Monday, February 11th 2019, 12:14 pm - May of 2018 is on its way to being one of the biggest non-events in recent memory when it comes to tornadoes on the Plains. What's going on with this year's tornadoes? Is it a sign of things to come? There are some surprising answers - and a lot of unanswered questions - when it comes to tornado research.
We've reached the middle of May - the traditional peak of the Plains tornado season - but Tornado Alley remains unusually quiet.
May of 2018 is on its way to being one of the biggest non-events in recent memory when it comes to tornadoes on the Plains. Tornado Alley, stretching between the Dakotas and central Texas, has long been known as the favored spawning site for severe storms over the central United States. In an average May, the country sees 276 tornadoes touch down, with those in Kansas, Oklahoma, and Texas making up 109 of the total. 2017 saw a confirmed 287 tornadoes for the month of May. Through May 11 this year, however, the preliminary total as reported by the Storm Prediction Center is a paltry 52.
What's going on with this year's tornadoes? Is it a sign of things to come? There are some surprising answers - and a lot of unanswered questions - when it comes to tornado research.
2018 - All quiet in Tornado Alley?
This spring has, thus far, been an outlier versus the climatological record. April went down as the coldest in more than two decades over the contiguous 48 states, with Iowa and Wisconsin setting new records for the coldest April in 124 years of records. Image courtesy NOAA.
While May has been more reasonable in terms of its warmth across the country, the overall pattern has still been off-kilter, particularly across the Plains and west of the Rockies, where the jet stream and its accompanying weather patterns have looked more like mid-to-late June than May.
One of the reasons May is prime-tornado-time for the central and southern Plains is that, while the atmosphere is in transition from its winter to summer states, we see remnants of strong synoptic (large-scale) winter-like storms diving down over the Rockies and colliding with building summer-like heat and moisture flowing back into the nation's midsection from the Gulf of Mexico.
In a lot of respects, weather across the middle of the country went from winter to summer with no break; no transition, no clash and, by and large, no tornadoes.
Where are the tornadoes? Is it climate change?
Well, not so fast. While there is some research into what severe weather scientists expect to see, with a changing climate over the United States, there are still a lot more questions than answers. Some trends seem to be emerging, though.
Researchers from Purdue University looked into what impact a warming trend over the U.S. in recent decades might have had on tornadoes, and found we may be seeing a shift away from the traditional Tornado Alley. Looking at tornado activity between 1954 and 2013, the study found a new maximum center for tornadic storms emerging in the Deep South - the rise of the so-called "Dixie Alley."
The study showed that while annual counts and the overall number of days with tornadoes in the central Plains were on a downward trend, the overall number of tornadoes reported annually in the country was about level. How? Because of a sustained increase in tornadoes in the Southeast. Two 'peak' seasons in the South. Image courtesy Bloomberg.
Part of the upper hand the South has in terms of boosting its twister count is that tornado season there has not one, but two 'peak' periods; one in the late fall, and one in the late winter/early spring. It's no coincidence that both of these coincide with when the transitional period between winter and summer-like jet streams takes place over the continent (we talked about this above). In November, we start to see the Bermuda High - a semi-permanent area of high pressure off the Atlantic coast - recede east across the ocean, allowing jet stream troughs to 'dig' more deeply into the Southeast, bringing that cold, dry air out of the Plains to collide with lingering moist, humid air in the Gulf States. The same happens in reverse in late winter, when the pattern starts to swing back the other way.
While this change is definitely noteworthy - and worth investigating, when you consider Dixie Alley storms have caused an average of about $600 million in property damage annually since 1997 (compared with $470 million in the Plains), and killed more than 40 people per year (compared with 13 in the Plains) - again, we don't know enough to point the finger entirely at climate change. For one thing, the South and Southeast are more densely populated than the Plains. Tornadoes in the Southeast are also harder to forecast, as they behave differently than their Great Plains cousins - something that projects like VORTEX-SE are researching.
So what can we say about climate change and tornadoes so far? “The geographical shift in tornado activity has been established through powerful statistical methods and is shown to occur during two successive 30-year periods moving from a colder weather pattern to warmer conditions,” says Ernest Agee, one of the lead authors of the Purdue University study. “More research is needed to search for changing climate trends responsible for tornado formation and this geographical shift, but climate change is a distinct possibility.”
As luck would have it, some of that research is already underway.
How can we 'predict' what tornado season will look like in the future? Increasing advanced warning time for tornadoes has been a focus for research since warnings began, and if changes to the season - and distribution - of storms like the one noted above are any indication, we'll need to continue to adapt our forecasting methods as we head into an uncertain future. There are some interesting leads out there; here's a small sampling of some of them.
Omens from the Ocean
A research group from the University of Miami turned their focus on the Gulf of Mexico as a warning sign for severe weather season, comparing the difference between weather in a 'cold' Gulf season and a 'warm' one. As was mentioned above, the Gulf of Mexico is one of the major heat and moisture drivers for severe weather and tornadoes in the United States, so it stands to reason that the general 'feeling' of the Gulf would impact what we see on land.
After looking at 30 years of data, the team found a possible link between sea surface temperatures (SST) in the Gulf and seasonal prediction of severe weather trends in the country. Perhaps not surprisingly, the average amount of atmospheric energy - the CAPE - seems to be linked to the water temperature in the Gulf; in years with warmer Gulf waters, we see more energy, and thus more favorable conditions for severe storms to develop. In years with cooler SST, we see diminished activity. While we can't use that to predict individual storms, it might work as an early-warning system to indicate whether we expect an active tornado season or a quiet one. While predicting specific weather systems is limited to about a week's lead time, models are pretty good at forecasting SST in the Gulf of Mexico about three months out.
Using large-scale models to predict small-scale storms
You might call numerical computer modeling the backbone of weather forecasting, with computer models ingesting tremendous amounts of data from points in the atmosphere and crunching the numbers before spitting various products out for forecasters to analyze. Longer-range climate models work similarly, albeit on larger and longer scales, with a focus on a different kind of answer; long term trends rather than specifics about weather-system scale storms.
That doesn't mean climate models are completely useless when it comes to thinking about severe weather, however. Several research papers have been published in the past few years looking at the way climate trends might relate to tornado activity in North America. These studies serve to underscore both what we know - things like the way tornadic activity relates to the available energy in the atmosphere (CAPE), and how changes in amount or distribution of energy will impact where we see severe weather - and what we don't know. Unfortunately, one of those lists is still very much longer than the other; I'll let you guess which one.
As one research team concluded, at this point studies into what climate change will mean for the intensity, number, and distribution of tornadoes largely serve to highlight the need for more studies. While it may be frustrating that we can't make a conclusive link between any one outbreak - or even any one season - between our day-to-day weather and a shifting climate, the picture isn't completely unclear.
“Can you link any particular season to climate change? The answer is no,” Dr. Victor Gensini from the University of Northern Illinois said in an interview. “But climate change is sort of what we say 'loading the dice' for these events to happen.”