In light of the recent mild start to the winter of 2006-07 for much of the eastern half of the United States, there has been a lot of discussion on climate change websites about how much of the anomalous warmth can be attributed to the moderate El Nino in the tropical Pacific and how much is due to global warming. The exceptionally warm start to the year and relative lack of snow across much of the United States and Europe has led some climate scientists to declare 2007 as the warmest year ever. An overlooked question in this climate discussion is just what has happened meteorologically since the beginning of December to make it so warm.
Actually, the reasons for the anomalously warm start to winter may be found from the previous fall. Many non-meteorologists have long since forgotten that September and October were actually cooler than normal across the United States. Many locations, including Chicago, had the earliest measurable snowfall on record and Buffalo, NY, was devastated by several feet of early-season lake-effect snowfall. At this time, the El Nino was relatively weak, which, combined with a cold phase of the PDO (PDO index = -0.95) allowed for a meridional (north-south) jet stream across the United States with a storm track that extended from the northern Rockies into the southern Plains and northeastward into the Ohio River Valley (Kentucky tied its September rainfall record). The north-south meandering jet stream allowed for several arctic outbreaks which accounted for the cooler than normal temperatures.
What does this have to do with the warm winter? In November and December, the El Nino rapidly strengthened into a moderate event, which produced a zonal (east-west) jet stream that brought mild Pacific air across the entire United States. In the first two weeks of January, the El Nino began rapidly weakening, which allowed for cold air to intrude into the western United States, which led to a series of powerful winter storms across Colorado and the southern Plains. Further weakening in mid-January allowed the meridional jet stream to again bring cold air into the entire United States. While south-central Kentucky has yet to see its first snowfall (Ironically, both Los Angeles and Phoenix have recorded more snowfall than Bowling Green this winter) of the year, temperatures have been roughly as cold as normal since 1/16.
The computer models and climate teleconnections such as the North Atlantic Oscillation (NAO) and Eastern Pacific Oscillation (EPO) suggest that the coldest weather of the winter is on its way for the next few weeks. I don’t expect that the Mid-South will bear the brunt of the cold air (that will be the Northeast), but I do expect that by the middle of February, many of us will be looking forward to spring.