Comparison of Observed Conditions with Stability Indices LT Cathy McDougall 20 March 2001

Introduction The forecasting of thunderstorms and related phenomena (hail, tornadoes, strong winds, heavy rain) is based primarily on the concepts of: a) conditional instability. b) convective instability. c) forced lifting of air near the surface.

Conditional Instability the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist- adiabatic lapse rates) d >  > s d = -9.8 C/km s = (-7 +/- 3) C/km the available-energy definition (i.e., a parcel possesses positive buoyant energy) + > - real latent instability - < + pseudo latent instability - only latent stability saturated parcel = instability unsaturated parcel = must evaluate CAPE and CIN

Stability and Instability Indices Showalter Index SI = T500 - Tp500 >= 3 No convective rain 1 to 3 Showers likely -2 to 1 Thunderstorms probable <= -3 Tornadoes probable K Index K=(T850 - T500) + Td850 - (T700 - Td700 ) Mod K = (aveTsfc-850 - T500) + aveTdsfc-850 - (T700 - Td700 ) K THUNDERSTORM PROBABILITY 0-15 0 15-20 20 21-25 20-40 25-30 40-60 30-35 60-80 35-40 80-90 >40 100

Instability Indices (Continued) Total Totals Index TT = T850 + Td850 - 2T500 >44 Thunderstorms >50 Thunderstorms with large hail and tornadoes. SWEAT Index 12D +20(TT-49) + 2v8 + v5 + 125(S+0.2) < 272: unlikely 273 to 299: general storms; slight risk of severe storms 300 to 400: storms approaching severe limits; moderate risk of severe storms 401 to 600: few severe storms with isolated tornadoes; strong risk 601 to 800: scattered tornadoes; high risk

Convective Instability Convective instability was originally defined by Rossby (1932) to be when one of the three following equivalent conditions is met over a layer of the atmosphere: (1) the lapse rate of wet-bulb temperature (Tw) exceeds the moist-adiabatic lapse rate (s) (2) the equivalent potential temperature (e) decreases with height, or (3) the wet-bulb potential temperature (w) decreases with height.

THUNDERSTORM EVENT 12 February 2001 at 4Z Closest Sounding to the event was 12FEB01, 00Z

Instability Indices from 12/00Z Showalter Index using -7 = -0.7772 Thunderstorms probable Showalter Index using Skew T= 3.3000 No convective rain Showalter Index using -4 = -10.7823 Tornadoes probable KIndex = 13.3167 0% probability of Thunderstorms ModKIndex = 24.4125 20-40% probability of Thunderstorms TTIndex = 55.4000 Groups of tornadoes are feared - ref 1 Severe Weather Probability Moderate - ref 2 SWEAT = 204.5875 Usefulness low because < 250

Comparison with Stable Atmospheric Conditions Sounding 05 Feb 2001 18 Z Fog

In contrast with Thunderstorm case Theta E increases with height in the lower levels indicating stability (no convective Instability).

Instability Indices from 05/18Z Showalter Index using -7 = 16.8038 No convective rain Showalter Index using Skew T= 10.9778 No convective rain Showalter Index using -4 = 13.4514 No convective rain KIndex = -52.3478 0% probability of Thunderstorms ModKIndex = -32.0259 0% probability of Thunderstorms TTIndex = 15.3744 Severe Weather Probability unlikely SWEAT =No wind data available

Conclusions Stability indices are only a forecasting tool. Don’t depend on a single index. Severe weather indices only indicate the potential for convection. Determination of the convective instability using e seemed to be a better indicator in our region.