12:00 PM | Stratospheric warming event and widespread snowpack are signals for a return to cold weather after next week's moderation
The weather pattern in recent days has been quite cold with below normal temperatures throughout the Mid-Atlantic region. In fact, it looks pretty cold right through Monday and we’ll likely have snow shower activity later Saturday night and early Sunday that could leave a coating in many spots. However, significant moderation in temperatures will take place next Tuesday and Wednesday and that new above-normal temperature pattern may continue right into the following weekend. What happens beyond next week’s break in the action? There are a couple of reasons to believe that a return to cold weather, perhaps even very cold, is quite probable for the Mid-Atlantic region during the middle and latter part of January.
To begin, there is a significant "sudden stratospheric warming" event now underway across the high latitudes. Temperatures over the next several days will climb rapidly in the stratosphere in the high latitudes and this type of event often results in the displacement of cold air that is normally bottled up in the polar regions to lower parts of the atmosphere (i.e. the troposphere) in the mid latitudes. In this particular case, it is likely that this warming event will result in a weakening and shifting of the polar vortex away from the North Pole and perhaps with a complete split of the vortex. Some stratospheric warming events in recent decades have, in fact, been followed by widespread very cold air outbreaks across southern Canada and the US some two to three weeks after the initial upper atmosphere warming. For example, such an event occurred during December 1984 and this led to an extremely cold January 1985 in much of the central and eastern US. (More info on recent major stratospheric warming events in the "Stratospheric Warming" video found in our "Meteorology 101" section). I believe the first sign to look for to tell if this stratospheric warming event is indeed having an effect on global weather patterns will be a change in the current slightly positive Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) index values to sharply negative readings by the middle of the month. This will likely occur as the polar jet stream generally weakens when the stratosphere warms and the polar vortex weakens. The change to these index values (AO, NAO) from positive to negative will be a pretty strong indicator that cold weather is going to return to the central and eastern US - perhaps in about two weeks or so from now.
In addition to the stratospheric warming event, another signal for cold air to return to the region after next week’s expected moderating temperatures is the extent of the snowpack currently across the nation and across the entire northern hemisphere. In fact, the nationwide snowpack of ~67% was the highest amount in the last ten years to begin a new year (and records for this only go back 10 years). Last year at this time there was only about 20% of the country with snow cover as we were experiencing a very warm winter nationwide. Even more important than the nationwide snow cover is the snowpack across the northern hemisphere which is currently running significantly above climatological normals for this time of year. Extensive snowpack like this in the northern hemisphere can have a direct contributing effect on the generation (or in this case the re-generation) of widespread cold air masses in our source regions for cold air (e.g. Canada, Siberia, Alaska and Greenland where there has been historic cold in recent days).
Stay tuned - we’ll continue to monitor all of this over the next couple of weeks.