What is the Significant Tornado Parameter?

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Significant Tornado Parameter

An index used by weather forecasters to focus their attention on the co-existence of conditions favoring the development of F2 - F5 tornadoes. Abbreviated STP and also Sig-Tor. “The STP was developed and tested as a tool to aid operational forecasters in discriminating between significantly tornadic and nontornadic supercell environments”.¹ “A majority of significant tornadoes (F2 or greater damage) have been associated with STP values greater than 1 within an hour of tornado occurrence, while most non-tornadic supercells have been associated with values less than 1 in a large sample of RAP analysis proximity soundings.”²

effective-layer STP = \frac{m l C A P E}{1500 joules / kilogram} \times \frac{E S R H}{150 meters / secon d^{2}} \times \frac{E B W D}{20 meters / second} \times \frac{2000 - m l L C L}{1000 meters} \times \frac{200 + m l C I N}{150 joules / kilogram}

The “ml” prefix in mlCAPE, mlLCL and mlCIN signifies mean conditions in the lowest 100 millibars.

ESRH is Effective Storm-Relative Helicity. See Thompson et al, “Effective Storm-Relative Helicity…”, cited below.

EBWD is Effective Bulk Wind Difference, thus a measure of wind shear. Set at 1.5 for EBWD > 30 m s⁻¹, and to 0.0 for EBWD < 125 m s⁻¹.See Thompson et al, “Effective Storm-Relative Helicity…”, cited below.

mlLCL. LCL is an acronym for Lifting Condensation Level. "The mlLCL term is set to 1.0 when mlLCL < 1000 meters AGL, and set to 0.0 when mlLCL > 2000 meters AGL."

mlCIN is Convective INhibition. "The mlCIN term is set to 1.0 when mlCIN > -50 J kg⁻¹, and set to 0.0 when mlCIN < -200 J kg⁻¹"

1. R. L. Thompson, J. A. Hart, K. L. Elmore and P. Markowski.
Close proximity soundings within supercell environments obtained from the Rapid Update Cycle.
Weather and Forecasting, vol. 18, no. 6, pages 1243-1261 (December 2003).

Page 1257. Original publication of the STP; superceded by Thompson et al, "An update…" (2005), cited below.

resources

R. L. Thompson, R. Edwards, and J. A. Hart.
Evaluation and interpretation of the supercell composite and significant tornado parameters at the Storm Prediction Center.
Preprints, 21st Conference on Severe Local Storms,
San Antonio, TX: Amer. Meteor. Soc., J11–J14. 2002.

R. L. Thompson, R. Edwards and C. M. Mead.
An update to the Supercell Composite and Significant Tornado Parameters.
Preprints, 22nd Conference on Severe Local Storms.
Hyannis, MA, 2005.

Richard L. Thompson, C. M. Mead, and R. Edwards.
Effective storm-relative helicity and bulk shear in supercell thunderstorm environments.
Weather and Forecasting, vol 22, pages 102-115 (February 2007).

Richard L. Thompson, Bryan T. Smith, Jeremy S. Grams, Andrew R. Dean, and Chris Broyles.
Convective Modes for Significant Severe Thunderstorms in the Contiguous United States. Part II: Supercell and QLCS Tornado Environments.
Weather and Forecasting, vol 27, pages 1136-1154 (October 2012).