My goal is to give you a general idea of what I’m seeing in our weather future this winter. But before I get into the nuts and bolts of the forecast, let me talk about a few of the factors that will have an impact on our situation this winter.
La Nina
The most obvious one that everyone seems to be talking about is La Nina. La Nina is basically the cooling of surface temperatures in the Central and Eastern Pacific Ocean waters. See the graphic below.
El Nino is the exact opposite of this. El Nino is the warming of these same waters, as seen in this graphic.
Right now, NOAA says that we are in a La Nina pattern, and will continue to be. Listen to what the folks at NOAA are saying, "Currently, La Niña is not as strong as it was in September 2010. Roughly one- half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter. Of these models, the majority predict a weak La Niña (3-month average in the Nino-3.4 region less than -0.9°C). In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950. However, the NCEP Climate Forecast System (CFS.v1) predicts a moderate-strength La Niña this winter (between -1.0°C to -1.4°C) and CFS.v2 predicts a strong La Niña (less than -1.5°C), which rivals last year's peak strength. For CFS forecasts made at this time of year, the average error for December-February is roughly ±0.5°C, so there is uncertainty as to whether this amplitude will be achieved. Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter."
Bear in mind, though, that we had a strong La Nina last year, but as you can tell from the graphics I provided above, our weather didn’t exactly match up with a classic La Nina situation. Why? Last year we had one other key factor play a larger role that dominated our weather – the NAO (see below). So even with La Nina forecasted to continue through our winter, it may not mean all that much if the NAO steals the show again, so to speak.
The North Atlantic Oscillation (NAO)
NOAA defines the NAO as “the climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic.” So in real terms, basically you have a high pressure zone and a low pressure zone that are wedged in between the Jet Stream. How strong or how weak these pressure zones are controls the path of the Jet Stream. See the graphic below.
In the Positive Phase of the NAO, the low pressure area near Greenland is strong and the high pressure area off the coast of Africa is strong. This pushes the Jet Stream farther north and thus keeps the snow storms more to our north because storm systems usually follow the Jet Stream.
In the Negative Phase of the NAO, the low pressure and the high pressure areas are both weak and that allows the Jet Stream to move farther to the south. That takes storm tracks in the winter time right into our area, increasing our potential for snow. And as it would turn out, the NAO is once again forecast to go negative.
In the chart above, the dashed line in the middle is neutral or the zero line. Right now you can see that the NAO is positive barely, but the red lines are the forecast ensembles which are trending the NAO more negative. In fact if the graphic were to go out further you would see that the models are actually taking the NAO more negative than the above graphic shows.
If this holds true, I think we will continue to see “blocking” becoming a major determining factor in the Jet Stream’s path. When the NAO goes negative, this weakened area of generally lower pressure allows a strong high pressure to form just off the coast of Canada. This serves as a blocking mechanism that shoots the Jet Stream south over our area, then back north up the Atlantic Coast. This is sometimes called a Greenland Block, as you can see in the illustration below. This Greenland Block was very prevalent last winter with the NAO being negative and I think it’s going to come into play in a major way once again this winter.
The Bottom Line
So how will all of this affect us? I think a strong Greenland Block will set up multiple times this year and will cause a large swath of cold, arctic air to be forced over the Eastern USA. The Jet Stream will be set up to drop snow makers into our area, but the temperatures will be the biggest concern. I think we’ll see below normal temperatures the first part of the winter for sure. The real question will be where will the freezing line be most of the winter during these precip events? I can’t rule out the possibility that we’ll see significant ice accumulation, but I’m expecting the ice to be more of an issue in Southern Kentucky, Tennessee and even into Virginia/West Virginia. I do think we will see above normal accumulations of snow.
So just for the heck of it, here are three “just for fun” predictions for the winter ahead. We’ll see how I do once the season is over.
Coldest Temperature: -3F
First Snowfall: Week of November 23rd
Total Snow Accumulation: 26”
So, what do you think? Let me know in the comments!
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