11:45 AM | January looks to be colder than normal in the central/eastern US - perhaps even significantly - thanks in part to a stratospheric warming event
There are reasons to believe that January will be a colder-than-normal month in the eastern and central US - perhaps even much colder-than-normal and the coldest in many years - and part of the reason for this outlook is a stratospheric warming event that is now underway over the northern latitudes. In addition to the stratospheric warming event that is forecasted to continue by the GFS computer forecast model (5-day polar stratospheric temperature forecast map shown above), supporting evidence for a colder-than-normal January comes from the very latest NOAA/NCEP Coupled Forecast System model temperature anomaly forecast for January which is significantly colder-than-normal in the central and eastern US.
The phenomenon of stratospheric warming was first discovered in 1952 and there have been about 30 events registered since then or about one every two winters. Research suggests that stratospheric warming events are a consequence of the interaction between the North Atlantic, the troposphere and the stratosphere, and there tends to be an increased number with a warm North Atlantic Ocean as currently exists. During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days. This sets off a reversal in the west-to-east winds and the partial or even complete collapse of the polar vortex.
In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take weeks to unfold and ultimately can lead to numerous Arctic air outbreaks in the central and eastern US. An in-depth video discussion on the stratospheric warming phenomenon can be found at http://thesiweather.com/meteorology-101/. A detailed video on the current situation is available (below). We’ll continue to monitor all of this in the days and weeks to come here at "thesiweather.com".