Will it snow this winter?
Now that we are past the heat of summer and temperatures are gradually cooling off, people start thinking about what the upcoming winter may hold. The question that is on the tip of everyone's tongue is 'Will we see snow this winter?'. Believe it or not, the answer to that annual query USUALLY lies with the temperatures of the oceans that surround North America.
We have several ocean temperature indices. The big 3 consist of the AMO (Atlantic Multidecadal Oscillation), the PDO (Pacific Decadal Oscillation) and El Nino. Most folks have heard of the El Nino cycle. It can be an indicator of a winter's average temperature and precipitation. However, after looking back at over 60 years of data from the El Nino, AMO, and PDO cycles, I've found that the best 'predictor' of possible winter snowfall is NOT the El Nino cycle. Instead it's a good idea to check out the AMO. While it may not guarantee that you will see snow, it can be a good indicator as to when you will NOT see snow.
Here's how I came to this conclusion. I found monthly snowfall data for Shreveport that dates back to the 1930s. So, I wrote down every month during which Shreveport received at least an inch of snow. (It's not real snow to me if the ground doesn't turn white.) From 1948 to the present during the winter months of December, January and February we have received 1" or more 26 months out of a possible 189 months. Unfortunately, I could only find monthly AMO data that goes back to the 1940s so that's why I started at 1948. There wasn't much of a correlation between snow and the PDO or El Nino cycles. I really couldn't find much of a correlation when you combine the three cycles. I did find that when you look at the 26 'snow' months, the AMO was negative 16 times. (62%) I admit that 62% is not as too impressive. What that means is that having a negative AMO doesn't guarantee that we will see snow. It just indicates that the chance is higher than normal.
Then I thought to myself, 'Is it possible to predict that it WON'T snow?'. Perhaps zero on the AMO index is not the magical line. If it's not then what is? The answer could be February of 1960 when the AMO sat at 0.231 yet we still received 2" of snow. That 0.231 is the highest that the AMO has been during a month that Shreveport has received at least 1" of snow. So let's pretend that .231 is that magical line. Consequently, we can say that since 1948 we have not received 1" or more of snow in Shreveport during any month in which the AMO was higher than 0.231. That leads me to this year. Will we see 1" or more of snow this winter? Based on the above it appears highly unlikely. The latest AMO index that came out in September had the AMO at a whopping 0.487. The trend is also going up as the AMO has increased for the past 8 months straight. Can I absolutely guarantee that it won't snow? No. Here's why. The AMO can change somewhat rapidly. Take February of 2011. You probably remember that it was a rather 'white' month. The AMO that month was .141. While that is below the .231 threshold mentioned above, the AMO from the September of 2010 was similar to our current AMO at 0.486. The difference in this year's AMO and the AMO from 2010 leading up to the snowy Feb 2011 was that the AMO trend in 2010 was already going down.
So the moral of the snow story for the upcoming winter is this. Based on the above information, until the trend in the AMO at least reverses itself, it is looking very unlikely that we will NOT see snow this winter. How unlikely is very unlikely? Based on the image above we have about a 14% chance of seeing 1" of snow in a TYPICAL winter month. Given the current AMO value and the fact that it is increasing means that our chance may be less than 5%.
The AMO index for October will be out soon. Check back here as I will post an update.
Update November 7: October 2012 AMO is out and has fallen to from 0.487 in Sept. to 0.376. That is amazingly similar to the AMO of October of 2010 which fell from 0.486 to 0.361. (Reminder from above that Feb of 2011 was a snowy month by ArkLaTex standards) That means that there is still some hope for you who want some snow. Interesting observation that the average temperature in Shreveport was below normal for the month of October. Could be a decent indicator of the correlation between the AMO and our temps. Check back here in early December to see if the descent in the AMO continues.
Here is the latest outlook for the winter (December-February) from the Climate Prediction Center. It shows that we should expect near or perhaps slightly above normal precipitation for the winter (top image) and near or slightly above normal temperatures.
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