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Severe Weather Parameters
   
Totals Totals Index (TT)
45 to 50
T-storms likely
50 to 55
T-storms likely, some severe
55-60
Severe T-storms likely

 
K Index (KI)
< 30
T-Storms w/heavy rain possible
  30-40
Better potential for heavy rain
 40
Best potential for heavy rain

 
Lifted Index (LI)
0 or higher
Stable; minimal convection
 0 to -3
Marginally unstable
-3 to -6
Moderately unstable
-6 to -9
Very unstable
-9 or lower
Extremely unstable

 
Showalter Index (SI)
0 or higher
Stable; minimal convection
 0 to -3
Moderately unstable
 -4 to -6
Very unstable 
-6 or lower
Extremely unstable

 
Severe Weather Threat Index (SWEAT)
 300 to 400
Severe thunderstorm potential
 400 or higher
Tornado potential 

 
Convective Available Potential Energy (CAPE)
 0 or lower
Stable 
    0 to 1,000
Marginally unstable
 1,000 to 2,500
Moderately unstable
 2,500 to 3,500
Very unstable
 3,500 or higher
Extremely unstable

 
 
Lid Strength Index (LSI)
  below 2
Convection unrestricted 
 2
Neutral cap strength
  higher than 2
Convection suppressed 

 
Bulk Richardson's Number (BRN)
 10 or lower
Extreme vertical wind shear; some develop possible 
10 to 45
Moderate shear; supercellular development favored
  45 or higher
Weak shear, high bouyancy; multicellular development favored

 
 
Bulk Richardson's Number Shear 
(BRN Shear)
 25 to 100
Tornadic supercell possible

 
Storm Relative 500 mb Winds (S-R Winds)
16 Knots
Lower limit for tornadic supercells
 40 Knots
Higher limit for tornadic supercells 

 
Storm Relative Helecity (Hs-r)
 150
Minimum supercellular development
 150 to 299
Weak mesocyclone (F0 or F1 tornado possible)
  300 to 449
Strong mesocyclone (F2 or F3 tornado possible)
450 or higher
Violent tornadoes possible (F4 or F5)

 
Energy Helicity Index (EHI)
 1.0 or lower
Supercells unlikely
 1.0 to 2.0
Weak short-lived supercells
  2.0 to 2.4
Moderate supercells
2.5 to 2.9
Supercells/Tornadoes more likely
3.0 to 3.9
Strong supercells/tornadoes possible (F2 or F3)
4.0 or higher
Violent mesocyclones/tornadoes possible (F4 to F5)

 
Wet Bulb Zero Height (WBZ)
 Surface to 5K AGL
Hail unlikely
 5K to 6K AGL
Small hail possible
 6K to 7K AGL
Larger hail more likley
7K to 10K AGL
Largest hail likely
10K to 11K AGL
Smaller hail possible
11K or higher
Hail unlikely