This Weekend & My Official Fall 2013 Forecast | Weather
After a very cool and wet winter / spring / summer of 2012 and 2013, the fall and winter forecast for 2013 - 2014 does not seem as clear cut as it appeared at this time last year. I am actually somewhat leaning in the direction the CPC is for rain, however I disagree with the CPC again when it comes to what I think the temps will be doing for fall. Before we get into what I am thinking I would like to explain what it is that I look at to make my forecast. First I need to of course talk about the weekend!
This weekend we will have our first rain in 17 days!! Remember when we couldn't buy a dry day? Now we have gone to the flip side of the coin and we can'y buy a drop of rain! That is all going to change this weekend. Look for a decent cold front to start moving into the N Georgia area tomorrow morning. This will bring rain across the area that will give most of us 1-2 inches through tomorrow night. The timing on this cold front will bring rain into the NW section of the state starting tomorrow at around 4am, then into metro Atlanta by around 6-8a. Rain wil come in pockets of light to moderate rain, sometimes there will be some heavier pockets. We are not expecting any severe weather at all tomorrow. Rain should come to an end in the early afternoon in Atlanta, earlier to the north, later to the south. Cooler air will then move in and bring our temps into the lower 70's by tomorrow evening, lower 60's for Sunday morning. Sunday afternoon will be gorgeous with afternoon highs in the upper 70's and lower humidity!
If you are heading to East Lake tomorrow for the tournament, bring a poncho because it will be raining on and off for the majority of the day through the evening hours. Temps tomorrow afternoon will be in the mid to upper 70's.
Tomorrow the rain will be coming to an end in the metro area by around 4pm so the evening concerts should be nice and dry, at least the air will be. Temps tomorrow night will be in the mid 60's with light winds from the north at around 10mph.
El Nino and the North Atlantic Oscillation are the two major indicies that I look at to determine what my forecast will be. Here is an explainer on what both are:
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 see, 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 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".
When the waters warm off the coast of S America it creates some serious problems for N America as we are normally sent into a pretty wild weather pattern for parts of the US. From my experience in what I have seen is that we sometimes in the central and eastern US we can get into a cooler and wetter than normal weather pattern during the summer.
Right now the Pacific is in an "Enso Nuetral" pattern which means that it is neither El Nino, nor La Nina. Region 3.4 has actually warmed up a bit and because of that some of the global climate models bought on this by bringing the Pacific into a weak El Nino pattern. However, if you look below you will see that the actual temps in that region are cooler than forecasted and the warming has actually stopped. It is for this reason that I think the fall will be based on a "Neutral El Nino". The thing about a neutral El Nino pattern is that there are such differences between years in terms of weather that it is impossible to make a forecast based on statistics. So, to make an accurate forecast for us is to base it on what is considered a normal fall pattern. That means big shots of cold air coming down the plains into the Southeast giving us an average fall in terms of temps if I were to base this strictly on the El Nino factor. For rain it is a different outcome. In quite a few neutral years that southern jet is very active and can bring us quite a bit of rain..... this again is normal for the Southeast.. Atlanta typically gets 4 inches of rain for all the months in the forecast, 4 for October, 4 for November, and 4 for January.
North Atlantic Oscillation (NAO)
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. Ever since December of last year we have been hanging in the negative side of the NAO. Look at the graph down below and you will see that we started raining in December of last year when the NAO went into the negative. It has recently stopped raining and has warmed up. This is also matched up with the fact that the NAO has gone back to the positive side.
All global models are trying to bring the NAO back into the negative side which I believe since we are going to stay in an ENSO nuetral period for the next 3 months. If this is case then we will be seeing cool shots coming down the plains into the Southeast. Again, this supports the ENSO based forecast on a typical fall for temps.
Not all El Ninos or La Ninas are the same, but they can give us a good idea on what is going to be happening in the SE. When we are in an El Nino pattern, statistically we have more rain and cooler temps due to the increased cloud cover. During La Nina years we don't see as much rain and we see warmer temps. This also leads to an increase in the chance of severe weather across the SE. These are all statistics, you can see below with some of the maps that there is a direct correlation between El Nino and rainy SE years.
Persistence is when we base climate forecasts on what has been happening, especially in extreme conditions. When we are in a big drought the chances are that we will stay in that drought barring some extreme measures... ie tropical system, major stalled out front.... Also at the same time if we are in a very wet pattern, we will typically stay in a wet pattern. The thing about this fall is that we have been in an extreme wet pattern for 9 months, but we have come out of that in the last 2 weeks with no rain for September! In other words persistence is somewhat difficult in this case.
The CPC Forecast
if you look below you will see the CPC forecast that calls for "Equal chances" of us having a warm or cold fall. LOL!! What kind of forecast is that?? It is also calling for us to have "Equal chances" of having a wet or a dry fall. What the heck??? What this basically means is that they won't commit to a forecast either way. I will admit, it is a tricky forecast based on the neutral El Nino and the ever oscillating NAO. However, I do have one last thing in my bag of tricks.... My gut and experiencing the weather in this part of the country for the last 20 years. No amount of statistics can replace that.
My Official Fall Forecast
The last thing I haven't talked about is my gut and what I observe. So far this fall we have already had 4 wedges. That is the pattern that sets up with cool cloudy air coming down the Appalachians. I think that will be a very persistent setup this fall, and it could happen quite a few times. So, based on all the above I have come to the conclusion that the wedge might be the biggest factor in our weather this fall. It makes sense with an active southern branch of the jet and cold air coming down from up north. This is the classic "Wedge" pattern always leads to 2 things: Cooler than average temps and less than average rainfall. The reason is that this wedge keeps cool air in all of N Georgia and that usually shuts down most of the rain by keeping it to the south. I can also say that I think it will be cloudy a lot over the next few months with all the moisture coming in from the Gulf and cooler air coming in from the north... This fall has the potential to be one of the more dreary ones in recent memory... When it comes to snow, I do think that this could be a fall that could have some snow in mid December!!
Remember that this is a long range forecast and the accuracy on these can be tricky. Over the years I have had some good ones and some bad ones. Luckily my winter and summer forecast for this year and last year were pretty good so lets all hope this one is too!