Violent Tornado Parameter
The Storm Prediction Center released a new composite severe weather parameter in 2018, known as the Violent Tornado Parameter, or VTP. In order to better understand VTP, it is helpful to first consider the Significant Tornado Parameter (STP), a dominant parameter in tornado prediction for many years.
It should be noted that severe weather parameters such as STP, VTP and several others are used as one of many tools in predicting severe weather and/or tornadoes. No parameter or index by itself is without flaws. The most effective way to predict tornadoes is to assess many variables that come into play, aside from just wind shear and instability. Some of these include frontal boundaries, forcing mechanisms, storm mode, etc., which cannot be readily assessed by simply looking at most severe weather parameters.
Broadly put, STP considers instability, cloud base heights (LCLs), storm relative helicity, effective shear and convective inhibition. Each of the five terms in the equation are normalized to 1, meaning that with increasing values over 1, the environment becomes increasingly favorable for a significant tornado. Likewise, significant tornadoes are relatively uncommon with STP values below 1.
(Y-axis: STP, X-axis: tornado intensity)
The concept behind VTP was to more effectively assess low-level instability, a key ingredient that often comes into play with the strongest (violent/EF-4 or stronger) tornadoes. Simply stated, the difference in an environment favorable for a relatively weak tornado and a much more intense or violent tornado, comes down to the amount of low-level instability.
(Effective) VTP calculation, where each of the seven terms is multiplied together.
VTP is essentially STP with two new terms. There is a term for mixed layer instability (MLCAPE) in the lowest 3km and 0-3km lapse rates. The terms are normalized, such that as each term grows larger over 1, the environment, in theory, becomes increasingly favorable for a violent tornado.
My initial concern was that by adding two low-level instability terms, the resultant value gets skewed when there is substantial low-level instability. This is especially the case as 0-3km instability gets large, particularly over 250 J/kg. While it is not always the case, large 0-3km instability is generally associated with steeper 0-3km lapse rates. In essence, VTP quickly becomes a bloated number, if there is an environment with steep low-level lapse rates and large low-level instability.
While it may seem helpful to offset STP with these terms, here is the big issue. While STP was set to 1 for a normalized value for significant tornadoes, VTP is not as clear cut. Research from 50 cases shows that the mean threshold value for a violent tornado using STP was close to 3, while the same value using VTP was roughly 5. The research acknowledges that VTP is often higher than STP and the thought is that a higher composite value flags the increased potential for a violent tornado. Within that dataset, the range for VTP is much larger than STP, leading me to question just how useful the parameter is.
On March 3rd, 2019, a violent EF-4 tornado effected Lee County in Alabama. As one might expect, there was substantial low-level instability in place at the time of the tornado, in addition to the background environment also being favorable for significant (EF-2 or stronger). Keep in mind that the Lee County tornado was the first (official) EF-4 rated tornado since April of 2017.
In the relatively short time I’ve had to assess VTP, there appears to be more false positives than anything. In the months from May of 2017 through all of 2018, there were no violent tornadoes in the United States. At the same time, there were countless examples of environments supporting large VTP values of >5 (many much higher), which is five times larger than the normalized STP value of 1.
Below are just a few notable examples of VTP not being very effective at tornado prediction, in chronological order from summer 2018 to early 2019.
On July 18th, 2018, a shortwave trough ejected southeast from the Northern Plains into the mid to lower Missouri Valley. Severe thunderstorms developed near a warm front in Nebraska, but sub-severe storms were observed farther southeast into portions of eastern Kansas and Missouri. The latter storms formed in the vicinity of substantial low-level instability, but relatively unfavorable shear. The offset in 0-3km instability was enough to spike VTP values to around 5 in the vicinity of discrete storms. Despite this, relatively weak shear was not supportive of tornadoes or any severe thunderstorms, for that matter.
Farther west, where shear was stronger, the analyzed VTP values were much higher, on the order of locally 12+. Although the environment in west-central Kansas was characterized by favorable shear and large instability, both large scale forcing and mesoscale convergence were insufficient for thunderstorms to form. This was a case where VTP across Kansas and Missouri was not a useful tool, given an array of factors considered. This is a good example of why just using one severe weather parameter and not considering the other mesoscale details is a flawed approach.
Two days later, the same storm system shifted south and east. Two corridors of severe thunderstorm activity were noted, with the most prominent being from the Ohio Valley into the lower Appalachians. To the west, additional severe thunderstorms were observed from the Ozarks into Mississippi. Between these two areas, there was a relative gap in severe thunderstorm activity, despite a large area of “high” VTP values. Over western Tennessee, VTP spiked over much of the area. Despite a few attempts at convective initiation along a sagging cold front, no robust convection survived into Tennessee. In this case, very large buoyancy was noted throughout the column, i.e. >5000 J/kg SBCAPE. Driven by steep low-level lapse rates, 0-3km MLCAPE was well above 200 J/kg, based on modifications to the 00z BNA sounding. Even though instability parameters were more than sufficient for intense thunderstorms, forcing was negligible and low-level wind fields were fairly weak. This is another case of low-level instability skewing VTP, despite multiple red flags arguing against severe thunderstorm activity.
July 23rd, 2018 was a case of terrain tending to skew VTP values. Violent tornadoes are extremely rare in eastern Colorado, with only one known example since 1950. With that said, VTP, driven again by low-level instability, has a tendency to blow up over the higher terrain in the area. On July 23rd, the large scale weather pattern was fairly benign. Although it was a very warm day in eastern Colorado, there were no storm systems or prominent shortwaves around to create an environment supportive of tornadoes. Even deep layer shear was marginal, aided somewhat by the elevation, supporting effective shear around 30 knots. Still, despite this, large low-level instability offset the otherwise marginal shear, supporting VTP values, locally over 5. Interestingly enough, there were a few severe thunderstorms farther west from this VTP bullseye, where even higher terrain worked to offset other negating factors. To the east, a discrete storm managed to form in western Kansas, but it was outside of the more seemingly favorable tornado environment. Regardless, that storm did not become severe.
A much-anticipated moderate risk setup was outlined on February 23rd, 2019. From far eastern Arkansas into northwestern Mississippi, a broad area of VTP values >8 developed during the afternoon and despite discrete storms in the area, this convection did not become severe. While low-level instability wasn’t extreme by any standards, what was already skewing this environment was very strong shear, with effective shear magnitudes in excess of 60 knots. In this case, STP was already elevated, in the 3-5 range over the same area. All it took was modest low-level instability, in terms of low-level lapse rates and 0-3km MLCAPE, to skew the resultant VTP values even higher. In the end, the strongest convection and bulk of the tornadoes were observed well to the east of the highest STP/VTP values. If STP was already a bit flawed in this case, VTP was even less helpful in highlighting an area with the greatest “potential” for a strong or even violent tornado.
In summary, the concept behind the Violent Tornado Parameter (VTP) is solid. It takes the somewhat dated and simplistic Significant Tornado Parameter (STP) and factors in low-level instability, as that is the key difference between environment supportive of weaker tornadoes (less low-level instability) and stronger or even violent tornadoes (greater low-level instability). With that said, using VTP is not as clear-cut as STP, as the latter parameter has a base of 1.0 for strong tornadoes. With VTP, the mean value is closer to 5 for violent tornadoes, but there is considerable variability in the data.
