2-week blitz of storms to bring damaging flooding, 12-plus feet of snow to California
After six years of coping with the state's worst drought on record, Californians are not used to rain. But they'd better prepare for it, fast, because an onslaught of storms the likes of which the state has not seen in at least a decade is coming quickly.
These storms have tropical connections, and are bringing with them extraordinary amounts of rain and snow, along with strong winds. A dizzying array of storm watches and warnings have been issued across the West, from freezing rain advisories (Oregon) to flash flood watches (California) to winter storm watches and warnings (Rocky Mountains), as the most intense storm approaches for Saturday through Monday.
These storms, known as "atmospheric rivers" for their extraordinarily narrow channels of eye-popping levels of moisture, and hence copious amounts of rain and mountain snow, could have deadly consequences, officials are warning.
The National Weather Service is cautioning millions from the San Francisco Bay area to Lake Tahoe to be prepared for potentially historic flooding as the storm systems wreak havoc with streams and rivers.
The weekend storm is forecast to dump at least 7 inches of rain in the Bay Area, with up to a foot of rain in some of the hilly areas nearby, while several more feet of snow fall in the highest peaks of the Sierra Nevada Mountains.
Of particular concern is the presence of 4 to 8 feet of new snow that fell this week even at lower elevations in the mountains of northern and central California.
Photos from ski areas in the Sierras showed a winter wonderland of fresh powder, which many skiers and snowboarders took advantage of before the next storms hit.
The weekend storm is forecast to bring with it a slug of mild air all the way from the tropics, which will turn the heavy, cement-like snow to rain from the surface all the way to 9,000 feet during the height of the storm. This means all but the highest mountain peaks will see rain for part of the event, before temperatures plummet again by Monday.
This raises the specter of snowmelt-induced flooding in areas like Yosemite National Park, the Lake Tahoe region and other parts of the Golden State.
Here's how meteorologists at the National Weather Service in Reno described the flood dangers this storm poses:
"It won't take much rainfall to generate flood impacts across the Sierra and western Nevada.Creeks, streams, urban areas and farmland are certainly at risk for flooding late this weekend."
By the time the next storm is over, the highest peaks of California's Sierra Nevada Mountains and parts of the Cascade range will have picked up over 120 to 150 inches of snow in the past seven days, with much more on the way as more storms take aim at the West next week and beyond.
In fact, computer models show storms with atmospheric river links stacked up one after the other like planes landing at O'Hare Airport during rush hour.
"The amount of rain over the next 7-10 days will likely be substantial if not historic," according to the National Weather Service (NWS) forecast office in San Francisco.
Using NWS predictions, the private forecasting firm WeatherBell Analytics calculated that these storms could dump 14.45 trillion gallons of water during the next seven days. Such a figure is far from exact, but it illustrates the potency of the atmospheric river.
The NWS is predicting up to a foot of rainfall from the weekend event alone, and has referenced a particularly damaging storm that hit on January 3, 1982 as a potential analog to this event. That storm caused mudslides in the Santa Cruz Mountains and north bay.
Mudslides, debris flows and river flooding are all threats from the upcoming storms throughout central California in particular.
"It's impossible to predict the exact impacts of this weekend but [it's] a good reminder of what can happen when the the [sic] sub-tropical connection off the Pacific kicks in," meteorologists wrote in a technical forecast discussion early Friday morning. "After several years of drought and dry January's its [sic] a stark reminder of what northern California winters can throw at you."
The weekend storm also appears to be comparable to an event that began in late Dec. 2005 and caused about $300 million in damage across the state, according to the U.S. Geological Survey.
The upcoming storm, as well as the storms that will follow it, is an example of a potent atmospheric river event.
Atmospheric rivers are narrow highways of moisture in the sky. These phenomena transport huge amounts of water vapor — the gas form of liquid water — and point it like a firehose at a relatively narrow area. When the moisture hits a source of lift, such as a storm system, a mountain range, or both, that moisture is wrung out in the form of rain and snow.
While other parts of the country receive precipitation from various types of storms — like hurricanes along the East Coast — California is highly dependent on atmospheric rivers for its annual precipitation. About six to 12 storms per year typically contribute nearly half of California's annual precipitation.
Some research shows that extremely prolific atmospheric rivers could become far more common — perhaps twice as common as they are now — in parts of California by the end of the century, though this is still a matter of scientific debate. If this occurs, water managers will have to adjust to accommodate more precipitation extremes and avoid severe floods.
Not enough to end the drought
In the next 7 days alone, California is forecast to average about 5 inches of rain and melted snow statewide, which is about 25 percent of the state's annual average precipitation and 1.2 inches higher than an average January, according to WeatherBell calculations based on NWS forecasts.
With heavy rain and snow having already fallen in parts of California at the start of January, the state is likely to far exceed the average January precipitation in the Golden State during the next 7 to 10 days.
Even with these atmospheric river events, however, California's drought won't be over.
It would take several more such events to make a more significant dent in the long-term precipitation deficit the state has accrued.
But there's no El Niño
One of the remarkable things about these Western storms is that they're hitting during a winter in which there's no El Niño present in the tropical Pacific Ocean. Yet during the past two winters, when one of the most powerful El Niño events on record was occurring, such storms were a no-show.
El Niño years tend to be more efficient at producing atmospheric rivers, since moisture and atmospheric heat are heightened above parts of the tropical Pacific Ocean at such times.
El Niño events are characterized by higher ocean temperatures in the tropical Pacific Ocean, altered Pacific trade winds and shifts in weather patterns from Australia to North America and beyond.
Atmospheric rivers can occur during other winters as well, but they may be slightly less frequent or potent.
So far, the winter of 2016-17 is turning out to be exactly what California was hoping for from last year's El Niño, it's just arriving as an unexpected, and potentially dangerous, guest.