The parade of storms that blasted California over the past week marked a strong start to the rainy season. Some parts of the state, including Napa, Santa Rosa and Sacramento, received half the rain in 24 hours that they got in all of the past year.
But with California locked in one of its worst droughts in modern history and some areas short two years' worth of water, a lot more wet weather is needed to mend the state's water woes.
The long-term forecast calls for more dry conditions across much of California in the months ahead. Climate scientists are watching a La Niña weather pattern emerge in the tropical Pacific, which can push the storm track to the north and divert needed rain from the state during the crucial wet season. This would make it even harder for California to get the above-average precipitation required to put an end to the drought.
"Pretty much everyone in my neighborhood was watching their Halloween decorations wash away this weekend," said Dave Rizzardo, a hydrology manager for the California Department of Water Resources. "It's easy to think we're out of the drought now, but this storm is clearing out. It's not going to rain much more this week and possibly beyond that."
The past two years have been two of the driest in a century, a situation exacerbated by record warm temperatures that have further choked the brown and brittle landscape. California saw one of its worst fire seasons this year, with nearly 2.5 million acres burned, in part because of the drought. Meanwhile, dwindling water supplies have prompted restrictions on drawing water in many watersheds, hitting the agricultural industry especially hard. Some cities and towns have also faced cutbacks, though no statewide reductions have been ordered by the governor.
In the northern Sierra, where precipitation is vital because its big reservoirs provide much of the state's water, up to a foot of rain fell in the past few days. The quantity is impressive, helping Lake Oroville, one of the largest reservoirs, swell 100,000 acre feet, or more than 10% its current volume, even before all of the runoff was counted. In some mountain areas, rainfall totals were 20% of what typically falls in a year.
This year, however, officials at the Department of Water Resources say the northern Sierra will need much more than average precipitation — perhaps 140% to 150% — to bring water supplies back to average, or at least close.
"Average doesn't pull us out of this drought," Rizzardo said. "We're trying to make up for several years. There's still a long way to go."
As of Sunday night, Lake Oroville was at only 25% of capacity, or about 41% of where it typically stands at this point in the year. Shasta Lake, the only reservoir that is bigger than Oroville, was just 22% full, holding about 37% of what it typically holds on this date.
"There's still a tremendous amount of rain needed," said Brad Sherwood, assistant general manager for the Sonoma Water agency, which delivers water to communities in Sonoma and Marin counties.
Lake Mendocino, one of Sonoma Water's two primary reservoirs in the Russian River watershed, added a third of its volume over the past week. However, the lake remained less than 15% full.
The heavy rain Sunday and Monday capped a weeklong siege of on-and-off wet weather in California, Oregon and Washington from the Pacific. The train of storms was largely triggered by a "bomb cyclone," an area of rapidly decreasing low pressure, that pushed the systems ashore, including a Category 5 atmospheric river over the weekend.
An atmospheric river is a giant plume of moisture that generally wrings out when it makes landfall. Similar to hurricanes, the intensity of the systems is measured on a 1 to 5 scale.
In addition to widespread flooding and mudslides in areas that recently burned in wildfires, especially in and around the scar of the northern Sierra's Dixie Fire, the atmospheric river drove many record 24-hour precipitation totals.
Downtown San Francisco received 4.02 inches of rain on Sunday, the highest daily total ever for October, according to the National Weather Service. Sacramento reported 5.44 inches, the most ever recorded in a 24-hour period there.
Also over the weekend, the city of Napa recorded a 24-hour rainfall total of 5.35 inches and Santa Rosa recorded a 7.83-inch 24-hour period. Both totals were more than half the amount of rain that fell during the past water year, from Oct. 1, 2020, to Sept. 30, 2021, according to the weather service.