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: https://www.tnvalleyweather.com/post/it-s-official-la-ni%C3%B1a-is-here
We are now going to follow up on this with a progress report for where we are with respect to La Niña and the other various things teleconnections we have been keeping an eye on, and then we are going to take that knowledge... and instead of looking at what it means for the winter... go ahead and look at what it may mean for severe weather for the first few months of 2021 from a large scale perspective. What we have been steadily seeing for the past few months in that regard is a bit unsettling, and after watching things play out, we now have enough confidence to publicly talk about it.
The latest sea surface anomaly chart shows that La Niña has definitely intensified a good deal since Kory's post a few months ago. We are firmly in the grips of a moderate (and not far from strong) La Niña. This is evidenced by the very cool waters over the equatorial Pacific. The latest readings released from the Climate Prediction Center show that the ENSO value has already reached -1.2 degrees Celsius, showing that this is firmly within the moderate La Niña range. We are nearing the peak intensity of this La Niña, and we will probably come just short of having an "official" peak in the strong category, but some additional cooling over the next few weeks IS possible, and in all honesty, this La Niña is "acting" like a strong La Niña event regardless. One thing of particular note that we will go more in-depth about in this article is that the coldest ENSO waters are shifting west, with areas back closer to South America warming. This is an important and unsettling sign in terms of the large scale pattern favorable for severe weather down the road, and we will talk more about that.
PDO (Pacific Decadal Oscillation)
Back in September, Kory talked about the PDO and how it was starting to organize in a more negative phase. After a blip in October, that negative PDO has really intensified, and the latest numbers from the Climate Prediction Center show a major negative PDO phase in progress (November reading: -1.77). The hemispheric weather pattern going forward with an intense Pacific jet and troughing over the Gulf of Alaska and northerly low-level flow down the West Coast of the USA toward the Baja California area of Mexico will only help to further intensify the already major negative PDO. At some point, these things often establish feedback on each other and become self-sustaining or perpetuating, and that is exactly what is happening with the negative PDO now and some of the other teleconnections. There is no reason to believe the negative PDO will do anything but hold its own or further intensify over the next few months. This will continue to help the large scale setup be favorable for an intense Pacific jet to be available to move into the United States, with the subtropical jet taking a big back seat once other forcing mechanisms get into favorable phases for it to relax. As we head into late winter or especially spring, this will help the large scale pattern over the United States be mainly driven by the polar branch of the jet stream.
TNI (Trans-Nino Index)
Earlier in the discussion, we talked about the colder ENSO waters shifting westward and the eastern area waters starting to warm. A positive phase of the TNI, or Trans-Nino Index, is setting up. A positive TNI phase often happens in the spring as a La Niña is in its weakening mode. This helps lead to a pattern over the North American and adjacent area with a low amplitude westerly upper-level jet over the US with mean troughing in the west/central portions of the country. This, in conjunction with mean subtropical ridging, positioned near and just east of the Bahamas, sets up large scale low-level southerly flow from the Gulf and the western Caribbean up into the Plains, Dixie Alley, and the Ohio Valley areas. This alone then sets up a large scale environment over the USA that is favorable for large scale severe weather events, with a pattern that ejects big storm systems out into the middle of the nation, a pattern that advects rich low-level moisture northward out of the Caribbean into the warm sectors of these storm systems, and a general large scale pattern characterized by vertical directional shear, with mean southerly flow in the low-levels and mean westerly flow in the mid/upper levels.
Research ( https://www.spc.noaa.gov/misc/CSWW-2015/afternoon1/002_csw_workshop_mar_11_2015_version2.0.pdf ) has shown that a positive phase of the TNI can be very dangerous for severe weather in the central and eastern United States, including the Tennessee Valley and the overall Dixie Alley area. This research has shown that the number of (E)F3-(E)F5 tornadoes nearly doubled in +TNI years when looking back on them in history. From 1951-2010 (before the 2011 Super Outbreak, so it wasn't included in this statistic, but it fits also), seven out of ten extreme tornado outbreaks in the United States happened with a positive TNI phase. Several historic tornado years, such as 1917, 1932, 1936, 1971, 1974, 1999, 2008, and 2011 were all associated with positive TNI phases.
The positive TNI phase is already developing now and should only further develop and intensify over the next few months as the La Niña begins to weaken and the eastern ENSO regions continue to warm and the remaining colder water shifts westward.
