Why so much snow? Can we blame the NAO?February 20th, 2013 at 5:26 pm by petemangione under Tony's Pinpoint Weather Blog
With the Blizzard of 2013 and several other snowstorms already in the books, it has been quite a winter so far! Here in the weather department, we are constantly being asked, “Why are we getting all of this snow? It is ever going to stop?” Those are very complex questions with many contributing factors such as air pressure patterns across the globe, sea surface temperatures, and the position of the jet stream.
One term you may have heard thrown around is the NAO, or the North Atlantic Oscillation. The NAO refers to the difference between two pressure centers….the “Azores High Pressure” and the “Icelandic Low Pressure”. The Azores High (see the H in the figures below) is a high pressure center located offshore from Portugal, and the Icelandic Low (see the L in the figures below) is a low pressure center located in the North Atlantic near Iceland.
The NAO is in a positive phase when the difference between the high and low is strong. This large difference in pressure is enough to pull the jet stream between the two. This has the effect of flattening and shifting the jet stream to the north in the eastern United States. If you think about the jet stream like a storm track, a storm track to our north usually means less storms and milder temperatures. Therefore, a “NAO Positive Mode” often leads to warmer, and less stormy conditions in the eastern United States (see the globe on the right of the figure below).
The NAO is in a negative phase when the difference between the high and the low is weak. This causes the average position of the jet stream to “dip” further to the south in the United States. This pulls in colder air and usually brings in a relatively higher amount of snow storms (see the globe on the left of the figure below).
Can the NAO be used to predict snow in southern New England?
Let’s look back at the NAO status over the years and compare it to some of our big snow storms here in New England. In the figure below, the blue represents periods that, on average, were in a negative NAO. The red represents a positive NOA phase.
Notice the large blue spike in 1996 indicating a strong negative phase of the NAO. Of course most of us remember the blizzard of ’96, and that the winter of 95-96’ was jammed packed with snow storms! Notice another blue spike for the start of 1978…..no one can forget the blizzard of ’78.
But there are many cases where a negative NAO did not lead to a big snow storm or a snowy winter. Notice the prolonged blue spike at the end of 2009 into 2010. While we did have a lot of snow in December 2009, the Providence area didn’t get that much from January 2010 into the rest of the winter. However, the mid-Atlantic (including the Washington DC area) was slammed with several monster snow storms; a lot of these storms missed us just to the south. Basically, an active storm track produced by a negative NAO doesn’t necessarily aim these storms right at New England. Because of the way that we stick out in the water, it is always difficult to forecast which storms will get us and which ones will be a near miss.
So what’s the outlook for the last month of winter?
Let’s start with the NAO forecast first. The chart below has a lot on it, so just focus on the first graph with the spaghetti plot of red lines coming out of it. These red lines represent the NAO forecast into the start of March. Because these red lines are BELOW the solid dashed lines, it indicates that the NAO will stay negative for a few more weeks.
This suggests, on average, we can expect colder temperatures and an active storm track. However, an active storm track can produce lots of snow but can also produce storms that just miss us. This weekend is a perfect example! We know a storm is coming for sure, but where exactly will it go? Stay tuned, you will be hearing this a lot over the next few weeks. –Pete Mangione