During the month of September weak El Nino conditions continued across the equatorial pacific ocean. The Climate Prediction Center is forecasting that the El Nino will persist through the 2009-2010 winter season and will likely peak at moderate strength. El Nino conditions generally produce well above average temperatures and near average precipitation across the northern plains. However…another factor is influencing the typical El Nino impacts...the Pacific Decadal Oscillation (PDO).
The PDO is a long-lived El Nino like pattern of northern Pacific Ocean variability. Unlike El Nino events which generally last 6 to 18 months...PDO events persist for 20-30 years. The most visible effects of the PDO are across the north Pacific Ocean and western North America. The PDO is currently in a negative phase, having shifted from a positive phase in the late 1990s. When the PDO is in a negative phase it has a tendency to dampen the effects of El Nino. Thus…we are not expecting temperatures to be as warm as one would expect when the PDO is in a positive phase.
Near average precipitation is expected across northeast Wyoming and western South Dakota. However…snowfall may be slight below average given the forecast of warmer temperatures. Precipitation on the plains of northeast 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.