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. In Los Angeles, the stormy weather has meant slick roads and traffic jams on Thursday’s morning commute.

Skiers and snowboarders hungry for powder are calling California’s 10-day forecast the “most insane forecast ever.” Already, in the past 48 hours, Mammoth Mountain has received three to seven feet of snow—and that’s just the beginning of a storm cycle that might continue until at least the end of next week.

How will these storms affect Los Angeles? 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 het 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 Southern California?

Right now, Southern California will see less moisture than much of the state from the AR event. In Los Angeles, rain is expected to arrive Saturday but the heaviest rainfall should fall early Monday morning, according to the National Weather Service. Los Angeles County should expect about an inch of rain, with the potential for higher amounts in select areas.

If heavier rain occurs, be on the lookout for flash flooding and mud debris flows, especially near recent burn areas.

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

Meteorologists are predicting rainfall totals of three to five inches in Fresno, upwards of eight inches in San Francisco, and a foot on the west side of the Sierra.

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 of snow.

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.

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 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.”

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 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 in Mammoth and Lake Tahoe unbury.

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.

The moisture is sorely needed in California, but the warm nature of the upcoming storm poses another problem.

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.