La Nina conditions have developed in the central Pacific Ocean. La Nina is defined as the periodic cooling of sea surface temperatures in the east-central equatorial Pacific Ocean. This typically occurs every few years, and is the cool phase of the El Nino/Southern Oscillation cycle. The current La Nina is forecast to strengthen through the fall and persist into 2011.
La Nina episodes usually affect weather patterns across the country. This is due to variations in the jet stream pattern caused by the cooler than normal sea surface temperatures. Historically for our part of the Midwest, La Nina episodes have resulted in warmer and drier than normal conditions in the fall, and wetter than normal conditions in winter.
To illustrate this point, below are several climate graphs from the Climate Prediction Center (CPC). They show 3 month temperature and precipitation distribution for El Nino, neutral, and La Nina phases for Central & Southeast Illinois. These are based on data collected since 1950. Note how fall (September, October, November or "SON") temperatures trend above normal, while precipitation is typically lower. For the winter graphs (December, January, February or "DJF") temperatures show no particular tendency, while precipitation trends above normal.
While the data above gives us some idea of what to expect in a La Nina year based on historical data, for the current situation we look to the climate experts at the CPC. They provide temperature and precipitation outlooks on monthly and seasonal timescales out to one year based on many data sets, computer models, and other tools. Below are the fall and winter outlooks. Current forecasts show an enhanced probability of above normal temperatures and below normal precipitation this fall, and above normal precipitation this winter.