Why Is Colorado Flooding?

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The extreme rains and flooding in Colorado beg questions: Why there and why now? Here are three common culprits and how they factor in this case:

Is it El Niño?

Because neither an El Niño of La Niña condition is present right now in the Pacific Ocean, they are not to blame. On the other hand, the equatorial Pacific is in a state that climatologists call “La Nada,” which could be worse.

This neutral, or “La Nada” event, has been stuck in place since the spring of 2012. Models aren’t predicting much to change for months to come, according to the National Weather Service’s Climate Prediction Center.

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“Without an El Niño or La Niña signal present, other, less predictable, climatic factors will govern fall, winter and spring weather conditions,” said climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory. “Long-range forecasts are most successful during El Niño and La Niña episodes. The ‘in between’ ocean state, La Nada, is the dominant condition, and is frustrating for long-range forecasters. It’s like driving without a decent road map — it makes forecasting difficult.”

In fact, some of the wettest and driest winters occur during La Nada periods, according to Patzert.

“Neutral infers something benign, but in fact if you look at these La Nada years when neither El Niño nor La Niña are present, they can be the most volatile and punishing. As an example, the continuing, deepening drought in the American West is far from ‘neutral,’” he said a week ago, we the conditions developed for the Colorado floods.

Is it because wildfires destroyed nearby forests?

This would seem logical, since there are giant burn scars and exposed soil after this and other summer’s fires in mountains above Boulder and other communities along the  base of the mountains. All that rain falling on the exposed ground, where forests used to be must be part of the problem, right? Well, sort of.

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“Many, many watersheds that were not burned, flooded,” said John Moody, a research hydrologist for the U.S. Geological Survey in Boulder who has studied how water runs off land that has recently burned. “In this case of extreme rainfall, three-year old burns probably only contributed a small increment to the total discharge.”

In other words, there is only so much a forest can protect you against. When 15 inches of rain fall in a few days where that constitutes six months of normal precipitation, there’s not a lot a forest can do to hold back the flood waters. This flood would have happened regardless of the summer fires.

Is it it because of global warming?

This is the trickiest one of all. Certainly more frequent extreme weather events are predicted by climate models and this flood fits that description. Likewise, the slow-moving or stalled weather patterns fit with some ideas that the ongoing warming of the Arctic can cause the jet stream to get very deep and stubborn loops that then create atmospheric rivers, like that carrying unusual amounts of water north into New Mexico and Colorado.

So the answer is a solid maybe.

Photo: The August 2013 image of sea surface heights in the Pacific Ocean from NASA’s Jason-2 satellite shows that the equatorial Pacific Ocean is now in its 16th month of being locked in what some call a neutral, or “La Nada” state. Image credit: NASA-JPL/Caltech/Ocean Surface Topography Team

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