Last week, as forecasters at the National Weather Service in the San Francisco Bay Area looked at a series of storms that would hammer the region, they predicted that some areas in the North Bay would see six to eight inches of rain between Saturday and Tuesday.
Instead, around a foot of rain fell at the wettest locations. Highways were flooded, landslides were triggered, and rivers swelled. By Tuesday afternoon, a house slid down a saturated hillside and fell into the Russian River in Sonoma County. Evening evacuation orders were issued, hours before the swollen river started spilling over onto a handful of roadways.
The storms, known as atmospheric rivers, are a common feature of West Coast winters. They’re thin ribbons of moisture carried by powerful winds, sometimes for thousands of miles, which can make forecasting them especially challenging.
“We got a little more rain than expected, and the rivers responded more than expected as well,” said Dylan Flynn, a meteorologist with the Weather Service. “A couple days ago we didn’t expect any of the rivers to flood.”
The rain could have been even more impactful if Northern California hadn’t seen an abnormally dry January. The first of the recent rains was mostly absorbed by the dry soil starved of rain, sparing the area from more severe flooding.
Another one of these moisture-rich storms is slated to sweep the region Thursday into Friday, bringing more rain and flooding concerns, but Mr. Flynn said this one is expected to be more “run-of-the-mill” than the last.
This prolonged spell of atmospheric-river activity in Northern California has brought rain to the valleys and coast and snow to the mountains. The storms’ effects have stretched as far north as Washington and into Southern California.
To pinpoint where these systems will land, meteorologists use tools far more sophisticated than what was available two decades ago, including higher-resolution satellites and weather models run on supercomputers. There’s even a program to fly hurricane-hunter planes from the National Oceanic and Atmospheric Administration into the storms to collect data.
Still, it is tricky to predict these systems with perfect accuracy.
A misforecast can result in surprises. Strong atmospheric rivers carry copious amounts of moisture, and if a storm unexpectedly shifts slightly one way or another, it can bring a lot of rain to one watershed versus another, consequently leading to one river to flood while another remains safely in its banks.
Chad Hecht, a meteorologist with the Center for Western Weather and Water Extremes at the University of California, San Diego, said pockets of intense rainfall can occur within these storms and lead to hazards such as urban flooding or mudslides in areas where wildfires have recently burned. “These pockets can form at such small scales that forecast models struggle to resolve them, and it’s hard to identify exactly when and where they will occur,” Mr. Hecht said.
As these storms began taking shape last week, there was some concern that they could shift south to Los Angeles, where devastating wildfires last month left the ground scarred. Instead, by early this week, forecasters said they expected only up to about an inch of rain for some parts of that region. And by Wednesday, they said, the storm was “underperforming” there, with up to half an inch of rain measured.
Atmospheric rivers are corridors of moisture transported through the atmosphere by powerful winds. They’ve long existed all over the world, but the term was only first used in scientific literature in the 1990s, and the media quickly latched onto the colorful description.
On the West Coast, the systems form over the Pacific Ocean in the subtropics. As they travel over the ocean, they pull in moisture and move across a vast, empty expanse where observational data is sparse compared with what the robust network of weather stations on land can collect. There are about 100 buoys managed by NOAA in the Pacific, the world’s largest ocean, that help predict the approach of a storm, versus 1,000 observational data points that can forecast the storm’s course once it reaches land, said Marty Ralph, the director of the Center for Western Weather and Water Extremes.
“Satellites help, but big gaps remain,” Mr. Ralph said.
Cliff Mass, a professor of atmospheric science at the University of Washington, said that today’s sophisticated satellites mostly make up for the lack of observational data available over the ocean. He’d like to see more high-powered, long-distance radar set up along the West Coast to collect data from the ocean as a storm approaches. “We can take the data from those radars and take it into our models, and then we could substantially improve our forecasts,” Mr. Mass said.
Even with all the tools available, a forecast is a precarious undertaking, because while the storms can be thousands of miles long, they’re also narrow at only a few hundred miles across.
“Because of that narrow plume of moisture, these system can literally thread the needle between observations,” said Brian Garcia, a meteorologist with the Weather Service.
When these storms hit land, they’re wrung out like sponges. Forecasters are tasked with pinpointing where the systems will unleash that moisture. One thing they know for certain is that these systems are likely to release the most rain when they interact with mountains on West Coast, where there are a lot of large ranges, including the Siskiyous and the Sierra Nevada.
“Because the mountains don’t move, especially relative to the weather, that makes it easier to predict where the heavy rain or snow is going to be,” Mr. Ralph said.
While the mountains don’t move, the actual storms can make slight yet unpredictable shifts because of disturbances in the atmosphere. Mr. Garcia said that atmospheric rivers travel through the air more like a Frisbee with a wobble than like an arrow with a straight path.
In the case of the system that swept California from Monday night into Tuesday, Mr. Garcia said it “wobbled” into the Bay Area, resulting in over a foot of rain on Mount Tamalpais in Marin County. Just across the Golden Gate Bridge, San Francisco recorded 2.8 inches of rain on Tuesday, breaking its same-day rainfall record of 2.2 inches set in 1882.
The region is expected to get more rain on Thursday and Friday, though that storm is expected to be less impactful than the previous two. By the end of the day on Friday, Mr. Garcia said, some of the wettest locations in the North Bay may have recorded up to 18 inches of rain over the last week.
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