[--MAIN HOME--] [--ALL HABYHINTS--] SEVERE WEATHER INDICES PAGE METEOROLOGIST JEFF HABY TOTAL TOTALS <44 Convection not likely 44-50 Likely thunderstorms 51-52 Isolated severe storms 53-56 Widely scattered severe >56 Scattered severe storms SPEED SHEAR 0-3 Weak 4-5 Moderate 6-8 Large 8+ Severe K INDEX 15-25 Small convective potential 26-39 Moderate convective potential 40+ High convective potential CAPE 1 - 1,500 Positive 1,500 - 2,500 Large 2,500+ Extreme SWEAT 150-300 Slight severe 300-400 Severe possible 400+ Tornadic possible SR HELICITY 150-300 Possible supercell 300-400 Supercells favorable 400+ Tornadic possible LIFTED INDEX / SWI -1 to -4 Marginal instability -4 to -7 Large instability -8 or less Extreme instability BRN <45 Supercells favorable <10 Too sheared teens Optimum EHI EHI >1 Supercells likely 1 to 5 F2, F3 tornadoes possible 5+ F4, F5 tornadoes possible NOTES: *Max uvv = square root of 2 × CAPE *BRN (Bulk Richardson Number) = CAPE / (0-6 km) Shear *Showalter (SWI) = used when elevated convection is most likely (cool season) *EHI = (SR HEL × CAPE) /160,000 *SWEAT = 12(850Td) +20(TT-49) +2(V850) + (V500) +125(sin(dd500-dd850) + 0.2) *Total Totals = (T850- T500) + (Td850 - T500)= vertical totals plus cross totals *K index = (T850 -T500) + (Td850 - Tdd700) *SR Helicity : determines amount of horizontal streamwise vorticity available for storm ingestion *streamwise = parallel to storm inflow *Important to look for thermal and dewpoint ridges (THETA-E) *For tornado, inflow must be greater than 20 knots *20 to 30% of mesocyclones produce tornadoes *Tornado types: rope, needle, tube, wedge *Look for differential advection; warm/ moist at surface, dry air in mid levels *Severe weather hodograph: veering, strong sfc to 850 directional shear * >100 J/kg negative buoyancy is significant *Good match: BRN < 20 and CAPE >2,000 J/kg *Strong cap when > 2 degrees Celsius *Study depth of moisture, TT unreasonable when low level moisture is lacking *KI used for heavy convective rain, values vary with location/season *Instability enhanced by ... daytime heating, outflow boundaries *Models generally have weak handle on return flow from Gulf, low level jet, convective rainfall, orography, mesoscale boundaries, and boundary conditions *Large hail when freezing level >675 mb, high CAPE, supercell *Synoptic scale uplift from either surface WAA or upper level divergence *Fair weather cumulus: cumulus humulus, cumulus mediocrus *T-storm warning when Hail > 3/4", wind > 58 mph, gate to gate shear > 90 knots *Sounding types: Inverted V, goal post, Type C, wet microburst