Questions about Severe Indices

[Alex Fisher]

I was wondering how reliable the things on the SPC Mesoscale Analysis Page are, such as Supercell Composite, Significant Tornado Index, and Craven Significant Severe Index. They often seem to disagree on the most likely place for significant or severe weather development and I was just wondering which ones are most trustworthy or at least most accurate. Thanks for any input.

 

[Jeff Snyder]

What would you rather have, a storm in a 1000 J/kg SBCAPE and 200 m2/s2 0-3km helicity, or an environment characterized by 6500 J/KG MLCAPE and 450 0-3k helicity but NO storms. Most indices can help characterize an environment, but applying them to an operation setting is worthless unless storms actually develop in the environment. Therefor, a region of 12 EHI and a Significant Tornado index of 8 means nothing other than an attempt to characterize the environment... For example, given the above EHI and SiggyTor, there appears to be an enhanced tornado threat in that environment, relative to an environment of 2 EHI and 0 siggytor, but they have nothing to do with actual thunderstorm initiation/development... Use indices with a grain of salt, since they can be deceiving. For example, take a siggy tor of 5. Now, that may actually be in an environment of 400 CAPE, but massive helicity and low LCLs (which can be common on the cool-side of a warm-front). Now, despite 5 looking good for tornado potential, it's much less likely given the fact that two of the components are so favorable that they make up for the low CAPE (which in reality, would likely inhibit tornadic storms)... Most do, however, hold some utility (statistical analysis has shown that several indices do perform pretty well, given initiation), but you just have to use them wisely...

 

[Alex Fisher]

Thanks for the info!