The ridge of high pressure that has been in place for days is moving east, and a strong low pressure system that formed in the Gulf of Alaska is taking its place. As winds flow counterclockwise around the low, they pull tropical moisture northeast from the Pacific Ocean — an atmospheric river that will pour down over much of California in the coming days.
Rain and snow will be heavy at times, especially in the northern Sierra Mountains, where winds from the west will be forced to flow over high terrain. As air rises, the water vapor (humidity) will condense into clouds and precipitation.
The National Weather Service is suggesting two feet of precipitation in the form of water could drench Northern California. In the cold high elevations, much of that will be frozen, which means multiple feet of snow.
Snow levels are starting low — just 2,000-3,000 feet, according to the National Weather Service — but will rise as the air gets warmer later this week. Nearly three feet had already accumulated at Kingvale, Calif., (elevation 6,000 feet) as of Tuesday morning.
“Three to four feet of snowfall accumulation can be expected above 4,500 feet with some areas possibly seeing 5 feet or more along the crest by Thursday,” the Weather Service wrote in a Tuesday morning discussion. “Needless to say, severe travel impacts are likely crossing the northern Sierra.”
Travel is being discouraged in the northern Sierras until the storm lets up. Road closures for major thoroughfares, such as Interstate 80 and Route 50, are possible over the passes during the heaviest snowfall through Wednesday.
By the weekend, the atmospheric river will shift south and bring rain to Southern California, a region that has been neglected by rain over the past year. While a large portion (around 15 percent) of Northern California saw drought relief in 2016, the southern half of the state was very dry up until December.
“Los Angeles marked a sober milestone last year, when the NWS announced that the last five years were the driest ever documented in downtown L.A. since official record-keeping began almost 140 years ago,” wrote Angel Jennings in the L.A. Times Monday evening. The L.A. Times has provided unparalleled coverage of the region’s historic drought since it began in 2011. “Precipitation during that period totaled just 38.79 inches — roughly half of the normal amount.”
Then, in a marked reversal, December turned out to be the wettest since 2010, and it looks like the pattern will stay wet through at least the beginning of next week. A few tenths of an inch of rain will fall across Southwest California through Friday, followed by a significant surge over the weekend.
The National Weather Service in Los Angeles is cautioning that they do not know exactly where the heaviest rainfall will occur, but that “there is potential for a heavy rain event across portions of all of Southwest California sometime between Saturday and Monday, which could result in significant flash flooding with mud and debris flow issues, especially across the recent burn areas.”
Even with all the travel headaches and flooding these storms will likely cause, Californians are welcoming the precipitation after years of drought and water restrictions.
“This is what we’re supposed to be getting,” Johnnie Powell, a meteorologist at the National Weather Service, told the L.A. Times. “After six years of a drought, I love saying that. This is normal rain and snow that we’re supposed to be getting in December and January.”