October always is an interesting month to determine whether the winter will start good or not. A combination of indications create a base for a cold or a mild winter. The end of October is approaching and it is therefore interesting to see what kind of indications October has brought us. As you may know, I am not a big fan of computer models forecasting for the long term, because it has to do more with gambling than with forecasting. I usually never write a long term winter forecast, but I make an exception this time. Especially because a number of important parameters are bringing 'good news'.
I immediately start with something pretty complex: the Polar Vortex. If you've followed my forecasts and PowderAlerts during winter 15-16, you've might read the term before. The Polar Vortex was pretty strong during autumn last season and a result it wasn't possible for cold air to head to the Alps. A strong zonal jet stream sent all its moisture towards Scandinavia. It was dry and mild in the Alps.
Remember: A strong Polar Vortex keeps cold air away from the Alps because all the cold air is sucked towards the Arctic. A weak Polar Vortex however allows cold air to travel to the south. And it seems like the Polar Vortex will get weaker and weaker. How that's possible has to do with the second point: the growing snow cover in Eurazia.
Meteorologists are looking with special attention to the growth of the snow cover in Eurasia in October. In particular because the growth of the snow cover south of 60 degrees northern latitude appears to be an important indicator for the development of the Polar Vortex in the coming months. If there's more snow than usual, then the probability of a weak Polar Vortex is realistic. And looking at the development of the snow cover we see that this is going in the right direction. This site from NOAA (US weather institute) allows you to monitor the development.
In particular, The growth of the snow cover south of the 60 degree line is significant and this could result in the development of a strong high-pressure area above Siberia. Such a strong high pressure area influences the Polar Vortex and as a result this could lead to a weak Polar Vortex and greater chances on a cold winter. That process looks like this:
A negative Arctic Oscillation (a negative AO) is important for the formation of the snow cover in Eurasia. That negative AO results in cold temperatures and more snowfall. And you've guessed it: that negative AO results in more snow for Eurasia and that further weakens the Polar Vortex.
An important additional factor is the relatively small size of the sea ice at the Arctic. Less sea ice and more snow fall results in cold temperatures focusing on the south of Siberia and not on the Arctic. This normally results in more snow for Siberia, a weaker Polar Vortex and a greater chance on significant cold weather in the winter on the northern hemisphere. The start of winter 16-17 will be colder on the northern hemisphere than the start of winter 15-16. That is: if all the indications won't fool us.
There is a high probability that the Polar Vortex will be pretty weak the next few months, which will increase the chances on a cold start of the winter. We can expect the first cold temperatures to arrive in early November.