” to “high” on Thursday afternoon. High-risk events are associated with a large percentage of deaths and damage associated with flooding. The high-risk zone includes portions of the southern Sierra Nevada region and central California coastal ranges.
Weather Service forecasters wrote that the flood risk had increased because computer models are now simulating higher snow levels and downpours. Up to 10 inches of rain in 24 hours could fall in some areas where there is snowpack, and so “the potential for widespread flooding is considerable,” the Weather Service said.
Rivers could show significant rises into the weekend, and several are forecast to exceed flood stage. Totals of up to 10 inches are possible in the Santa Cruz Mountains, along the Big Sur coast and in the southern Sierra by Saturday. Broad swaths of the state, particularly the western foothills of the Sierra, are also at risk for flash flooding, rockslides and mudslides. Massive snowfall — well over 6 feet — is again possible at the highest elevations.
The National Weather Service office in Sacramento said up to 5 to 12 inches of rain could fall in the foothills and mountains through Sunday, with the heaviest into Friday.
“Flooding of roadways, rivers, creeks, streams, and other flood prone areas will be possible, especially in areas that have poor drainage due to snow blocking drains and culverts,” the Weather Service wrote, noting that rockslides and mudslides are also possible in susceptible areas.
Relentless snowstorms have battered the state since late February, burying mountain towns while helping to build a nearrecord snowpack. The wet winter has had a remarkable impact on the California drought. According to the latest U.S. Drought Monitor released Thursday, much of central California is now droughtfree, including areas that were in the most severe drought category before winter started. But that relief comes with escalating flood potential and snow-related impacts.
UCLA climate scientist Daniel Swain called the latest storm a “classic ‘pineapple express-type’ atmospheric river event” with moisture originating near Hawaii.
“This is going to be a pretty warm storm immediately on the heels of what were historically cold storms, at least in terms of the ability for these moist air masses to drop a lot of snow at low elevations,” he said in a video update on Wednesday.
Rain-on-snow flood threat
With warmer air moving into California, the snow line will be much higher than in previous storms the past few weeks — as high as 8,000 to 10,000 feet.
“This means all coastal ranges and much of the Sierra foothills will receive heavy rain on top of snow, increasing the threat of rapid runoff due to snowmelt,” the Weather Service’s Weather Prediction Center wrote in a discussion.
Most snowmelt is expected at lower elevations, where temperatures will be warmer.
“Rain that falls on shallow, low-elevation snow will melt it quite quickly,” said Anne Heggli, a doctoral student who researches rain-on-snow events at the Desert Research Institute in Reno, Nev., in an email. “Rain that falls on deep snow will increase the density of the snow by taking up the pore space between the snow crystals.”
Although snow higher in the mountains is unlikely to melt, it’s an open question just how much rain the massive snowpack will absorb.
“Given our recent cold storms, and very deep snowpack, I think middle-upper elevations have a reasonably good capability to absorb a lot of the upcoming moisture,” said Benjamin Hatchett, an assistant professor of atmospheric science at the Desert Research Institute, in an email.
However, rain can also flow freely to the ground through vertical channels in deep snowpack, quickly reaching the land and flowing over it.
“We know we have saturated soils, which are really the kicker in runoff production (not snowmelt as commonly thought), so any water making it to the land surface will flow into stream channels,” he wrote.
Middle elevations are a big question mark — and could end up contributing substantial runoff in the coming days.
“This can be the difference between nuisance flooding and major problems,” Hatchett said.
Storm could ‘prime’ the snowpack for future floods
Flood risk could escalate if additional atmospheric rivers arrive next week, as forecasts suggest. This week’s rain could carve pathways in the snowpack that could funnel water to the surface during subsequent storms. It could also increase the density of the snowpack, making it “ripe” for future melting.
“My concern is not so much this storm as the next one,” Hatchett said. “This will very likely act like a priming event, similar to January 2017, which was the key event in why the February 2017 storms caused so much havoc.”
In February 2017, intense atmospheric rivers produced heavy flows into the Lake Oroville reservoir, with the watershed already saturated from previous winter storms. Fears of an impending dam failure required the evacuation of 188,000 people downstream.
Heggli said that widespread flooding similar to January 2023 or February 2017 isn’t expected this week, but that could change if the storm onslaught continues.
“There will simply be less capacity in the watershed to take on additional rainfall with each consecutive warm and wet storm system,” she wrote.
The California Department of Water Resources signaled this week that it may soon shift into flood control mode for some reservoirs, a move that is expected given the wet winter and large snowpack looming above. For example, it may begin to release water through the main spillway at Lake Oroville as early as Friday.
“It does suggest that the inflows are now big enough and that the reservoir levels are now high enough, that they don’t want any more water stored in them for safety reasons,” Swain said. “So, they’re going to start to release more of that water from a number of these reservoirs as flood control operations begin.”