[GFS model forecast of the Arctic Oscillation (AO) index to the middle of October]
There is an index called the Arctic Oscillation (AO) which is defined by surface atmospheric pressure patterns and it is tracked closely by meteorologists during winter seasons as it can provide clues as to whether Arctic air has the potential to be transported from the northern latitudes to the middle latitudes. When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions, weaker zonal winds, and greater movement of polar air into the middle latitudes. While the AO index and its closely-related cousin called the North Atlantic Oscillation (NAO) are primarily used during the winter season, trends in October can provide important clues about the soon-to-follow winter season. Indeed, the AO is about to plunge into territory not seen during the month of October in over a decade and there is an outside chance it reaches record low territory for the month of October.
The map above shows the observed AO index value (black) up to the current date and then the forecasted values (in red) to the middle of the month. The AO is forecasted to plunge over the next couple of weeks to around -5 (legend at left) which is very close to the lowest level ever seen for the AO index in the month of October which was -5.09 recorded on October 18th, 2002. In fact, the last time the AO was lower than -3.5 during the month of October was in 2002 and there have only been five times where the AO index has been below -4.0 in October. By the way, the all-time low for the AO index came during the brutally cold month of January 1977 with a value of -7.43.
As it turns out, when we take a look at the winters since 2002 that featured an AO index below -2 during the month of October, almost every one (6 out of 7) experienced above normal snowfall in much of the Mid-Atlantic region and usually below-normal temperatures as well. Stay tuned…Vencore Weather winter outlook coming in November.