Two storms aimed at California are lined up back to back this week — the first is ongoing, and the second more significant storm is slated to arrive this weekend. On the one hand, wet and snowy weather is “normal” for winter in California. On the other, the state is still struggling with a historic drought, and it hasn’t seen storms like these in years.
Through Monday, up to two feet of moisture could dump on California’s high elevations. If the majority of it falls as snow, it could mean over 10 feet along the peaks of the Sierra Nevada. Heavy rain is expected to douse the lower elevations which could lead to flash flooding and debris flow.
The precipitation in these storms is so extreme because they are generating atmospheric rivers — huge plumes of moisture (humidity) that usually originate over warm ocean water, like the tropical Pacific. When you look at the water vapor maps, it becomes obvious where the name came from — it’s like a fire hose aimed at the West Coast.
As a ridge of high pressure shifts east away from the West Coast, low pressure systems are taking its place. Winds flow counterclockwise around low pressure, which pulls warm moisture northeast from the tropical Pacific Ocean. That moisture will pour down over much of California as rain and snow in the coming days.
Precipitation will be heavy at times, especially in the northern Sierra Mountains, where winds from the west are forced to flow over high terrain. As air rises, the water vapor condenses into clouds and precipitation (rain if it’s warmer than 32 degrees, snow if it’s colder).
The National Weather Service is suggesting two feet of water could drench Northern California. In the mountains, where the temperature will be below freezing, much of that will fall as multiple feet of snow.
The first storm, still ongoing, has dumped as much as five inches of rain on the Bay Area.
Interstate 80 over Donner Summit was shut down Wednesday morning because of heavy snow and extremely low visibility but has since reopened. Chain control is still in effect on major highway passes.
— Squaw Alpine (@squawalpine) January 4, 2017
The snow will keep falling through Thursday morning, and locations near Donner Summit have already received multiple feet of snow as of Wednesday morning. Squaw Valley/Alpine Meadows Ski Resort reported 4.5 feet, and Tahoe Donner reported 4 feet as well as a delay in opening due to heavy snow (you know it’s snowing hard when the ski resorts need time to catch up).
On the south side of Lake Tahoe near Route 50, which usually sees slightly less snow because of its lower elevation, Heavenly reported three feet.
After a brief cessation Thursday and Friday, the precipitation will pick up again Saturday as another storm aims a more significant plume of moisture at the West Coast. The National Weather Service in San Francisco writes:
Starting on Saturday another atmospheric river setup of much greater concern will bring widespread rainfall to our region. Models have been highlighting the weekend into Monday for multiple days now as an area of concern. [Precipitable water] values in excess of 1.30″ along with IVT values possibly above an incredible 1000 kg/m/s will be along our coast for around 48 hours.
That acronym, IVT, stands for integrated water vapor transport. It combines the volume of water with the speed at which it’s flowing overhead. The forecast value, 1,000 kilograms of water per meter per second, is an extreme amount of moisture in a short amount of time.
During that time moderate to heavy rain will blanket our region with very high rainfall amounts likely. Early guidance indicates the potential for many urban areas picking up two to five inches with much higher amounts likely for local hills and mountains.
In other words, parts of the Bay Area have the potential to tally eight to 10 inches of rain by this time next week, and flooding will definitely be an issue this weekend. Snowfall in the weekend storm is probably not going to be as significant since the air temperature will be warmer. In fact, it’s possible that the snow level could be around 9,000 feet in the Sierra Nevada, which means everything that falls below that elevation will be rain.
The weekend storm will be a significant rain and snow maker for Southern California, which unlike its northern counterpart, is still struggling with an exceptional drought.
The Weather Service office in Los Angeles is still cautioning that they do not know exactly when and where the heaviest rain will set up, but that flash flooding with mud and debris flows are likely this weekend into Monday, especially in areas that have recently been burned by wildfires. Strong winds may also bring down trees which could lead to power outages.
Forecast models suggest a brief break after Monday, but all signs point to a continued rainy and snowy pattern for California in January.
“After the weekend storm, there are several more systems lined up going through the end of next week and potentially beyond,” the Weather Service in San Francisco wrote Wednesday. “Needless to say, rainfall totals by the middle of January are going to be quite impressive.”