Negative AO Winters Almost Always End Up Snowy in Southeast

[FoothillsNC]

Hard to believe, because we've been in a negative AO pattern since October and are still above normal. We are coming off the warmest year ever (2012) so it's understandable, but in the end, if you bet the streak and believe in overall odds, it's hard to deny this study:

 

 

The Arctic Oscillation (AO) occurs in a negative state every few years on average. This is when higher than normal pressures build over the North Pole, which helps to relax the Polar Vortex, or even split it, along with the Westerlies, and that results in cold air in the Arctic being allowed to move southward into the continents of North America and over Asia.  When the AO is positive, the opposite usually occurs and the continents are warmer than average.

 

 

The purpose of this study is to investigate negative AO patterns and how those patterns correlate to above average snowfall in the Southeast states.  For this study, I’m including observations from the core Southeast states of Tennessee, Mississippi, Alabama, Georgia, North Carolina, South Carolina and Virginia.  The investigation indicates that when there has been a 3 month average of negative AO in the Winter months between November and March, then the odds of above normal snowfall is dramatically increased, regardless of all other ENSO states or other prevailing indices.  Although, all indices work in tandem with negative AO patterns, such as Negative NAO (North Atlantic Oscillation), this becomes the “chicken-egg” question, since the atmosphere is completely connected and works as a system.  But in the end, it’s irrelevant because just looking at all the 3 month average negative AO patterns alone in Winter strongly indicates above average snow regardless of all other states or indices.  Whether there is negative PDO, or positive, whether the ENSO is Nina, Nino, or neutral, I didn’t look at the exact states of all these indices or the other indices like QBO, EPO, WPO, etc because the negative state of the AO clearly shows a Winter Storm pattern almost always results either before, during or just after the negative 3 month mean.

This is the long term AO record going back to 1950. It was difficult to find snow records in the 1950′s, so I began with the year 1960 and went into the current time frame.  As you look at the years, the blue spikes show the years when the AO mean was negative for 3 months on average, and it’s those times during the Winters (and sometimes late Fall and early Spring) that the Southeast stands the best chances for major snow or ice.  In looking at the data overall for the Southeast, almost all of the snowiest Winters occurred either during, or just before or just after a 3 month period of negative AO occurred.  There are a few exceptions, but the data clearly indicates that a negative AO period almost always results in Winter Weather.  On the converse, a few times Winter Storms occured during Positive AO periods, but was few and far between and usually more localized. (January 2000) is an example.

 

The current state of the AO has gone strongly negative and since October, the index is probably going to be negative every month. That means that October, November, December, and now January 2013, it appears that we’re going to have a 3 month or more period of negative AO numbers and in this case, very negative AO.  It appears the more negative the AO, the better chance of above average snow and ice, but overall, any 3 month mean is likely to generate above normal, perhaps not as wide an area though as the stronger negative AO seasons.

 

The current outlooks for the AO are indicating a strongly negative AO in January, and possibly almost record strong, rivaling the big negative AO years of 1960, 1969, and 2011 when the AO went into extreme negative territory.

 

Coincidentally, some of the stronger negative AO years has been a strong indication for the extreme record snow amounts in some regions of the Southeast:

 

Below are just a few of the seasons in the Southeast when above normal snows occurred, going back to 1960.  As you can see by comparing the snowfall individual maps they line up almost perfectly with negative AO patterns.  There are a few seasons when above normal snowfall occurred WITHOUT a negative AO, but when a negative AO occurred in the 3 month means in Winter, it almost always was accompanied at some point with above average snowfall for the season.
(Images courtesy of NOAA)  This is NOT a comprehensive snowfall map for individual seasons, and doesn’t and makes no regard to positive or negative AO patterns. It just strongly indicates that when a 3 month mean of negative AO occurs, then above average snow is likely in that season.

