Back in mid-September, Kory Pilet of our forecast team did an in-depth blog article here talking about the developing La Niña and how we expected it to intensify, and just what that would mean. For reference, you can see that here...
I will just post the blog article I wrote for our website so that I don't have to type all that out again, and so that I don't have to worry about hosting multiple pictures on here. I will then add to it below...
Back toward the latter part of summer, I did some local area digging in our region here for past seasons where a significantly -PDO, a moderate or stronger La Niña, and a positive TNI phase were all in place simultaneously. I used a PDO threshold of -1.5 (November of this year is already down to -1.77 as we talked about in the 2020 severe thread), an ENSO peak threshold of -1.0 (we've already hit -1.2 as a trimonthly peak, and weekly readings have been lower) and then a positive TNI number (ENSO 4 is already colder than ENSO 1+2, making the TNI now positive, and that will only increase in magnitude). I also only looked for first-year cold ENSO events because, as we are seeing now, these are less prone to the subtropical ridge dominating the weather all winter. That is an important distinction that has to be taken into consideration. These are the seasons, since 1950, where all those things were in place simultaneously:
2010-2011, 2007-2008, 1998-1999, 1973-1974, 1970-1971, 1955-1956
Every. Single. One. of those... not one single failure... had at least one F4+ tornado specifically in the area of AL/MS/TN between I-20/U.S. 80 and I-40 (JAN-MGM to BNA). THERE WAS NOT ONE SINGLE FAILURE. And even in cool seasons where the numbers were close but didn't quite meet those specific thresholds, there were STILL violent tornadoes in the spring in that specific area of AL/MS/TN. It has been almost ten years since the northern half of Alabama has had an officially violent rated tornado, and it has been since 2008 since the southern TN counties of my viewing area have had one. With the exception of far west Alabama being clipped on 4/28/2014, it has been that long since the same area has had an SPC High Risk. With respect to both things there, we are in rather uncharted territory.
The current large scale pattern in place that continues to further assemble itself is what is in place when large scale bad things happen in Dixie Alley. It doesn't always mean they WILL happen every time a pattern similar to this is in place, but this seems to be the specific set of large scale conditions that are in place when major, bad events here do happen. This is the first time since 2010-2011 we have had the combination of a significant first-year Niña in place, a significant -PDO, a +TNI, and cool-season pattern suppression that is keeping the subtropical ridging from taking over early. By no remote stretch of the imagination am I calling for a repeat of 2011, 2008, 1974, 1932, or any other such event... but maybe I am saying that this might mean that the door isn't quite as locked shut for something like that as it would be in any other year with different large scale conditions in place.