(Bloomberg) – Engineers are racing to lower water levels at Lake Oroville in northernCalifornia before storm clouds open up again, addingnew strain to the nation's tallest dam.

When it comes to unthinkable disasters involving dams, one mightthink of war torn Iraq, where the beleaguered Mosul Dam is incritical condition after years of war and neglect. Suddenly, aninfrastructure calamity was possible in America. How did thishappen, seemingly all of a sudden?

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Events of the last week unspooled like the beginning of adisaster movie, with a partial collapse of the Oroville Dam's mainoff-ramp for high water, and dangerous erosion beneath an emergencyspillway. Almost 200,000 people downstream were evacuated — aprecaution in the still unlikely event the movie ends verybadly.

Evacuation warnings lifted

Though evacuation warnings have been lifted, the dam isn't outof the woods. Forecasters predict up to a foot of new rain over thenext week. Dam operators have spent days preparing for new storms,continuing to spill enough water out of the lake to accommodateincoming rain, according to the California Department ofWater Resources. More than 125 construction teams have setabout 1,200 tons of rocks and boulders in place to shore up theemergency spillway, and the department is using drone video inaddition to on-the-ground monitoring.

The fast pace of events though has obscured a more basicquestion: Why is California flooding? After all, the last severalyears have seen combined heat and drought that became so bad thatGovernor Jerry Brown declared a state of emergency in 2014.

Weather whiplash, it turns out, is normal in the Golden State,even if this year falls toward the extreme. Conditions in the stateare so variable that it's common to see departures in precipitationof as much as 200 percent from long-term averages.

The state experiences most of its rain between Octoberand April. And when it comes down during those months, itcan drop in sheets. Meanwhile, April to September aregenerally dry. It has no analog anywhere else in the continentalU.S.

“Drought usually ends in flood in this part of the world,” saidDaniel Swain, a University of California, Los Angeles climatescientist, “because that's what it takes to end them.”

'Pineapple express'

Ask a meteorologist and they'll explain that California's bigstorms come when an “atmospheric river” drops out of the sky. Thephenomenon is called the “pineapple express,” a wet, narrow band ofair that forms over the tropical Pacific and dumps its wares onWestern mountains as snow or rain. The accumulated snowpack actslike a reservoir for the rest of the year's water, until springtimetemperatures melt it, and it flows down into artificial reservoirs,like Lake Oroville.

This winter has been on par with 1982-83, when a powerfulEl Nino poured moisture into the atmosphere.Currently, California has almost 180 percent of the averagesnowpack for this time of year.

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But snow isn't the big problem — the big problem is rain. By thebeginning of 2017, the state had received more thanits normal share of precipitation, but just 50 percent ofits average snowpack, according to Michael Dettinger, a researchhydrologist for the U.S. GeologicalSurvey. That much rain creates problems for California's waterdelivery system: Snow sits there until it's time to melt. Rain justslides right back down. Into Lake Oroville. All of a sudden.

The new year has brought a relentless pounding. California hasbeen hit by about 10 atmospheric river storms since Jan. 1. By Feb.7, part of the dam's main spillway gave out. And this weekend,engineers were concerned that water tipping over an emergencyretaining wall was eroding the land beneath it. Imagine, forexample, pulling the stopper from a 60 million cubicmeter bathtub.

California drought over?

The drought that's now over for half of California wasn't theworst the state has ever seen, but it was bad. A high-pressuresystem — meteorologists jokingly call it the “ridiculouslyresilient ridge” — parked itself offshore and diverted California'ssnow and rain into British Columbia and Alaska. What made itfeel like the worst, though, was that the drought coincidedwith some of the hottest years in recorded history, leading toincreased evaporation, a 500-year snowpack low, and subsequentstatewide misery.

Yes, climate change plays a role. Heat records areon the march, warmer air holds more moisture, and moisture isammunition for big-precipitation storms. But the year-to-yearvariability in California's climate is so great that it easilyswallows up more subtle signals of global warming. Researchers dosee these storms intensifying in the future, but for now, Dettingersaid, they still fit California's “really wildly variableprecipitation regime.”

April 1 is traditionally the benchmark for measuring snowpack,because it used to be a solid proxy for the seasonal peak. Over thelast few decades, snow has begun to melt a week or two earlier,according to Sarah Kapnick, a climate scientist for the National Oceanic andAtmospheric Administration's research laboratory in Princeton,N.J.

America's aging infrastructure

It didn't take long for the Oroville Dam to become a symbol forAmerica's aging infrastructure. But the greater challenge is that,in addition to getting older, the nation's dams, bridges, androads were constructed for a world that's receded intothe past. Like many U.S. scientists, Kapnick works onimproving projections for how the climate may behave justover the horizon — a couple weeks to a couple years — so thatCalifornia and other regions can better prepare for meteorologicalmadness. But climate change is making a tough job even tougher.

“Everything's built for a climate from when the majorinfrastructure projects were happening — decades ago,” shesaid.

—With assistance from Brian K. Sullivan in Boston.

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