In the higher elevations, the same systems have been dumping feet of snow. Parts of the Sierra Nevada are bracing for another 3 to 5 feet, with blizzard conditions likely at the highest mountain summits.
The next in a conga line of atmospheric rivers lapped at the coastline of Oregon early Friday and was poised to drop into California. Flood watches cover much of the Golden State, including central and northern parts of the San Joaquin Valley along with the coastal plain.
In many areas, wet conditions this week have saturated the soil, making flooding more likely.
“We’re definitely seeing some wet soil,” said Andy Bollenbacher, the lead forecaster at the National Weather Service office in Hanford, Calif. “We had a decently sized atmospheric river come through the area on Tuesday. That gave the Valley between an inch or two in the foothills, with three or four in the Sierra Nevada and up to five inches in the mountains beneath the snow line.”
So far, flooding has been modest, but this could shift during what is likely to be a busy travel period surrounding New Year’s Eve.
“The soil won’t absorb water nearly as well as it normally would,” he said. “That will bring nuisance flooding in poor-drainage areas and perhaps some rockslides in the mountains. You still have some water on roadways from Tuesday’s storm.”
About 1 to 2 inches of rain is likely in the San Joaquin Valley, with the atmospheric river north of the Bay Area, with lesser amounts — around a half-inch, — in central and southern portions of the Central Valley.
Atmospheric rivers carry most of their moisture a mile or more above the ground, which is why heavier precipitation is usually limited to the higher elevations. The Coastal Range could receive 2 to 4 inches of rain, with 3 to 7 inches in the Sierra Nevada below the snow line.
San Francisco is running 1.59 inches above average for the month to date, and Sacramento is about 1.65 inches ahead of the mean.
Above 7,000 feet elevation, mainly snow is expected. A general 1 to 3 feet of snow is likely, with 3 feet or more above 8,000 feet of elevation and potentially as much as 5 feet or more above 9,000 feet.
Several low-pressure systems have triggered the atmospheric rivers. The first, a mature low west of Vancouver Island, is tugging a strip of moisture from the tropical Pacific eastward, thanks to its counterclockwise spin. That moisture plume will be aimed more directly at California on Saturday as a more intense low develops west of the state and moves southeast.
It will slip over the Great Basin of Nevada into Sunday morning, kicking off the new year with stormy conditions including heavy rain, winds gusting 40 to 60 mph and mountain snow. Reno is under a flood watch and a wind advisory.
The rains are welcome news in the West, where the U.S. Drought Monitor reports that 64 percent of the region is experiencing a “moderate” or worse drought. In California, 7.16 percent of the state is facing a top tier “exceptional” drought — level 4 out of 4 on the scale — which is nine times more territory than was affected this time last year.
Fortunately, more atmospheric rivers are on the way, particularly Tuesday onward.
“In this case, it looks like the jet pattern is much more [west to east]which suggests that more of the moisture tap, the “Pineapple Express,” is going to be coming here rather than Oregon, Washington or western Canada,” said Bollenbacher. “That’s the big difference-maker we’ve been seeing. It looks to be that way for the next couple of weeks, and we didn’t have that this year.”