On the left is the NOAA 30-day temperature forecast for December, 2009 and on the right is how temperatures have turned out for the first 27 days of the month.
In the forecast, "A" means "above normal" temperatures and B is the reverse. On the "actual" temperature map, greens/blues/purples are colder-than-normal values.
The forecast could hardly be more wrong. In eastern Montana, where the highest statistical chance of warmer than average temperature was forecast, temperatures have been 15° below normal!
The purpose of this post is not to criticize NOAA for this forecast but rather to illustrate the folly of pretending we can make skillful 30-year and longer forecasts. If we have no consistent skill at forecasting monthly and seasonal conditions, we have no skill at forecasting the weather decades into the future.
If you would like to read more on this topic, please go here.
UPDATE December 30: Great Britain has the same problem with poor seasonal forecasters claiming great confidence in their forecasts for future decades. More here.
UPDATE December 30, 4pm: Roger Pielke, Jr. also weighs in on the topic of overly confident climate forecasts.