Parts of central and northern California have seen record-low precipitation since March but that’s set to change with the incoming drenching. It’s the start of an active weather pattern leading into the rainy season, which typically starts around November.
“It’s an early start opportunity for us — and it’s a big one,” said Marty Ralph, director of the Center for Western Weather and Water Extremes in La Jolla, Calif.
Many feet of snow are possible in the high elevations of the Sierra Nevada as well, replenishing a depleted snowpack and improving water resource challenges that have plagued the beleaguered area. The heavy rain and snow should also end the fire season in central and Northern California.
“This is definitely a robust event,” said Andy Bollenbacher, a meteorologist at the National Weather Service in Hanford, in California’s Central Valley. “This is probably the strongest system I’ve seen in October in years. Usually we see this more toward November, December. This is a great start to our water year.”
The event reminds Ralph of heavy rains that drenched California in October 2016 when a network of gauges in the northern Sierras and near Mount Shasta, for example, took in six inches over a few days. “[It] got the water year off to a really great start,” he said, before ticking off the ensuing months’ wave after wave of deluges. “And that year turned out to be the wettest year on record.”
The impetus driving the waterlogged pattern is a zone of low pressure at high altitudes sitting just west of British Columbia. Nestled within a dip in the jet stream, it’s spawning a conga line of strong storm systems. The next, slated to brew between Wednesday and Thursday, will become a “bomb cyclone.” It’s a scientific term reserved for mid-latitude systems whose air pressure plummets rapidly, signaling a quick intensification.
The developing low will tug a stream of moisture-rich air toward the coastline, the narrow plume known as an “atmospheric river.” One is already producing rainfall from the Pacific Northwest to central California.
It’s producing light rain in Sacramento, which saw its first measurable precipitation in 212 days Monday — ending a record-long dry streak. Sacramento’s forecast includes rain chances every day through early next week.
Through Wednesday, this lead atmospheric river will continue to produce light to moderate amounts of rain and high-elevation snow
By midday Thursday, a second, stronger atmospheric river steered by the developing bomb cyclone will parallel its predecessor, soaking the Pacific Northwest first before working down the coast on Friday toward central California. Forecast models simulate a third atmospheric river, perhaps the most intense, arriving Sunday.
Ralph and the Center for Western Weather and Water Extremes, which rates atmospheric rivers on a 1-to-5 scale, are predicting top-tier level 5 impacts in central California, linked to the third atmospheric river Sunday into Monday. The rating is assigned based on forecast integrated vapor transport — or how much moisture the river is moving through a given volume of air. Each atmospheric river is like a conveyor belt.
The trio’s combined effects will lead to significant and, in some places, dangerous precipitation totals.
“We could see some mudslides,” Bollenbacher said. “We’re looking at a medium risk for debris flows. We’ve been messaging that for the burn scar of the Creek Fire. The Ferguson Fire had a really nasty burn area.”
Widespread flooding is not anticipated, because the ground is relatively dry, the Center for Western Weather and Water Extremes wrote in a briefing.
Because atmospheric rivers contain most of their moisture several miles above the ground, the greatest rain and snow totals will be observed over the higher peaks in the mountains.
Precipitation totals into the double digits (from rain and melted snow) can’t be ruled out in the northern Sierra Nevada; the Coastal Range will see maximum totals closer to five inches.
In central California where Bollenbacher’s office is located, the heaviest precipitation will fall from Sunday evening into Monday.
Equally impressive will be the systems’ wintertime impacts in the Sierra Nevada, where weather models simulate up to 100 inches of snow through early next week at the highest elevations. Snowfall rates topping three inches per hour and isolated thundersnow are possible at peak of the storm’s fury Sunday into Monday.
Snow levels will initially be quite high in California, or around 9,000 to 10,000 feet, Bollenbacher noted. But, by Monday, snow levels will decrease to around 5,500 feet.
Given all of the precipitation forecast over the next week, Daniel Swain, a climate scientist at the University of California at Los Angeles, tweeted that he’s “increasingly convinced” it will mark the end of fire season from Interstate 80 northward, which runs through the San Francisco Bay area and Sacramento.
The bulk of the rain will stay north of Southern California, where the fire season is far from over. That said, some beneficial rain is expected to fall there early next week.
While the big rains in October 2016 were the harbinger of a record-setting rainy season in the West, which nearly eradicated its drought at the time, it’s unclear whether 2021 will follow suit.
Last week, the National Weather Service declared La Niña had returned, which tends to favor plentiful precipitation in Northern California but limited amounts to the south.
“La Niña is typically associated with wet Northwest-dry Southwest, and Northern California is at the pivot point, like a seesaw with a fulcrum in the middle,” Ralph said. “But a couple of big [atmospheric rivers] in the middle of a La Niña in Southern California could make it relatively wet there.”