Even though we are still in Atlantic tropical season, and the tropics are indeed getting quite active, why not take a look ahead at the prospects for the upcoming winter in the Mid-Atlantic region? When dealing with long-range forecasting outlooks such as with respect to the upcoming winter season, it is generally best to start with a look at the oceans as they have a much higher heat capacity than the atmosphere and the biggest ocean by far is the Pacific Ocean. In fact, a significant transition is now occurring in the tropical (equatorial) part of the Pacific Ocean with respect to sea surface temperatures from colder-than-normal La Nina conditions to El Nino. El Nino, which is characterized by warmer-than-normal sea surface temperatures in the tropical Pacific Ocean, influences the winter weather across the US. It now appears that the upcoming winter will likely feature a weak-to-moderate El Nino and this could very well play an important role in both precipitation amounts and temperatures that we’ll experience here in the Mid-Atlantic region.
First of all, in terms of precipitation, El Nino winters generally tend to feature active sub-tropical jets which, in turn, often lead to enhanced storminess along the Mid-Atlantic coast. This was not the case last winter as La Nina conditions persisted in the tropical Pacific, and there was certainly a lack of the “classic east coast storm” that we experienced in, for example, the winter of 2009-2010 when El Nino last appeared.
As far as temperatures are concerned, I have examined all of the years since 1950 that featured weak-to-moderate wintertime El Ninos and compared them to years with strong El Ninos and the results were dramatically different with respect to impact on temperatures in the eastern and central US. Historically, strong El Ninos during the wintertime have often flooded much of the US with warmer-than-normal air, but interestingly, weak-to-moderate El Ninos have generally produced colder-than-normal conditions in the eastern US. The strength of the El Nino cycle is not the only important factor as far as impact on Mid-Atlantic weather conditions is concerned, but its exact location in the Pacific Ocean has been found to often be critical as well. Specifically, history has shown that when an El Nino is strongest in the central Pacific (central-based) as opposed to just off of the South American coast (east-based), colder-than-normal winters in the eastern US are more common. The latest 30-day trend in sea surface temperature changes in the tropical Pacific tends to favor the development of a central-based El Nino in the upcoming months rather than an east-based.
While there are many other factors that are critical to the upcoming Mid-Atlantic winter such as the snow pack that develops in North America during the fall months, there certainly are plausible reasons to believe that a cold and stormy pattern for the upcoming winter is on the table for the Mid-Atlantic given the increasing signs for a weak-to-moderate and central-based El Nino in the tropical Pacific. Stay tuned for updates over the next few months.