What happened in 2012, and what makes this winter so difficult to predict?
Mark Twain once noted that there’s only one thing certain about the weather: "there is going to be plenty of it." Although he made that statement in 1876, it still rings true more than a century later. Farmers are thankful for the forecasting improvements since Twain’s day. Even so, predicting the weather and climate remains one of the more fickle sciences.
"This winter is especially hard to predict," says Al Dutcher, Nebraska state climatologist. "We have seen some significant changes to the forecast throughout the past few months. That is a signal of a low-confidence forecast."
One key hang-up is whether an El Niño event will materialize by mid- December. El Niño and La Niña patterns are determined by Pacific Ocean sea surface temperatures, which in turn shape the jet stream that passes over North America.
What Are El Niño and La Niña?
During an El Niño event, warmer-than-normal sea surface temperatures in the eastern tropical Pacific Ocean reroute the jet stream as it passes over North America, pushing the major storm routes through the Southern U.S. and into Canada. La Niña begins with cooler-than-normal sea surface temperatures in the same Pacific region. This time, the jet stream drags most of the major winter storms through the Upper Midwest, with a drier, milder Southern U.S.
Iowa State University ag meteorologist Elwynn Taylor says it should be no surprise that U.S. weather is largely dictated by what happens thousands of miles away in our oceans.
"Our oceans have so much heating capacity," he says. "They can really soak up or give off heat."
During El Niño, the southern split of the jet stream drags across the southern half of the U.S., bringing with it a warmer, wetter winter, while the Midwest stays milder and drier. A typical La Niña jet stream flows much more northerly, usually across the Upper Midwest. The result is a generally drier South, with bursts of cold, wet weather in the Midwest and Northeast.
Dutcher says conditions are favorable to develop a weak El Niño pattern, but it’s not a lock. If it does establish, a normal El Niño event will last for about a year. El Niño visits more often than his sister—about seven out of every 10 years.
- December 2012