The Climate Prediction Center (CPC) released an updated winter forecast last Thursday. Most of the year they were forecasting a higher than normal probability of a warmer than average winter, but with their latest forecast, they are now predicting a higher than normal probability of a colder than average winter.
Why the change? I can not say for sure, but it is probably because the El Nino the CPC thought was going to form, in turn, has not yet formed and there are significant doubts one will form in the upcoming months. An El Nino, the warming of the equatorial waters in the Pacific Ocean, often brings warmer than average winter temperatures to this area, although, the last El Nino in 2009-2010 did not.
Seasonal weather patterns tend to be dominated by a number of factors, not just one and most of those influences are very difficult or impossible to forecast more than a couple of weeks in advance.
Here is the latest forecast from the CPC.