After multiple weeks of mostly dry weather, California saw a weather shift this weekend with the first in what will be a parade of storms arriving off the Pacific Ocean and pulling in a river of moisture.
The lashing of rain, snow and wind could continue for seven days, with forecasters warning of travel delays as well as flooding in portions of the Sierra Nevada and Northern California.
The storms are forecast to bring more rain than snow to mountainous regions, as well as bursts of strong winds. With colder weather on the way next week, a white Christmas is possible, but not definite, at popular holiday travel spots in the mountain range, including the Lake Tahoe Basin.
“We will see snow falling through the duration of Christmas Eve and Christmas Day in and around Tahoe,” said Heather Richards, meteorologist with the National Weather Service office in Reno, Nev. “It will be sticking at the higher elevations but it remains to be seen how much of it will stick around the lake.”
Why these storms are concerning
These storms will pull atmospheric rivers of moisture off the Pacific Ocean. When the plumes of water vapor smash into the coastal mountains and the Sierra, they can release large amounts of rain in short periods of time.
Too much rain all at once can create hazards, such as flash flooding and landslides.
These storms will bring beneficial precipitation to most locations, boosting the state’s water supply and wetting the landscape, which reduces wildfire risk. But forecasters warned that the prolonged rain and heavy bursts of precipitation could cause problems in some areas.
A large portion of the state was under a flood watch through Monday, including the North Bay, the Sacramento Valley, and the northern portion of the San Joaquin Valley.
This does not mean that this entire area will flood, but some flooding could occur. Watches mean that hazardous conditions are possible; people should prepare and watch for warnings, which mean that flooding is occurring or imminent.
Forecasters are especially concerned about a small area in the Sierra and its foothills in Northern California, where the Weather Prediction Center issued a Level 3 out of 4 risk for flash flooding Sunday to early Tuesday.
The timing of the rain
Scattered rain was falling across Northern California on Saturday with the first of the storm systems moving in and another one coming right behind it.
The heaviest rain and snow are expected across Northern and Central California through Monday before the precipitation pushes southward Tuesday into Thursday.
Another storm is predicted to affect the state from Tuesday through at least Thursday. This system is likely to be colder.
Over the next week, Northern California is forecast to record three to seven inches of rain across coastal areas and valleys, and seven to 10 inches of rain in the coastal mountains.
In Southern California, desert areas may get half an inch to two inches of rain, with coastal areas recording up to three inches, and coastal mountains five to 10 inches.
The Sierra Nevada in Northern California is predicted to pick up seven to 20 inches of rain, with snow at higher elevations.
Both rain and snow expected in the Sierra
Through the weekend and into Monday, snow is forecast to fall at locations above 8,000 feet in elevation. Mountain passes will get mostly rain rather than snow.
By Wednesday, colder weather is expected with snow at locations as low as 5,500 feet in elevation, through Christmas and potentially the day after. The Weather Service said travel delays are likely on highways because of snow and reduced visibility.
In the Tahoe Basin, locations at 8,000 feet and above are predicted to record one to two feet of snow while locations at 6,000 feet could pick up as much as 10 to 12 inches, according to Ms. Richards.
She said that there is uncertainty about how much snow the Sierra will record at the lower elevations because it’s unclear how cold it will get.
No matter, next week’s cold system will bring a much-needed boost to the snowpack, particularly in the northern half of the state, where it’s a mere 4 percent of normal for this time of year.
The Sierra snowpack is a key source of water for California and Nevada, and the snow that gets built up over winter helps replenish reservoirs as it melts through the spring and summer.
Michael Anderson, California’s state climatologist, said that some large storms in October and November bolstered the state’s water supply. But, he added, the “snowpack is still far below average due to both the lack of December precipitation and the above-average temperatures in recent weeks across much of the state.”
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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