December continues to run well above normal temperature-wise in the Mid-Atlantic region and thoughts now turn to whether this temperature pattern can flip in upcoming weeks and, if so, when. There is a possible stratospheric predictor of dramatic wintertime changes to much colder weather in the US and it is connected to an atmospheric phenomenon known as “stratospheric warming”. It is rather normal for the atmosphere to feature an upper-level vortex at or near the North Pole during the wintertime, but recent experience has shown that when the polar vortex weakens, or even reverses to become a high pressure area, a dramatic temperature change to much colder can follow in several weeks across much of the US. The chain of events that can cause this dramatic flip to colder weather in the US begins with the stratospheric warming near the North Pole which is then followed by the development of a blocking pattern in the upper atmosphere across the Arctic region. This then, in turn, ultimately leads to an infiltration of cold air into the continental US - sometimes with dramatic results. There have been about 30 registered stratospheric warming events since the phenomenon was first discovered in the early 1950’s. Two rather recent events, in fact, led to dramatically colder weather in the US several weeks after the process began. The first such stratospheric warming event began during November 1984 and it was followed by a very cold month of January (1985) in much of the US. In fact, it got so cold during that month that all outdoor activities for Reagan’s second Inauguration Day (January 20th) were cancelled in Washington, D.C. The second example started during November 1998 and it also resulted in a very cold month of January (1999) across much of the country. Both of these events took several weeks to evolve from the first stages of the stratospheric warming near the North Pole to the dramatically colder temperature patterns in the US. Stay tuned for updates over the next few weeks on this winter's chances for this type of atmospheric event. There are currently a few indications that this atmospheric phenomenon could take place in the coming weeks, but it is far from certain.