STORM2K

I have never properly understood what it means for the STI percentage, I have assumed very favorable conditions for tornadoes, but I want to be absolutely sure that it's the correct way to properly understand it so I can know when the next possible setup for tornadoes may happen and how favorable it could get.

Here is an example of a 75% STI from the 9z SREF

https://s11.gifyu.com/images/SOXmc.png

The STI is the product of probabilities of individual tornado "ingredients" occurring, which in turn are calculated based on how many of the SREF's 22 members are showing those ingredients occurring at a particular position. That is,

STI = 100 * (Probability of MLCAPE ≥ 500 J/kg) × (Probability of Mixed-Layer LCL Height ≤ 1.5 km) × (Probability of 0-1 km helicity ≥ 100 m2/s2) × (Probability of 0-6 km Shear ≥ 40 kt) × (Probability of 3hr convective precipitation amount ≥ 0.01")

Each of these ingredients are in some way favorable for discrete supercells and low-level rotation. If all SREF members show all of those ingredients present, then STI will be 100 (regardless of the actual magnitude of those ingredients). So, the higher the STI value, the greater the likelihood of these particular thresholds of those ingredients being met, at least according to SREF. You might notice that if any of those ingredients have a low probability of occurrence, then the STI can decrease substantially. If all ingredients showed up on 95% of the SREF members, then the STI would actually be 77.