5 things to know about the massive storm headed to Northern California

Rain and snow are forecast to pummel California this weekend, all part of a 10-day storm cycle that could bring inches of rain and upwards of 20 feet of snow to some mountain regions. This follows a tumultuous week of weather—especially in Lake Tahoe—where 4 to 7 feet of snow has already fallen and some roadways are flooded.

Skiers and snowboarders hungry for powder are calling California’s 10-day forecast the “most insane forecast ever.” That’s because precipitation totals in the Sierra are forecast to be approximately twice the monthly average for January. But all that moisture has a downside too, with some forecasters predicting that California will see the worst flooding since 1997.

How will these storms affect San Francisco? Here’s the breakdown:

1. What is an atmospheric river?

All of this precipitation is caused by an atmospheric river (AR), a narrow band of moisture that can transport huge amounts of water vapor towards the West Coast. Think of an atmospheric river as a fire hose that funnels moisture from the tropical Pacific towards California.

Thanks to a blocking high pressure system near the Bering Sea and a downstream plunge of the jet stream off the Pacific Northwest Coast, the atmospheric river is set to stall over California for at least two or three days starting on Saturday.

When an atmospheric river stalls over land, you can expect big precipitation totals.

2. How much rain will fall in San Francisco?

After a brief dry out on Thursday and Friday, rain is set to return to Northern California on Saturday and could continue for the next week. From Saturday to Monday, the city of San Francisco can expect between 2-5 inches of rain, with 5-9 inches of rain forecasted in the surrounding hills and mountains.

High winds are also a possibility, which could cause downed trees and power lines. Be aware that in some areas, heavy rain can cause flash flooding and mud debris flows.

Courtesy of the Weather Channel

3. What about rainfall and snow totals for the rest of the state?

Meteorologists are predicting rainfall totals of about one to two inches in Los Angeles, three to five inches in Fresno, and a foot on the west side of the Sierra.

While that’s a lot of rain to be sure, the snow forecast for the California Sierra Nevada mountains is even more intense. Most forecasters predict the Sierra Nevada will see at least 10 feet of snow over the next few days, with some forecasting as much as 20 feet.

In Lake Tahoe, how much snow the ski areas receive depends on the freezing levels. According to meteorologists at Open Snow, on Saturday night the snow levels will rise to about 9,500 feet, which is near the summit of resorts like Squaw Valley. This means that most of the precipitation that falls will be rain, not snow. Snow levels should drop on Sunday night into Monday, which will be good news for ski areas.

4. Why is it such a big deal?

This 10-day storm cycle is bringing a huge amount of moisture into the state. That means increased risks of flooding, mudslides, and debris flows in both the San Francisco and Los Angeles metro areas, as well as hazardous traveling conditions across the entire state. Head over here to see the specific flash flood and winter storm warnings already issued by the National Weather Service.

Atmospheric River events are important to California’s annual water totals, but they can also cause huge disasters. According to the National Weather Service, the flooding with this event may be the greatest since December 2005. In 1997, an atmospheric river caused over over a billion dollars worth of flooding across California. Wired science reporter Nick Stockton pointed out that some rivers and streams will “reach similarly epic levels this weekend.”

The flooding could get especially bad because the storm that delivered snow and rain earlier in the week has already saturated the snowpack and swollen rivers. The National Oceanic and Atmospheric Administration’s early forecasts predict that the Merced River in Yosemite National Park will surge to near or above a record 23.7 feet. That could be devastating to the park; a January 1997 flood with similar river levels caused $176 million in damages.

For ski areas, the new snow will no doubt be a boon to business and a thrill for those who love to ski powder. But forecasted temperature fluctuations—and the forecasted rain—mean that the snow will likely be quite wet, and that much snow will impact operations. Expect wind holds and terrain closures as ski areas unbury.

Courtesy of the National Drought Mitigation Center

5. What does it mean for the drought?

All of this moisture is good news for a state that has entered its sixth year of drought. A recent survey of the Sierra Nevada snowpack stood at just 53 percent of average, about one-third as much water as the same time last year.

The moisture is sorely needed in California, but the warm nature of the upcoming storm poses another problem. Forecasters in Lake Tahoe and Mammoth predict that the freezing-level over the weekend will be as high as 10,000 feet, meaning that much of the precipitation will fall as rain instead of snow.

That’s bad news for California’s snowpack. When warm rain falls onto existing snow it can cause flooding and diminishes the snowpack overall. The state needs a healthy snowpack that slowly melts in the spring and summer to refill reservoirs in the dry summer season.

California needs to keep as much high-altitude snow as possible through spring. That can’t happen if warm rain washes away some of the existing snowpack, which is what might happen this weekend.

Still, any moisture is good news at this point. In an ideal situation with colder temperatures, this storm could build Sierra snowpack without too much low-elevation flooding. Stay tuned.

• California will be pummeled by atmospheric river event this weekend [Weather.com]
• Atmospheric river events will dump 15 feet of snow on California [Mashable]
• Despite recent storms, California’s ‘snow drought’ continues [The Sacramento Bee]
• California’s huge storm could cause disastrous melting in the mountains [Wired]