Friday, November 4, 2011

How good are our winter forecasts?

I've written several stories, and you've undoubtedly seen several predictions about winter 2011-2012.

It always makes great conversation, and meteorologists' ability to predict an upcoming season has improved mightily in recent years.

But will we ever be able to really get the winter forecast right?

It wasn't long ago that the Farmers' Almanac, the woolly worm and the groundhog were all we had to forecast winter. Then along came vast improvements in computer modeling and scientists' understanding of the various factors that create weather, and that gave winter forecasts a bit more clout.

Still, there are some obstacles.

Last October and November, all the talk was about La Nina, and what it would mean for winter in the South. The standard thinking is that La Nina means dry and mild winters in the South.

Lo and behold, arctic air blasted into the South on Dec. 2 last year, and for 11 weeks, the eastern two-thirds of the United States shivered. Snow fell several times in the South. Charlotte got its first Christmas snowfall in a half-century, and December 2010 was among the coldest months in recent years.

Some meteorologists had warned us that there were other factors in winter weather -- specifically, the Arctic Oscillation (AO) or North Atlantic Oscillation (NAO). When high pressure builds near the arctic, low pressure tends to form at middle latitudes (i.e., the United States). That sends arctic air blasting into the United States, and it also provides a nice little track for storms to form.

It's why there was a series of winter storms last year from Texas to the Southeast coast, and then up the East Coast into New England.

It was what meteorologists call a Negative NAO.

Around Valentine's Day, the Negative flow went Positive, and La Nina took over. The weather got mild, and in late winter and early spring, the track of storms went across the South. That led to the series of tornado outbreaks that killed hundreds.

The question is: Can we predict a Negative NAO or AO?

"Right now, it's really not something we can forecast more than about a week in advance," said Mike Halpert, deputy director of the Climate Prediction Center for NOAA since 2007. "It's the fly in the ointment for our winter weather forecast."

Halpert said that if scientists knew the NAO would be neutral or positive, then they'd forecast a mild, dry winter for the Southeast, due to La Nina. Instead, the prediction is for "an equal chance" or above-average or below-average temperatures.

As you might expect, scientists are working hard to improve their ability to forecast the Arctic and North Atlantic oscillations. There are theories, Halpert says. One links the amount of snow in Siberia in October and November. More snow means colder air and higher pressure near the Arctic -- hence, a Negative AO.

Halpert says another theory links the oscillation to the amount of sea ice or to sea water temperatures in northern latitudes.

"We're looking at the research," he says. "This is an area of study for us."

Some meteorologists, like Brad Panovich at WCNC, the Observer's news partner, have predicted this winter will have a similar pattern to last year -- a very chilly start, especially in December and early January. Then the indication is for a milder trend later in the winter, followed by the possibility of another bad tornado season in early 2012.

This AO and NAO situation is a reminder that meteorology is a science, but one in which there are still many questions without answers.