La Nina (Spanish for little girl) is back, bringing an arctic-chilled jet stream on a southeasterly flow across Iowa for the winter. That means a succession of cold air masses and snowstorms. The Upper Midwest, including Iowa, will bear the worst of her wrath.
La Nina is characterized by periodic huge pools of cool water in the tropical Pacific. Both La Nina and its opposite, El Nino, affect the position of the jet streams, which in turn affect the track and intensity of storms.
In the Upper Midwest, a winter La Nina means below-normal temperatures and above-normal snowfall.
Last year, Iowa experienced its 34th-coldest and ninth-snowiest winter in more than 130 years of record keeping. It was the fourth straight colder-than-average winter and the fifth straight snowy winter.
Normal snow accumulation for the season is 31.5 inches in Eastern Iowa. Last year, snowfall totals were 39 inches. For the winter ending in 2010, 43 inches; 2009, 46 inches; 2008, 57 inches; 2007, 22 inches; and 2006, 35 inches.
Expect a chilly air-mass pattern to establish itself in November. Predictions call for the first half of winter to be the worst, but the second half will be colder.