ATLANTA -- Every year around this time of I dig into my bag of tricks and put out my "Official Winter Forecast for 20XX" and every year I learn and see something new. Part of long range forecasting is looking at the right signals the atmosphere is giving you, and part of it is just plain luck. It is not an exact science so as you read this and put it into your memory bank, as will I, know that there is a tremendous amount to be learned about long range forecasting. But also know that I am a student of mother nature short term, mid range, and long range and I am always studying to try to figure out why she does what she does.
First of all, let me tell you what the main things are that I look at:
- Persistence and trends
- Current Global Patterns
- El Nino / La Nina
- The North Atlantic Oscillation
- To some extent I also listen to local folklore
Persistence and Trends
This is exactly what it sounds like. Persistence in the world of weather essentially means that the weather you are having right now will most likely persist for a while. In other words when you are deep into a drought, you will most likely stay in a drought. When you are in a heat wave, you will have a better chance of staying in that heat wave than you will have getting out of it. That's just how mama nature works. The main thing that has been persistant this fall is what has been going on in Canada... My goodness it is COLD up there. Take a look at the image below and you will see that this is abnormally cold for this time of year! This has been the trend for the last few months, a much colder fall for a large part of Canada! That region along the Canadian Rockies is the area that mid latitude cyclones (the storms that affect us here in the US) tap into to and bring down the cold air and with it being this cold already, this winter for them could be brutal. This is the air we will be tapping into.
When you get temps down to -30 and -40 this time of the year it almost impossible to warm that part of the world until spring due to the lack of sunlight in that part of the country and the snow pack. Right now most of N America is below normal for this time of year for depth of snow, but also below normal for temps in the US east of the Rockies!
If this trend continues it means that storms coming across the US will have a much richer mass of cold air to tap into. This means also that storms originating in the mid lattitudes will be stronger with more cold air!!
Just a brief history on what El Nino is: El Nino was discovered by fishermen off the coast of Peru a very very long time ago. They noticed that every once in a while the fish that they needed to survive on sometimes died off..... Basically some years the fishing would be so bad that they would actually go hungry until the fish came back.... What was happening was a change in the trade winds in S. America. Normally the winds blow from the land out to sea, this would create what is known as upwelling. The winds coming off the land would push the layer of water at the top out to sea so that the cooler water from below would come up to replace it. This happened most of the time, but every once in a while the winds would change direction and the upwelling would stop. When the upwelling stopped the natural mechanism that kept the water off of S America cool stopped also, and the water warmed. Warm water is not good for nutrients for fish, so they would all die.... Since the fishermen in Peru and Chile noticed this around Christmas every time it happened they named it El Nino for "The Christ Child"..
ENSO, or El Nino Southern Oscillation, this year has been going back and fourth from slightly negative to slightly positive. When it is negative that means the water temperature in region 3.4 is slightly cooler or more towards what would be considered La Nina. When it is warmer in that region it is considered to be more towards El Nino. The water temperature must be more than 0.5 C warmer or colder for three consecutive months for it to be considered an El Nino or a La Nina. If it is neither of these it is considered to be "ENSO Neutral" and that is where we are right now, in an ENSO neutral pattern. La Nina and El Nino patterns actually have statistical correlations, but ENSO neutral is not as highly correlated. From a case study though the NWS was able to find some recognizeable patterns and you can see it below.
ENSO Neutral patterns tend to be very volatile across the eastern US with the southern jet active and rainy, but the northern branch also very active with cold air coming down from Canada. So with the cold air we have already seen this year you can imagine if that air persists and this pattern holds true.
North Atlantic Oscillation
The NAO is an index that is taken from the difference in pressure between the area near Iceland and the Azores Islands in the south Atlantic. When the index is negative, the eastern US typically sees cooler than average weather and there are more Nor'easters. When the index is positive we can see warmer than average temps and less rain. For the last few weeks the NAO has been on the negative side as we have been below normal in N Georgia for temps and few east coast storms have developed. What makes the NAO such a strong patter is this: a large blocking high forms over the North Atlantic in the upper levels of the atmosphere that forces a big east coast trough. This takes the storm track from Canada and pushes it down into the eastern US creating lots of east coast storms and also tapping into that colder canadian air. The NAO was a big negative for both winters in 2010 and 2011 which were both much colder than average for the eastern US and all of N Georgia. The only thing that is making me a little iffy on my forecast is the NAO is forecasted to stay slightly positive for the next 2 weeks. At the same time though, some of the coldest air in a while will be coming down this way this weekend and stay down here for a while! I think that the NAO is still not as well understood as El Nino.
The North Atlantic Oscillation is not as reliable of a forecasting tool as En Nino since it is based on sea level pressure rather than ocean temperatures. Ocean temps change much slower that air temps, just like the air in your house warms faster than water in a pool, so changes in the NAO happen rapidly and without explanation... kind of like every day weather.
The "Official CPC Winter Forecast"
The CPC is forecasting equal chances for a cold / warm / wet / dry winter!! In other words they see all the factors that I have talked about above to very difficult to forecast what they tink will happen over the next 3 months accurately. Like I said at the beginning of the blog, it is pretty tough to nail a forecast for 3 weeks much less 3 months!! I do remember last December that had forecasted the winter to be dry and warm...... same with spring..... and same with summer. In a nut shell, the CPC thinks that we will be about average temperature wise and average rain wise....
My Official 2013 / 2014 Winter Outlook
Based on persistence, and ENSO Neutral pattern, the potential for a negative NAO pattern I am thinking that we will see a colder than average winter!! I think also that with the southern branch of the jet becoming active we will see multiple chances to see snow across N Georgia! Now I can't go so far to say how many events we will have, but I think there will be chances from December through February to see winter events as far south as Atlanta!
Again, there is an incredible amount of uncertainty that goes into this forecast and remember that this is for the entire winter (December, January, and February) so some weeks will be warmer than average and some will be colder than average. But at the end of the winter I think that this will be a pretty accurate forecast. Through the years I have had some good ones, some bad ones, and some horrible ones.... We will see in March where this one stacks up!
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