【Nanda van Bergen】

It's hard to believe that after five years of record drought,Nanda van Bergen many Californians are hoping that this winter's weekly deluges will come to a swift conclusion. The latest storm, which struck over the weekend and ended on Tuesday, has forced thousands to evacuate, including urban residents in San Jose, as floodwaters continue to rise and levees are strained.

This winter demonstrates how too much of a good thing -- in this case, rain and mountain snow -- can lead to severe problems.

SEE ALSO: Insane drone footage shows massive damage and flooding at California's Oroville Dam

This is increasingly important, since human-caused global warming is already causing both ends of the weather extremes spectrum -- too little water and too much -- to occur more frequently and with greater intensity.

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So, what is it about these California storms that separates them from ordinary low pressure systems that affect other parts of the country?

The Oroville Dam spillway releases 100,000 cubic feet of water per second down the main spillway in Oroville, California on February 13, 2017. Credit: JOSH EDELSON/AFP/Getty Images

First, there is the amount of moisture they're carrying, and where this moisture originates. The cause of California's drought-busting flood woes lies thousands of miles off the Pacific coast, near Hawaii.

It's in this area that so-called "atmospheric rivers" are born, and for this reason, such storms are often referred to as "Pineapple Express" events, owing to the produce closely identified with Hawaii. These storms account for the majority of California's heavy rain events, both this winter and throughout history. (Atmospheric rivers also affect other parts of the world.)

From Hawaii, upper level winds channel air laden with water vapor — the gas form of liquid water — like veins in the atmosphere's circulatory system. Once this moisture butts up against the Golden State, it is lifted by storm systems and terrain, producing clouds and precipitation.

Original image has been replaced. Credit: Mashable

Atmospheric rivers can produce extraordinary precipitation totals.

The National Oceanic and Atmospheric Administration (NOAA), for example, found that a strong atmospheric river, such as the one that hit California last weekend, can transport "an amount of water vapor roughly equivalent to 7.5 [to] 15 times the average flow of liquid water at the mouth of the Mississippi River."

This winter has featured an unusual abundance of moderate intensity atmospheric rivers that in some cases have overwhelmed flood defenses, including at the Oroville Dam, which is the nation's tallest.

Water flows into the iconic Glory Hole spillway at Monticello Dam, in Lake Berryessa, Calif.  The unique spillway operates similarly to a bathtub drain. Credit: AP/REX/Shutterstock

An emergency spillway for the dam suffered erosion that led to the hasty evacuation of nearly 200,000 from areas downstream on Feb. 12. The situation at Lake Oroville has since deescalated, but other more dire situations are playing out in other parts of the state, offering a case study in the collision of climate change, extreme weather and aging infrastructure.

Many points across central and northern California have already recorded more precipitation than they get on average in an entire October-to-September water year. San Francisco, for example, has already exceeded their typical annual rainfall total.

And there may be more strong storms on the way through early March and before the wet season ends in April.

Global warming is amplifying droughts and floods

During the past several years alone, California has seen an epic drought and record warm summers followed by an unexpectedly prolific and destructive rainy season. Experts say this should be seen through the prism of global warming, otherwise infrastructure planners may not learn the lessons needed to better fortify the extreme weather-prone state.

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A bicyclist rides along a flooded street as a powerful storm moves across Southern California on Feb. 17, 2017 in Sun Valley, California. Credit: David McNew/Getty Images

Over the longer-term, precipitation extremes in many parts of the globe have become more frequent and intense, studies have shown.

In the coming years, this whiplash may only get worse as human-caused global warming amplifies both droughts (by raising temperatures and increasing the loss of surface moisture), and floods (by increasing the incidence of heavy precipitation events).

Michael Dettinger, a research hydrologist with the U.S. Geological Survey in Nevada, said the climate whiplash during the past few years, going from a paucity of precipitation to plenty, is a preview of what's to come.

"The combination of this winter and the recent hot-drought form a pretty reasonable analog for thinking about the challenges to come," he said in an email.

Original image has been replaced. Credit: Mashable

However, many uncertainties remain in how regional precipitation patterns may shift in a warming world, particularly in the Southwest U.S., where many computer models show an increasing likelihood of drought in the future.

Scientists are still studying how global warming may alter atmospheric rivers themselves, although there is agreement that as temperatures rise, more heavy precipitation is likely to fall as rain in higher elevations instead of snow, which would cause more strain on water infrastructure like the Oroville Dam.

Dettinger and other experts said sudden swings from drought to flood may be the new norm, which will require a fundamental rethinking of infrastructure design for new dams, levees, bridges and other critical infrastructure across the West and the rest of the country.

Vineyards and farmland along the Russian River are inundated by widespread flooding on Jan. 11, 2017, near Healdsburg, California. Credit: George Rose/Getty Images

"The current situation in California —specifically, the dramatic swing from extreme drought to water overabundance and flooding — is indeed a preview of California's likely climate future," said Daniel Swain, a climate scientist at UCLA.

"There is now quite a bit of evidence that future droughts here will be warmer and more intense, yet will be interrupted by increasingly powerful 'atmospheric river' storms capable of causing destructive flooding." 

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. 

Most atmospheric river events are not of the same powerful intensity as the one that struck the state over the weekend, with pineapple express storms considered a subset of overall atmospheric river events. According to NOAA, a total of 42 atmospheric river events hit California between 1997 and 2006.

Swain and Dettinger both say there is an increased expectation for more frequent and potentially stronger atmospheric river events in the future.

"One important point that the present situation highlights: while climate change sometimes leads to "unprecedented" weather extremes that have no historical precedent, it even more commonly increases the risk of extreme weather events at the upper end of the historical intensity spectrum," Swain said.

River in the Sky Keeps Flowing Over the West https://t.co/6LkcKnZtY0 #NASA pic.twitter.com/v5veClj0nv

— NASA Earth (@NASAEarth) February 22, 2017

Dettinger's work supports this conclusion, too.

"... What has really been going on is that we’ve just had so many [atmospheric river events] arrive in such a short time this winter. This is actually in keeping with one of the early findings re: West Coast atmospheric rivers and climate change… that the numbers of atmospheric rivers will increase faster than their (average) intensities," he said. 

This is also worrisome, since reconstructions of historic flood events --like the Great Flood of 1862 -- as well as a simulation of what would be a devastating flood both involved many weak to moderate atmospheric river events hitting in rapid succession; the same thing that's happening now.

Noah Diffenbough, a climate scientist at Stanford University, says California's wet winter demonstrates how we're already hitting the limits of water infrastructure that was designed in a completely different climate.

“We know that the climate is different now than when our water infrastructure in the West was designed and built,” Diffenbough said.

He said adapting to climate change means building a more resilient, less risk-prone water management system in California. "We’re already incurring great costs for the climate that we have now,” he said.

“Most of what we would do to protect ourselves now would help us be prepared for the future.”