Significant Tornado Parameter
A severe weather forecast index called the Significant Tornado Parameter (STP) is often used when predicting the potential of tornadoes.
There’s actually an algebraic equation that’s used to come up with STP:
STP = (mlCAPE/1500 J kg-1) * ((2000-mlLCL)/1000 m) * (ESRH/150 m2 s-2) * (EBWD/20 m s-1) * ((200+mlCIN)/150 J kg-1)
It looks complicated, but basically the equation factors in the following…
mlCAPE – A measure of Convective Available Potential Energy (CAPE).
mlLCL – The height of the mean parcel Lifted Condensation Level (LCL).
ESRH – The Effective Storm Relative Helicity.
EBWD – The effective bulk wind difference (wind shear).
mlCIN – Convective Inhibition or amount of preventative energy.
What we’re looking at here is for higher STP values. Higher CAPE values indicate more potential energy in the atmosphere, favoring thunderstorm development. The LCL is the height at which air becomes saturated as it rises. The LCL is approximately the height of cloud bases and lower LCL’s/cloud bases are common when tornadoes form. SRH is a measure for the potential of updraft rotation within a thunderstorm. If rotation is great enough, a tornado can form. Wind shear is the change of wind speed with increasing height in the altitude. If wind shear is great enough, this can also promote the formation of tornadoes. Finally, Convective Inhibition (CIN) is basically the opposite of CAPE. If there is too much CIN to overcome, it will be difficult for thunderstorms and tornadoes to form.
The mlCAPE term is divided by 1500 J kg–1 and that’s not an arbitrary number. Values of mlCAPE below 1500 are considered moderate or low, but values of 1500 or greater are considered high. So, a moderate mlCAPE value of 1500, when divided by 1500, equals 1. If mlCAPE is lower, the term falls below 1, but if mlCAPE is higher, the term is greater than 1.
The mlLCL term is a little bit more complicated. If the mlLCL height is 2000 meters (m) or higher, it is extremely unlikely that a thunderstorm or tornado would form. The entire term is set to 0 when the mlLCL > 2000m. Likewise, if mlLCL < 1000m, the term is set to 1. This is the case because mlLCL heights below 1000 meters are considered highly favorable for thunderstorm development and especially tornado formation.
The ESRH term is divided by 150 m2 s-2. The significance here is that SRH values below 150 are less favorable and values of 150 or greater are more favorable for thunderstorms and tornadoes.
The EBWD term is divided by 20 m s-1. Wind shear greater than 20 m s-1 (or about 39 knots) is considered highly favorable for rotating updrafts and tornado formation. The term is set to 0 when wind shear is less than ~24 knots. It is not common for tornadoes to form when wind shear is that low. On the other hand, the term is capped at 1 when wind shear reaches ~58 knots. Although wind shear does not often reach levels much higher than that, an extreme wind shear value could scale the equation, so that’s part of why the term has a cap.
The mlCIN term is also a bit more involved. The equation starts with 200 and adds the mlLCL and then that resultant value is divided by 150 J kg-1. Ideally, for tornadoes, there will be no mlCIN in place. Also, mlCIN is measured in a negative value. Increasing mlCIN results in a lower number. If the mlCIN value is less than -200, the entire term is set to zero. If there is that much CIN to overcome, it will be unlikely that a thunderstorm would develop. Likewise, once the mlCIN value is greater than -50, the term is set to 1.
Once all of this is considered, a value of 1 becomes fairly important. It’s a good benchmark and research shows that most significant tornadoes have resulted in an STP value of greater than 1. [link]
In Connecticut, an STP value of 1 is very high to extreme based on my research. In fact, the average STP value for historical tornadoes in Connecticut is about 0.3. The EF-1 tornado that hit Windsor Locks, Conn. on July 1st of this year had an STP value close to 0.5 in place, which was above the climatological average for historic tornadoes in Connecticut.
What does that mean?
Basically, if STP is non-zero, that should get your attention in Connecticut. A value of 0.3 is considered moderate, 0.5 is high and anything over 1 is highly unusual to see in this part of the country.
Most of Connecticut’s tornadoes have been relatively weak, or EF-0 to EF-1 on the strength scale. Like the STP equation says, an STP value over 1 supports significant tornadoes. STP values over 1 in Connecticut are rare, but in the case of such a high value, the probability of a significant or powerful tornadoes increases.
Tornadoes between 2007 an 2010 in Connecticut:
I have done a re-analysis of every tornado in Connecticut since 2007. Of the six tornado cases from 2007 to 2010, the STP values were between 0.1 and 0.5 in four cases and the other two cases were on the extreme end, with 1.1 and 1.2. It’s yet another example that you don’t need an STP value of >1 for tornadoes to form in Connecticut.
It’s not just about the STP.
Just because there is a high STP value does not mean that a tornado will form. Often, there needs to be other factors that come into play to increase the likelihood of tornadoes. A trigger mechanism, such as a frontal boundary, is very important.
Here are some websites that offer STP forecasts and observed values:
SPC SREF (SREF forecast)
College of DuPage (NAM and RAP forecasts)
SPC Mesoanalysis (Observed values and RAP forecasts)
