Predicting the weather a few days away is not easy here in Upstate New York — especially during the winter season when the weather is more active. There are so many small-scale features that impact the weather. From the Great Lakes to the Finger Lakes, Southern Tier hills to Adirondack Mountains. So how can we predict what is expected for an entire winter?
Well we can’t give you a daily forecast of what to expect each and every day, but we can look at planetary scale features around the globe to get an idea of what to expect locally. In other words, will we have above average temperatures, or below? More snow or less snow as the long winter rolls on?
A few of these features we use are the North Atlantic Oscillation, Eurasian snow cover, as well as ENSO, or El Nino Southern Oscillation. These features have an impact on the weather patterns around the globe. There are others as well, but these are the main ones we focus on for our prediction. The global weather pattern absolutely has an impact on what we can expect to see here in Upstate New York.
We then look at the current state of each as well as how they are expected to change over the next few months. We then compare that to how they were in previous years. From there we get a top five list of similar previous winters and then determine which one of those is our analog year. That one winter, whether it was a few years ago or 50 years ago that matches what we are seeing today with these global features. Then we make our prediction.
The NAO is the North Atlantic Oscillation. It has a negative and positive phase. It is the difference in pressure from near Iceland and the Azores. When negative, it can buckle the northern branch of the Jetstream. The Jetstream is the "river of air" aloft that drives weather systems around the planet. When the northern Jetstream is forced south of us during the negative phase of the NAO, the Arctic air drops in. Conversely when the NAO is in a positive phase, the northern Jetstream is to our north and the Arctic air stays north of us.
Now in order to have bitterly cold Arctic air masses develop, you need snow. You need lots of deep snow over a large area in Northern Europe and Asia, essentially Siberia. That is our Eurasian snow cover indicator. When expansive and deep, the cold air builds. When below average and thin, not as much cold develops.
Finally we look at ENSO in the South Pacific. ENSO can also have a negative and positive phase. When it is in a negative phase it is called La Nina. When it is in a positive phase it is called, El Nino. ENSO takes into account the water temperatures in the South Pacific. When colder than average it is negative or La Nina. When warmer than average it is positive or El Nino.
The phases of ENSO have a big impact on the southern branch of our Jetstream. When it is an El Nino year, the southern branch is typically stronger. It can bring big rains to California and keep southern portions of the United States wet. In a La Nina year the southern branch of the Jetstream is typically weaker, with dry conditions in California and the Southern United States. A strong southern branch of our Jetstream, El Nino can also cause a ‘zonal flow’ across the United States. That is when the weather patterns are moving west to east, rather than north to south. The air masses, that ultimately determines our temperatures here, originate off the Pacific Ocean, not the Arctic. The zonal flow will keep the Arctic air from penetrating into Upstate New York. We end up with warmer than average temperatures. It is essentially the opposite when in a La Nina. Although ENSO impacts our weather, the NAO has a bigger impact and that is what we focus most on for our prediction.
After analyzing what the current state of these indices are in November and how we expect them to change, we determine what the analog year is. This winter is shaping up much like the winter of 1994 to 1995. We think December will be a little warmer than average with a positive NAO being the main reason why. As the winter progresses into January and February, the NAO is expect to become neutral to negative. That would buckle the northern branch of the Jetstream and allow the Arctic air to flow in. So we expect January to be about average and February to have below average temperatures.
Since we are going to be in a strengthening El Nino pattern as well, watch out when that NAO goes negative. That is when the two main branches of the Jetstream can work in unison and we get the big Nor’Easters working up the coast. Tropical originating lows clashes with Arctic air! Good times! In essence February may bring big snows and bitter cold to Upstate New York. Oh then there is that Lake Effect Snow that inevitably occurs.
No matter what the winter season brings, embrace it, because there is nothing that can be done about it.