My Official Winter Forecast | Weather
Winter weather in N Georgia conjures some pretty bad images, the most recent one being January of 2011.. AKA Smowmageddon.. That was a pretty tough winter for most of the eastern US with 2 major snow events for the Atlanta metro area and temps well below the average of 55.2 degrees for the entire winter! Last year was the total opposite with the winter of 2012 being one of the warmest of all time for Atlanta, so what do I think will be happening this year? Let’s first explore the contributing factors that lead into the forecast.
First of all let me preface this entire blog by saying that this is a 3 month forecast, December, January, and February, and there is an incredible amount of uncertainty involved. A lot of this is based on the 20 years of experience I have in forecasting the weather for this part of the country. A lot is a “Gut” feel I have based on all the signs that I look at. But the majority of the winter forecast is based on statistics that come from 2 major indices that I look thoroughly at…..
IF YOU WANT THE SUMMARY JUST SCROLL DOWN TO THE BOTTOM OF THE TEXT FOR THE FINAL OUTCOME
The Contributing Factors: El Nino/La Nina, North Atlantic Oscillation, Persistence
By far the biggest contributor to what happens for our winter comes from that wicked current in the pacific that we weather guys refer to as El Nino. Let me refer to a previous blog to let you know what El Nino is all about:
What is El Nino?
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"..
El Nino Forecast
When El Nino kicks in the pattern for the entire US changes, but statistically it really changes the most for the southern Gulf States where we get into a very wet and warm pattern. What is does is creates an enhanced southern branch of the jet that brings a very active storm track for these states (see image #1). Originally a strong El Nino was looking possible as the region that we look at in the Pacific was showing signs of that happening, but the temps out there have backed off. You can see below (see image #2 and #2.1) that the pacific waters have actually cooled a bit to bring what we call an ENSO neutral region. This means that we will have a weak El Nino pattern this winter in my opinion bringing somewhat of an enhanced storm track in the Gulf Region. We have already seen a modified version of this with several storms coming out of Texas and Louisiana that have headed in our direction back in October and the few Nor’easters in November.
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 (see image #3). When the index is positive we can see warmer than average temps and less rain. For the last few months the NAO has been on the negative side as we have been below normal in N Georgia for temps and several Nor’easters have formed. As a matter of fact the NAO was the major part of my forecast for Sandy and the direction she took. What happens is 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 (you can see this series in images 4,5, & 6) creating lots of east coast storms. 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 (see image #7). This winter all computer models are hinting at a negative NAO, also that is the pattern we have been in for the last few months which is called persistence (see below image #8 for the NAO forecast).
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.
Persistence is just that, persistence. Sometimes we can look at the trends we are seeing now and be able to project into the future given that no major changes happen. For November we have been 1.5 degrees below normal for the high, and 4 degrees below the normal for the low. As long as there are no major changes in the pattern (the ENSO and the NAO) there is a good chance that we will continue to see this trend.
The CPC Forecast
From the images below (see image #9 and #9.1) you can see the “official” climate prediction center forecast has the Northern Plains as having a better chance of seeing a colder winter, and the SW as having a better chance of having a warmer than normal winter. Of you notice how these images are oriented, the cold forecast is on the northern side of the jet that is forecasted to stay in place and the warmer forecasted area is south of that jet. This places N Georgia in the area that has “equal chances of seeing a warmer or colder than normal winter”. As for rain, it actually has N Georgia in an area that has a “better than average chance of having a wetter than average winter”
My Official Winter Forecast
Based on these three things, the ENSO, the NAO, and persistence I am thinking that we will have a modified El Nino / negative NAO pattern. This basically brings a blend of both patterns:
For a majority of the winter we should have a somewhat active southern branch of the jet that will be bringing active weather right across the N Georgia area courtesy of this “weak El Nino” pattern. At the same time we are going to continue to see a negative NAO pattern that will bring cold air down in this direction…. So if you imply from those two things we are going to be cooler than average by a small margin and wetter than average there is a chance that we could see some snow this winter as well (see image #10 and #10.1).
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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