In Kory's previous article, he talked about how a stereotypical La Niña is often associated with positive phases of the NAO and AO (he also explains what those are in his article if you need to go back for that). Sometimes, however, some of those teleconnections like to conflict with what is typical of La Niña climatology, and we are seeing that now. Despite having a significant La Niña and negative PDO in place, and many aspects of the global atmospheric circulation responding to that, the NAO and AO are running negative. This, along with associated tropical forcing features, is one of the main reasons we have been in a colder weather pattern the past several weeks. While that may fluctuate some at times, the more we look at extended large scale data for at least the next few weeks, the more it looks like this general idea may not change a whole lot. That is keeping the large scale pattern over North America suppressed, with the storm track/jet stream shifted south and of a lower amplitude, and not allowing the subtropical ridging off the East Coast to really flex its muscle like it often can in a La Niña winter. That's a troubling sign to us. We have seen this in previous La Niña winters (one example being 2010-2011) where that pattern suppression holds for at least part of the winter, and then the subtropical ridge is delayed in taking over as we head into spring, allowing the polar jet to be pointed at Dixie Alley during the heart of March and April, the heart of our spring tornado season. As we look ahead and see just how long this pattern suppression may continue, we are starting to get a sneaking suspicion that this may be the case again this time. We have seen this same pattern suppression the past few years, regardless of ENSO phase. It comes as no real shock that it is happening again, but it is concerning.
Believe it or not, another thing that plays into this and has us concerned is the big drought in the western half to two-thirds of the nation. This is the most recently updated Drought Monitor for all of North America, last updated on Halloween, but the drought has expanded and intensified even more since then. This drought is concerning two us for two reasons:
The location of the drought is right over the source region for the classic elevated mixed layer (EML) during severe weather events in the Plains and eastward. The EML is a layer of warm, dry air in the mid-levels of the atmosphere that advects overtop low-level moist air masses within warm sectors of developing storm systems, leading to the large scale instability needed for severe weather events. Having significant drought in these areas sets up an environment favorable for significant EML plumes to develop and be carried eastward as storm systems eject out into the center of the nation.
The eastward expansion of significant drought into the Plains can often help the dryline shift east into the Mississippi Valley as storm systems eject out. This often helps focus severe weather events east of the Plains, into Dixie Alley and the general MS/TN/OH Valley areas, but it also offers a boundary to initiate storms in events that usually automatically favors a cellular storm mode versus linear because of more subtle forcing along a dryline compared to a cold front. The dryline being swept east of the Plains into the Mississippi Valley doesn't always result in big Dixie Alley outbreaks, but it is often one of the calling cards present when a big outbreak here does occur.
What does all this mean?
The main message to take away from this is that the coming late winter and spring may be active for severe weather events in our area, and you need to take time now to prepare, while the weather is relatively quiet. Stop thinking that it can't happen to you. You have time now to develop a severe weather safety plan for your family that includes what to do when you are at home, work, school, ball games, church services, out shopping, etc. Develop those plans now while the weather is quiet. Make sure you have a readiness kit in your shelter location. Make sure everyone in the family knows the safety plan and the shelter location. If you live in a mobile home or any kind of manufactured housing, NOW is the time to develop a plan for where you will go to seek shelter. You cannot stay in a mobile home during a tornado. They offer wonderful affordable housing, but a mobile home is a death trap in even a lower-end tornado. Get a NOAA Weather Radio so that you have an alarm in your home that will sound that can wake you up in the middle of the night if a tornado warning is issued. DO NOT RELY ON A SIREN to wake you in the middle of the night during a raging storm. This line of thinking gets people KILLED.
The general large scale pattern already in place is one that is unsettling because it assembles all the large scale ingredients necessary for major severe weather to happen east of the Plains of the USA, and it is the same large scale pattern that has been in place when some of our biggest events in this area's history have happened in the past. HOWEVER, that does not instantly guarantee that another major event WILL happen. This is just the global weather pattern setting the table. It is then up to an individual storm system to take full advantage of that, and we have NO WAY of knowing right now if that will happen. This is the set of large scale conditions that were in place for years like 1932, 1974, 2008, 2011, etc., but that does not instantly mean that another event of that magnitude WILL happen. It just means that maybe the door isn't quite as locked shut against it as it is in other years. Ultimately, all it takes is one small tornado coming down your street to be YOUR next 2011, 1974, etc. It doesn't take something historic on a large scale to change YOUR life forever. That is why you MUST be prepared.
Unfortunately, tornadoes are a way of life in our area of the country. We must be alert and prepared, and the Tennessee Valley Weather Team... with our Live StormTrack Doppler Radar, VIPIR storm tracking system, the Tennessee Valley StormTracker mobile weather vehicle, our SkyCam Network... will be right here to walk you through whatever nature brings our way over the next few months.