What the stratosphere and tropical Pacific can tell us about December
For the better part of the last three weeks, our forecasters have been discussing and monitoring the potential for a pattern change throughout the hemisphere. Medium range model guidance has often been gung-ho with the pattern changing within 7 days. As is often the case, forecast models were too quick to change the pattern. Much of this has to do with poor forecasting of the stratosphere and tropical Pacific ocean. Not coincidentally, these two features can give us clues as to where the pattern is heading in the weeks ahead.
As it stands this afternoon, model guidance and ensembles are in good agreement that a -NAO (North Atlantic Oscillation) pattern will develop over the next two weeks. This pattern is often defined by large ridges or higher than normal heights in the atmosphere over Greenland and the North Atlantic Ocean. -NAO patterns can allow cold air to be displaced farther south into the Northern 1/3 of the United States, particularly the Central and Eastern United States.