How Atmospheric River Will Boost California's Snowpack

The enormous atmospheric river storm currently slamming California, nicknamed the Pineapple Express, is set to significantly increase the state's snowpack levels, easing concerns about drought conditions and water shortages during the summer months.

A huge river of moisture hit the West Coast on Tuesday evening and swept heavy rains and snowstorms across California, which are expected to continue into the weekend.

The heavy snowfall from the storm is expected to top up the snowpack in the Sierra Nevada, which has been dwindling at below average levels for this time of year.

"Heavy Snow will expand from the Shasta Siskiyous to the Sierra Nevada tonight before spilling out over the Great Basin on Thursday and eventually portions of the Intermountain West and Rockies on Friday. The heaviest snowfall will occur over the Shasta Siskiyous and Sierra where 1-3' feet (isolated 4') are expected. Generally 1-2 feet are expected over the higher elevations of the western mountains with isolated 2'+ amounts possible," the NWS said in a January 31 forecast discussion.

The snowpack in the Sierra Nevada on January 30 was only 58 percent of the average for that date. On the same day last year, the snowpack was 214 percent of the average, in large part due to California being buffeted by several atmospheric rivers. The current level may rise in the coming days thanks to the snowfall from the storm.

"It does appear that colder air on the backside of the storm will drop snow levels considerably (below pass level and hopefully at least down to Lake Tahoe level) early enough to allow for some meaningful snow accumulation, though, later in the event," Daniel Swain, a UCLA climate scientist, wrote on his Weather West blog.

The Sierra Nevada snowpack provides around 30 percent California's water needs every year, meaning that if a snowpack is low, the state may be threatened by drought conditions in the summer months. Last year's higher snowpack meant that the drought afflicting California lifted significantly as the snowpack melted in the spring.

"Drought recovery has been occurring in the West. Particularly for California, this is primarily due to the excellent winter precipitation and snowpack season they had. Some areas had record-high snowpack. Most of the drought recovery happened before summer," Becky Bolinger, Colorado's assistant state climatologist, told Newsweek.

Atmospheric rivers are long, narrow streams of water vapor that form over the tropics, usually over the Pacific or in the Gulf of Mexico. Atmospheric rivers that originate near Hawaii and hit the U.S. West Coast are often nicknamed the Pineapple Express.

According to the National Oceanic and Atmospheric Administration (NOAA), the average atmospheric river system carries the same amount of water as the average flow at the mouth of the Mississippi River, but the most powerful can contain up to 15 times that.

While this year's snowpack has been less than impressive, California has seen a wetter winter: the state's overall precipitation since the start of the wet season on October 1 is 82 percent of average. The lack of snow in favor of rain may be due to storms being warmer.

"Despite strong El Niño conditions in the Pacific Ocean, a high-pressure system and several other climate factors have led to below average conditions so far and most storm impacts have been focused along the coastal regions," Michael Anderson, a state climatologist with the California Department of Water Resources, said in a DWR statement. "Many of these storms have also been warmer than average and produced more rain and less snow, a far cry from last year's near-record snowpack and once again demonstrating how California can swing from one extreme to another."

While this atmospheric river is expected to bring plentiful snow, it is also forecast to bring large amounts of rain to the lower altitudes, with several inches of rain being predicted across the state. This may lead to flooding in some areas.

"The WPC has issued a Slight Risk of excessive rainfall over parts of Southern California from Thursday into Friday morning. The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable," the NWS said in a February 1 advisory.

A second atmospheric river is due to arrive early next week, and may lead to even further rainfall and higher winds across the state.

"Storm #2, which will likely arrive on Sunday but may linger into Monday or even Tuesday, is still a little more uncertain but has a considerably higher ceiling in terms of rain (and wind) impacts–especially in Southern California," Swain wrote in his blog.

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