The outlook for November through February calls for above average precipitation for most of northeastern Wyoming and western South Dakota. Temperatures are expected to be below average, especially across northwestern South Dakota.
A weak La Nina has returned to the equatorial Pacific Ocean. A moderate La Nina is expected for the 2011-2012 winter season. Based on this forecast, the overall weather patterns this winter will likely produce more storm systems originating over the Pacific Northwest. These systems will produce more frequent precipitation and cold air outbreaks across northeastern Wyoming and western South Dakota -- especially northwestern South Dakota.
Previous winter seasons with similar La Nina conditions have been reviewed, as well as years with weather patterns like those of the summer and early fall months of 2011. These were used to predict the overall weather pattern for this winter.
Winter temperatures will likely be slightly below average across northeastern Wyoming with below average temperatures expected across western South Dakota. The coldest temperatures are expected across northwestern through central South Dakota where occasional strong arctic intrusions are expected.
Precipitation will likely be above average on the plains of northeastern Wyoming and western South Dakota. The greatest confidence for above average precipitation is across the Bear Lodge Mountains, the northern Black Hills, the northern Foothills and northwestern South Dakota where more frequent snowfall events may occur.
Precipitation on the plains of northeastern Wyoming and western South Dakota for November through February averages about 1.5 to 2.5 inches, which is only around 10 percent of the annual average. In the upslope areas of the northern Black Hills and Bear Lodge Mountains average precipitation for the winter season is 4 to 6 inches.
A stronger La Nina would tend to produce cooler and wetter conditions than forecast.