Where's Winter??? An Explanation...
December 18, 2011 8:24 PM
It’s no secret that this year has been much different then the last several with the driest fall on record and plenty of mild sunny days. In fact, Sunday we smashed more records with temperatures soaring into the 60’s in some areas, which is some 30 degrees above average. Now, even though our really warm weather will be ending Monday as highs drop back to the 30’s, it still looks above average through much of this week. I could say that La Nina has played a factor in all of this weather, but it really hasn’t. As you probably remember, last year was a La Nina winter and it was pretty brutal… lots of cold, too much snow, and very little sunshine in between. But La Nina isn’t the only variable that plays into our weather. Now, granted La Nina is probably one of the largest and arguably the most well known, but there are dozens of other factors that controls how much snow we receive and how low our thermometer will drop. And so far this year, there are two other variables that have had a much greater impact on our weather then La Nina.
One of these variables is far easier to understand then the other, so let me start you off easy. The first variable just has to do with the specific pattern that we are currently in. Every weather pattern goes through cycles, which is why we go through dry periods, and wet periods, and hot periods, and cold periods… a lot of it has to do with patterns and it can be very hard for Mother Nature to change these patterns just like it would be very hard for you to stop doing something after it becomes habit. Well, the current pattern we are in is a dry and mild one and let me show you why. So over the last several months I have talked a lot about cut off lows. These low-pressure systems get dislodged from the main steering current in the atmosphere (called the jet stream) and typically end up sitting in one spot for a while, cause there is nothing to move it. But when it does finally begin to move, these often move erratically which makes them very hard to predict. Well these lows have been sitting in the Desert Southwest and then rolling due east as they get back into the steering current. Lets take the current situation for example. Below are 3 images from forecasting models and are composed of what meteorologists call vorticity, which basically means “spin” in the atmosphere. This is able to show us exactly where spinning areas of low pressure are located.
Notice in picture 1 (which is what the atmosphere looked like Sunday) there is a cutoff low in the southwest that I have circled and labeled it as 1. This low will be responsible for the huge storm in the Southern Plains in the next couple of days. The jet stream is the blue line that I have drawn in at the top portion of the picture.
Now over the next couple of days, this cutoff low will move east, but as you see in picture 2, which is Monday night, another one forms and sits outside of the jet stream.
This low moves slowly east, but is replaced by yet another low in the last picture, which is Friday morning. So you can see that this pattern is pretty repetitive and is partially responsible for our area being so dry over the last few months.
Now the second variable that has kept our area mild and dry the last couple of months is a bit more complicated. It is called the North Atlantic Oscillation. Let me start you off with the “dictionary definition”: a 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. Basically, what that told you is that the North Atlantic Oscillation or NAO as its called, is a weather phenomena which controls where cold Arctic air is located and where storms will track across the Atlantic Ocean and ultimately interact with this Arctic air. So now, if you aren’t confused already, you are wondering…What does this have to do with our weather??
The easiest way to understand the NAO is to know that it has 2 phases. It can either be at positive phase or negative phase. We determine which phase it is at by sea level pressure over the North Atlantic. That pressure is put into a chart and it spits out a graph like the one you see below. This is the NAO index graph since November 1st. Notice the dates are on the bottom and it has both positive and negative numbers on the left with a black line right in the middle indicating the zero mark of the graph. Now, if that blue line is above the black line, the phase is positive. If the blue line is below the black line, the phase is negative. Each phase correlates to something different for our weather right here in South Dakota. Now typically, the further away from neutral that blue line goes, the greater the effects of both the positive and negative phases.
If the NAO is positive then the cold arctic air gets trapped at the North Pole because of a general southerly flow across much of the Northern Hemisphere. This correlates to generally mild temperatures over the U.S. and can often lead to very tranquil and very mild conditions much like we have been experiencing over the past several months. This is likely the main reason for all of our nice dry weather since August.
If the NAO is negative then the cold arctic air gets dislodged from the North Pole by what is called a “Greenland High”. This high pressure basically sets up a blockade preventing eastward movement to storms, which means that storms will have to go around it to get by. This leads to dramatic temperature swings sending very cold air south into the United States and very warm air north to the Arctic. This type of pattern is likely what aided in our extremely snowy and cold winter last year.
So when we talk about La Nina and El Nino from now on, you know that those are not the only two things that control the weather in the U.S., it is just one of the more obvious and typically the most dominant, however, so far not this year. So your question is now going to be, when will we finally get some snow?? Well, as I have just shown you, that is a very complicated question. This year, we will likely finally get some snow when our cutoff low pattern ends, and our NAO is in the negative phase. When will that happen?? Who knows!!! But when it does, prepare for the snow.