September 24, 2010

La Nina conditions are expected to peak during the December-January time frame. The Climate Prediction Center indicates that much of the Pacific Northwest, northern Rockies, and extreme northern Plains will experience below normal temperatures in the December to February period. Above normal temperatures are projected for the southern one-third of the U.S. In between, no predictable temperature trend is indicated (Figures 1 and 2).

I expect that much of the central U.S. through the northeast will see extreme temperature volatility, with the greatest likelihood in December and January. Extreme temperature volatility is defined as extended periods (7-14 days) of much above normal temperatures followed by much below normal temperatures. Transitions between these periods usually result in strong snow and/or ice storms. The most likely corridor will run from the central Plains east-northeast through the eastern Great Lakes region.

Most La Nina winters bring above normal snowfall to the northern Rockies southward to northern Colorado. Below normal snowfall generally occurs across the southern Rockies and Sierras. In addition, dry conditions are common from New Mexico east through Georgia. Some of the most significant droughts from Texas to Georgia are the direct result of below normal moisture during the fall and winter months of an established La Nina event.

Current model consensus is that the current La Nina event, which is developing faster and strong than predicted three months ago, will end by mid to late spring. Over half of the historical La Nina events have lasted more than a year. Severe to extreme drought conditions likely will materialize across the southeastern third of the U.S. and extend west through southern California by the official end of winter.

Allen Dutcher
Extension State Climatologist

See Spring Forecast

Also

• La Nina Conditions
• Fall Forecast