Here we are at the end of November and it seems like it is Spring around here now that most of the snow has melted after our significant early season snowfall and all of the rain that has fallen the past two weeks. Green grass has even begun to appear. But the low sun angle and a look up at the mountains tells the true story, winter is right around the corner. After spending much time doing research and comparing sea surface temperatures, snow cover, La Nina and other factors I am cautious to write what I think this upcoming winter will hold for us in far Northwest Montana and the Northern Rockies.
While there has been much talk about La Nina and many claiming that because we are in a La Nina year we will have a big winter La Nina in and of itself does not guarantee anything. Yes it's true that there is an overall bias towards heavier winters during La Nina years in our part of the country that is not always the case. There are several other factors involved that need to come together to produce heavy winters across our region. One of those factors is on a much much smaller scale as many of the long time locals know. It is common for our region to be affected by small scale features such as snow bands and tracks of surface lows along with cold air intrusions from the north and east that all have to line up perfectly to give us heavy amounts of snow. For the sake of this article, I will define a "heavy winter" as one in which valley areas receive greater than 80 inches of snowfall except for the Bull Lake/Heron region in which their annual snowfall is about 80 inches so if you live in these areas a "heavy winter" would be over 100 inches of snowfall. The Climate Prediction Center, known as the CPC for short is predicting that our region can expect slightly greater odds of below normal temperatures and greater odds of above average precipitation for the winter months of December, January and February as can be seen in the below graphics.
There is a bit of uncertainty here in the temperature department as indicated by a slight bump in our region but still some confidence in below normal temperatures. The precipitation department has greater odds of above normal precipitation. This is the official forecast from the CPC. I have also looked at some other data and done some research that is interesting. For the month of October and November so far this year, the pattern across the North Pacific and North America has been remarkably similar to one of the heaviest winters in recorded history for not simply our region but the entire Western US, that being the Winter of 1951-1952. At this time though, I can confidently say I do not think that will happen this winter. Looking at some other data, we can see that there is in fact a rather large area of colder than normal sea surface temperatures across the Central and Eastern Equitorial Pacific indicating a La Nina as can be seen in the graphic below by the blue coloring along with cooler than normal conditions along the Northwest Coast and the Gulf of Alaska with warmer than normal Sea temperatures up in the Bering Sea indicated by the yellow and red shaded areas.
This is important as areas that typically have warmer than normal sea surface temperatures during the winter months favor upper level ridging while areas that have colder than normal sea surface temperatures favor troughing. So far this fall we have seen a rather persistent and strong upper level trough over the Gulf of Alaska and a blocking upper level ridge over the Bering Sea. The position of the upper level trough has shifted however and that is what has caused our pattern to change from a snowy and chilly one at the beginning of the month to a mild and wet one at the middle and end of the month so far. The other factor I have looked at is the current snow and ice cover across North America. The map below reflects conditions as of Sunday, November 26, 2017 on the left with conditions as of November 26, 2016 on the right.
The maps may look very similar but there is a big and very important difference regarding snow cover across the Western US and snow and ice cover over Eastern Canada and Hudson Bay. As of yesterday, almost half of Hudson Bay is frozen over and most of the smaller lakes of Eastern Canada are frozen over whereas last year at this time, snow cover was more plentiful across the West and Hudson Bay along with the smaller lakes of Eastern Canada were largely ice free. So far this fall, there has been a rather persistent and somewhat strong Hudson Bay low that has continued to funnel colder than normal temperatures for much of the Upper Midwest and Northeast region of the US. The location of the Hudson Bay low is critical for our region as was evidenced last winter when the low set up much farther to the west than normal due in part to the extensive and widespread snow cover that developed early on over Western Canada and the Western US along with a late freezing over of Hudson Bay.
So what does all this mean for us? Well here is my prediction for our winter based on the above information and more of which I did not include. I believe that here in far Northwest Montana we will see near to slightly below normal temperatures with above normal precipitation or similar to what the CPC is forecasting. I do think we will see some rather cold arctic air outbreaks that may last short periods of time over a few days to perhaps a week but not the weeks on end of below freezing temperatures we saw last winter. Where the Hudson Bay low eventually sets up will be critical to our weather this winter. If it remains around Hudson Bay as it has so far this fall then we will likely see longer stretches of drier weather along with extended periods of milder and wetter weather with higher snow levels. If the Hudson Bay low ends up retrograding (moving westward) and sets up further west as in last winter then we may be in for some brutal cold snaps and periods of heavier snows when those cold snaps moderate and warm up. While last winter was also a La Nina year, it was a weaker La Nina than this winter and as of this writing it is possible that this years La Nina will strengthen into a moderate strength or possibly even a strong La Nina. Again, La Nina is only ONE factor and cannot be used to justify anything, it is simply a factor in our weather. I believe that most areas may come just shy of that 80 inch snowfall mark for the winter or if you live in Bull Lake and Heron the 100 inch mark but on the other hand, it would not surprise me if we exceed that. And it's always possible that we could have a repeat of the Winter of 1951-1952 although that does seem unlikely. The best advice is to be prepared and keep the shovel, sand and snowplow ready to go.