Many have wondered and asked about the heavy winter most of us in the Northwest part of the country including Northwest Montana and what exactly caused the winter of 2016-2017 to be one of the heavier ones? I have heard several rumors and myths circulating about the cause of the abundance of low elevation snow and longer than normal periods of cold. I hope to shed a little light on this and perhaps give some insights into how the winter of 2017-2018 may shape up for Northwest Montana.
It would be a wrong conclusion to decide that the reason we had much above average snowfall across Northwest Montana was due to La Nina last winter. While it may be true that La Nina had some role in the winter's outcome, there are several other factors that need to come together to provide the conditions we did see. In doing research on weak to moderate La Nina which is what we had last winter, of 10 weak to moderate La Nina's historically there is a split between cold and snowy, mild and wet, cold and dry and mild and dry for our part of the country. I would like to point out what I believe was the determining factor for our colder and snowier than normal winter of 2016-2017.
Take a look at the map below which shows snow cover over Canada and the United States from November 30, 2016.
The white areas are snow cover while the yellow areas are ice. The blue represents unfrozen water and green is obviously land not covered by snow. The 2 standout areas to look at are the amount of snow cover present across much of British Colombia and Alberta south through much of the mountainous terrain of the western US and the mostly unfrozen Hudson Bay over Eastern Canada although almost all of Canada is covered in snow otherwise. Why is this important? Typically, Hudson Bay freezes over fairly early in the fall season and much of Eastern Canada has more snow cover than Western Canada. This allows for upper level lower pressure, what is referred to as the "Hudson Bay Low" in meteorology, to develop mostly over Hudson Bay and a corresponding upper level ridge to build over the Western US into Southwest Canada. This is a very stable pattern and results in what is called a "Positive PNA pattern." This type of pattern is often a mild and wet one for us in the Northwest including Northwest Montana or a dry one with an abundance of valley fog and low clouds. But in November of 2016 we see much more snow cover over the Western part of North America and an unfrozen Hudson Bay. This change allowed for the Hudson Bay low to develop much farther West than it normally would setting up more over Alberta and Western Saskatchewan. The greater snow cover over the west and an unfrozen Hudson Bay meant that while much of Eastern Canada had snow cover, Hudson Bay was still relatively "warm" and unfrozen which allowed an upper level high pressure area to develop. By having the "Hudson Bay low" displaced much farther west than normal allowed for reinforcing cold air and storm systems to move in off the Pacific to continue to produce widespread snows over the the entire Western Northern Hemisphere. Below is another picture taken of snow cover on February 1, 2017
Here again we see that most of the Western US is covered in snow while much of the Eastern US is not. While Hudson Bay is now frozen over, the Great Lakes are not. Once the Hudson Bay low takes up residence over a particular area and once an upper level ridge takes up its own area during the winter months, they are very difficult to dislodge. With the early season set up of the Hudson Bay low over Western and West Central Canada during the Winter of 2016-2017 the pattern continued to repeat itself with continuous cold air dropping down from the north and systems moving in off the Pacific unabated by the removal of the upper level ridge which was displaced much farther east over the southeastern US. The result was largely a continuous negative phase of the PNA pattern which is shown below next to the positive phase of the PNA.
Strong positive phases of the PNA often result in cold air being forced south into the Eastern US while the negative phase has the Western US receiving cold and stormy conditions. So how do things look right now? Below is the current snow cover over Canada and the US as of October 24, 2017.
While it is still too early to tell what may happen a few things to consider here again is the greater coverage of snow over Western Canada, especially over British Colombia and once again the unfrozen Hudson Bay and lack of snow cover over Eastern Canada. Typically as we go into November the Hudson Bay low will begin to take up residence before being "anchored" in place typically by the end of December. Where that will happen is still not yet known for sure. I am confident enough though that if we see a continual early build up of Western Canadian snow cover and a late freezing over of Hudson Bay, the Hudson Bay low may set up further West once again. As for the winter season, we are officially in another weak La Nina and the Climate Prediction Center is calling for greater odds of below normal temperatures and greater odds of above average precipitation but that is still far from being a slam dunk. In any case, if the Hudson Bay low sets up further West, the odds of the Northwest having a colder and snowier winter increase substantially. If it sets up further East the odds decrease quite a bit for colder and snowier but increase for milder and wetter. The setup of the Hudson Bay low is NOT the only factor in predicting what kind of winter we will see but it is ONE of them and I believe is often overlooked. The best approach would be to take the advice of hardy Montanans. Be prepared for anything!