11:45 AM | An unfolding stratospheric warming event provides supporting evidence for cold weather ahead

[Polar view of current stratospheric (10-millibars) temperature pattern and the 10-day forecast using the GFS model; courtesy NOAA]

Discussion

One of the ways to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from northern Canada into the central and eastern U.S. Indeed, there appears to be a significant stratospheric warming event in the offing over the next 10 days or so (above) centered near the North Pole that provides supporting evidence for cold weather conditions ahead in the central and eastern U.S.

During the winter months in the lower polar stratosphere, temperatures are typically lower than minus 70° Celsius (purple area above). 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° in just a few days. This sets off a reversal in the west-to-east winds and the 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.

The plot (below) shows the stratospheric (10-millibar) temperature pattern for 2013 and 2014 in the high latitude region of 90°N to 65°N. 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. Interestingly, last winter season featured a sharp spike in stratospheric temperatures at just about this same time of year (i.e., late December, left circled region below) and the following couple of months were indeed much colder-than-normal in the central and eastern US. This year saw a sharp, but temporary spike in stratospheric temperatures during the latter part of November and perhaps that atmospheric event contributed to the cold and snow of Thanksgiving week in this part of the country.

A detailed video discussion on “Stratospheric Warming” can be found on the “Meteorology 101” page of the weather web site.

[Stratospheric (10-millibars) temperature plot for 2013 and 2014 in high latitude region of 90N to 65N; courtesy NOAA]