Don't get too comfortable: Extremes like last year's heat wave are expected to recur -
Published Wednesday, July 3, 2013 at 1:00 am / Updated at 8:05 am
Don't get too comfortable: Extremes like last year's heat wave are expected to recur

• Read more about the emerging research on weather extremes in Nancy's Almanac.

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The drought and wildfires in the West are the flip side of a stubborn weather pattern that is bringing Nebraska its cool, pleasant start to summer.

In the short term, there's no worry that the intense heat in the Southwest will be unleashed on the Central Plains. Much of Nebraska and all of Iowa are expected to average cooler-than-normal weather through the first half of July.

However, the overall pattern of unrelenting heat in one area and prolonged coolness or rain in another is indicative of how the changing weather is delivering profound impacts.

A year ago, Nebraska was in the midst of its hottest, driest year on record. The drought jeopardized the livelihoods of many ranch families, and mega-fires consumed more than 600 square miles of the state. In Omaha, the first week of July 2012 brought three successive triple-digit days and the month itself was the driest July on record.

The year before, drenching rains and heavy snows in the upper Missouri River basin resulted in historic flooding the length of the river valley.

While extreme variability is a normal part of weather, emerging research is knitting together an explanation of why these extremes are becoming worse.

One of the things that ties recent years together is a jet stream that loops more steeply than it typically has, resulting in stalled weather patterns that allow conditions to intensify.

The jet stream matters because it separates cold Arctic air from warm tropical air, and it serves as the highway that most storms ride along. As the jet stream drops south, cold air flows into the U.S. As it shifts north, warm air fills in behind it. After the jet stream bypasses a region, less rain or snow falls.

Robert Oglesby, a climate scientist and professor at the University of Nebraska-Lincoln, points to the jet stream to explain the past few years.

“Our weather appears to be exactly the opposite of last year, but the two years have a lot in common,” Oglesby said. The common denominator: The jet stream has been traveling an exaggerated wavy path. In scientific terms, it's called amplified circulation.

This year, the jet stream has dipped deep into the central U.S., allowing relatively cool, dry Canadian air to flow into the Central Plains.

At the same time, a steep hump over the western U.S. has allowed warm air to flow northward. Because the jet stream hasn't shifted, the hot air in the Southwest has built upon itself, getting hotter and hotter.

Last year, that sort of hump and stalled pattern prevailed across much of the central U.S., resulting in the historic flash drought.

As summer hits full stride, Nebraska and Iowa certainly should see their usual hot weather. Nebraska, which is still in drought, remains vulnerable to sharp increases in temperatures.

Oglesby said it's too early to draw definitive conclusions about what's happening with the jet stream, but research at Rutgers University and elsewhere may have uncovered the underlying explanation: Rapid warming in the Arctic.

Even though the Arctic seems worlds away from Nebraska and Iowa, it's crucial to this area's weather, said Rutgers atmospheric scientist Jennifer Francis. That's because the Arctic plays a big role in establishing the sharp temperature contrast that creates the jet stream.

In her research, Francis is seeking to answer this question: Is accelerating warmth in the Arctic affecting weather across the mid-latitudes, which include the lower 48 states? And if so, how?

“We are definitely in a new normal because of a variety of climate change-related factors,” Francis said. These factors include a warmer atmosphere, warmer oceans, higher sea levels and more water vapor, which when combined provide more energy and moisture for storms. Added to this mix is the accelerated warming in the far North, which scientists refer to as Arctic amplification.

“While we still can't say for sure that AA (Arctic amplification) is the main factor or even an important factor ... evidence is piling up that suggests that it is,” she said.

If the research bears out, Nebraska's and Iowa's famously erratic weather could become even more erratic, Oglesby said.

“There could well be a climate change lesson to be drawn from this,” Oglesby said. “In Nebraska, we like to take pride in our extremes of weather. The (scientific) suggestion for us, at least in the summer, is that the extremes will become even more extreme.”

Current conditions and forecast

Contact the writer: Nancy Gaarder    |   402-444-1102    |  

Nancy writes about weather, including a blog, Nancy's Almanac. She enjoys explaining the science behind weather and making weather stories relevant in daily life.

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