Seasonal forecasts are available from a number of sources, including the Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI). Today the Illinois State Climatologist, Jim Angel, wrote in his blog about another CPC source of monthly and seasonal forecasts at http://www.cpc.ncep.noaa.gov/products/NMME/seasanom.shtml.
The North American Multi-Model Ensemble provides forecasts from different climate prediction models and combines them together into an ensemble forecast. This is a typical technique used to provide forecasts with a higher degree of confidence, and is used in hurricane forecasting and weather forecasting as well as seasonal forecasts. The web site shows the predicted patterns for anomalies of surface temperature (Tmp2m) and precipitation (prate) for the globe and the US for each of the seven models in the ensemble. You can look at the results of individual models and compare them to find the areas of highest agreement.
The map below shows the precipitation forecast for September-October-November (this is season 1 on the table). The highest probabilities of above normal precipitation in SON are in the Southwest. In the Southeast, the probabilities are for a wet fall but with lower probability than the Southwest. If you look at the results of the individual models, you can see that 5 of 7 show a wetter fall, but two show normal or drier conditions. There is no way to tell in advance which model will do best in each situation, so by looking at the range of possibilities you get a better sense of what might be likely to occur.
If you go to the home site http://www.cpc.ncep.noaa.gov/products/NMME/, you can also get ensemble forecasts for individual months.