A milder weather pattern continues here in the Mid-Atlantic region and it will remain into the early part of next week with a peak in temperatures to occur this weekend. By the middle of next week, however, another pattern change will begin to be seen in the northeastern US and it could become quite dramatic. A major “sudden stratospheric warming” (SSW) event has occurred during the last week or so and it has resulted in a breakdown of the stratospheric polar vortex. In fact, the normally “single-entity” upper-atmosphere vortex has split into two as a result of this dramatic SSW event.
A major SSW event such as this is seemingly setting off a series of events that will soon lead to a resurgence of cold air masses into the central and eastern US. Specifically, the SSW event suppresses and cools the troposphere and weakens the polar jet stream which, in turn, helps to generate high latitude blocking in the atmosphere. Once the blocking becomes established, usually near Greenland, it acts to force the coldest air normally bottled up over the Arctic to move southwards into the mid-latitudes. This whole process of Arctic cold spilling southward to the surface levels in the mid-latitudes typically occurs some 10-20 days after the initial sudden stratospheric warming and the change to a cold pattern can last for a few weeks. The blocking in the atmosphere at high latitudes can be tracked through the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) indices which, when negative, indicate blocking is occurring in the North Atlantic region. Indeed, these indices are in the process of changing from slightly positive readings to negative values as this SSW event continues to unfold and impact the global weather patterns.
The bottom line, after a significant warm up that peaks this weekend in the Mid-Atlantic with a “spring fever alert”, the overall weather pattern here will likely change to colder-than-normal towards the middle and latter part of next week and that change to cold could persist for awhile. The first waves of cold air as a result of this pattern change will strike the western and central US, but by later next week, they should reach the northeastern states. There is the potential that during the time period from mid-January to early February parts of the US experience some of the coldest air seen in quite some time and snow chances will no doubt be on the rise. One final note, on the other side of the pole, brutal cold air will invade a very large area from Europe to Asia over the next couple of weeks – perhaps some of the coldest air seen there in several decades.