NOTE: This article includes information and text from the National Weather Service.

(WGHP) – Several rounds of “atmospheric rivers” have dumped rain and snow on parts of California over the last couple of months. Now, the state is dealing with dangerous flooding conditions and record snowfall. 

What is an atmospheric river and why are they repeatedly in the forecast for the West Coast? 

What are atmospheric rivers? 

Atmospheric rivers are long, narrow channels that run through the atmosphere transporting water vapor. According to the National Oceanic and Atmospheric Administration, the water vapor they carry is roughly equivalent to the average flow of water at the mouth of the Mississippi River. 

What's the weather?

Atmospheric rivers are particularly prevalent in the West Coast and are incredibly important to the water supply in the western United States. The snowpack in the mountains is also important for drought relief and filling reservoirs. 

According to a study by NASA, Sierra Nevada atmospheric rivers are 2.5 times more likely to result in destructive “rain-on-snow” events, which is when rain falls on a snowpack and freezes, than other types of winter storms. 

A well-known atmospheric river is the “Pineapple Express” which brings moisture from Hawaii over to the West Coast of the mainland United States. According to the National Oceanic and Atmospheric Administration, in previous years,  the Pineapple Express has dumped up to five inches of rain on California per day. 

About 30% to 50% of precipitation that falls in California each year comes from atmospheric rivers. They also contribute to 40% of the Sierra Nevada snowpack and more than 80% of the state’s major floods. 

This year, the high rainfall amounts have helped alleviate some of the state’s serious drought concerns, but also brought significant snowstorms and catastrophic flooding throughout the state. 

While atmospheric rivers can be incredibly beneficial in terms of water supply and improving drought conditions, the high water content they produce can result in landslides, power outages, flooding, and other life-threatening natural disasters. 

What impacts has California had from several atmospheric river events this year?  

This week, California saw its 11th atmospheric river of the season, dropping more rain and snow, and leaving more than 300,000 customers without power.

State reservoirs that had been concerningly low are now filled above average for the first time this year due to increased rainfall and snow. 

The rise in water levels comes after a three year drought that plagued California. 

California saw rainfall records on Tuesday with Los Angeles International Airport, Downtown Los Angeles, Santa Monica Airport and Long Beach airport all recording new daily rainfall totals. 

Santa Barbara Airport had the highest rainfall record with 2.54 inches of rain. 

Over a dozen communities along major rivers saw flooding including areas along the Salinas, Sacramento and Merced rivers. The Pajaro river suffered a levee breach last week and has continued to spill water onto nearby communities. 

According to the National Weather Service, over 90 flood watches, warnings and advisories have been in effect statewide as well as avalanche warnings in the Lake Tahoe area. 

Officials are concerned that the severe flooding may lead to significant crop loss. 

While some areas of California have been seeing catastrophic flooding, the Sierra Nevada mountain range has seen significant snowfall. 

As of March 15, snow levels in the range are more than double what they typically are in the spring when they’re at their peak. California’s most recent atmospheric river has melted some portions of the snowpack in the lower elevations. 

The snowpack can usually absorb rain with its high elevation, reaching 14,000 feet, but snow on its lower peaks can begin to melt which could lead to more flooding.  

The snowpack in the Sierra Nevada mountains has reached record-breaking levels due to several rounds of atmospheric rivers. According to the California Department of Water Resources, the Southern Sierras have a snowpack 257 percent greater than the average for this time of year. 

Dodge Ridge Mountain Resort has broken its all-time season snowfall record with 654 inches of snow. Bear Valley Resort has also received over 600 inches of snowfall this season. 

The concern with high snowfall amounts in the Sierra Nevada mountains is avalanche risk, strain on structures and equipment and problems with travel not to mention, eventually, all the snow is going to melt. The melting of such a large snowpack could lead to even more flooding in portions of California. 

Nearly a third of the state’s water supply is derived from the Sierra Nevada mountain range. 

The Golden State is expected to see its twelfth atmospheric river of the season next week.