 

 

 

I looked at several sites scattered in the Southeast core states to observe the top 10 snowiest Winters.  Almost all sites strongly correlate a negative AO pattern with above average snowfall. That’s not to say that snow can’t occur during postive AO patterns, but the general idea is that when a negative AO pattern occurs for 3 consecutive months in the Winter, then odds strongly favor above average snowfall.  Below are the top ten snowy Winters for Tupelo, Birmingham, Memphis, Nashville, Raleigh, Charlotte, Greenville-Spartanburg, Blacksburg and Richmond.  All of the values are official NWS observations.

 

Birmingham

 

Memphis:

 

Tupelo:

 

Nashville:

 

Raleigh:

 

Charlotte:

 

Greenville-Spartanburg:

 

Blacksburg:

 

Richmond:

 

In conclusion, we are coming off of the Warmest Year in history here in most of the United States, and there’s never any guarantees in what the future holds as far as forecasting weather. But statistically, odds heavily favor the Winter of 2012-13 as going down for Above Average Snowfall in many regions of the Southeast core states because of the nature of the AO and the precedence of it’s history and Winter Weather in this region. Who knows, only time will tell if this season is the exception the the rule.   Only time will tell if the past conditions can carry forward in this season.  I'd be really suprised if a stretch from northern Mississippi across the Tennessee Valley, Apps to NC, SC and the Mid Atlantic ended up below normal in snowfall again this Winter since the Negative AO history indicates that Winter Storms are likely in these periods where AO is negative for 3 consecutive months in the Winter.

 

 

 

 

[metalicwx366]

Very interesting and great post!

[BIG FROSTY]

Excellent study Robert! Thanks for sharing, very encouraging to a snowman that's about to have a meltdown!!!

[calculus1]

Great stuff, chock-full of info, as always, sir!

 

The trend of colder than normal northern European and Asian temperatures during a -AO has definitely come to fruition so far this winter season.  Now, if we can just get the colder air to also bleed down on our side of the globe...

[Brick Tamland]

You would think the rubberband has to snap back sometime. But people said that last winter, too. I know this winter is different, but we keep waiting and waiting for the rubberband to snap back, and it might not do it until winter is over and then it'll just be too late for snow around here.

[Poimen]

Thanks, Robert. Excellent work, as always!

 

One interesting thing I noticed is that for RDU (I didn't look closely at other cities) 6 of the 10 snowiest winters occurred in back-to-back seasons. i.e 58-59 with 59-60; 61-62 with 62-63; and 78-79 with 79-80. I don't know what to take away from that in regards to this winter, but I found it interesting nonetheless.

[GaWx]

Robert,

 Excellent post as usual. I love to read studies like this.

[tonysc]

Yes many thanks Robert, it give us snow weenies something to hang our hats on at least. And I do remember many of those big snows in those years, especially March 25th 1971, the 5th day of Spring I might add and one of the biggest snowstorms I can remember in the Upstate of South Carolina, snowflakes as big as golf balls and at times tennis balls all day long, and if memory serves me correctly, a complete surprise to the mets. February 1973, one of the biggest snowstorms to ever hit South Carolina and Georgia, and a complete surprise again.  February 1979, short sleeve shirt weather on Friday, began snowing late Saturday afternoon and it was 15 degrees all day long on Sunday with heavy snow, now you don't see that very much in the South, it was forecast though and we couldn't believe it standing outside just after dark in short sleeves on Friday evening and the NWS having put out a Winter Storm Watch for Sat. through Sun. and boy were they right that time. January 7th 1988 and it was forecast a good four days in advance, upper teens at mid afternoon on that Thursday and heavy snow. Oh well those are just a few of them that stand out in my mind and please excuse me for going on about it but Roberts post, and thanks again for all the effort Robert, just brought up some good ole memories of some outstanding snowstorms here in the Upstate of SC. It is nice to think of those again and hope for more like them. Here's hoping this can be one of those winters before it's over with.   Tony