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General thunderstorm (instability) indicators

Index Weak (low) Moderate Strong (high)
Surface based CAPE * 300 to 1000 1000 to 2500 > 2500
Vertical Totals     > 28
Cross Totals < 18 18 to 19 => 20
Total-totals > 42 > 48 > 50
K-Index 20 to 25 26 to 35 > 35
KO-Index  * > 6 2 to 6 < 2
Surface based Lifted-Index * 0 to -2 -3 to -5 < -5
S-Index < 39 40 to 46 > 46
Showalter Stability Index > 2 =< 2  
Rackliff Index => 29    
Bradbury Index < 3    
Modified Jefferson Index > 27    
Boyden Index > 94    

 

 

Severe thunderstorm indicators

Index / Environment ** Weak (low) Moderate Strong (high)
Bulk Richardson Number > 35 (multicells)   10 to 50 (supercells)
Cross Totals 22 to 23 24 to 25 > 25
Surface based Lifted-Index * 0 to -2 -3 to -5 < -5
SWEAT Index < 300 300 to 500 => 500
Thompson Index 24 to 34 35 to 39 => 40
Total-totals 48 to 49 50 to 55 => 56
Severe thunderstorm Index > 3.5 2.5 to 3.5 < 2.5
Storm Relative Helicity 0-3 km ** (supercell potential) 100 to 150 m²/s² 150 to 300 m²/s² >300 m²/s²
0-6km shear ** (supercell potential) 35-40 kt   >40 kt
0-3km shear ** (bowecho potential with wind damage)     23 to 37 kt

 

Surface based CAPE */ 0-6 km shear ** =<30 kt 31-49 kt =>50 kt
=< 1000 J/kg ordinary cell ordinary cell / supercell ordinary cell / supercell
1000 to 2500 J/kg ordinary cell ordinary cell / supercell ² supercell ²
>2500 J/kg ordinary cell ² ordinary cell / supercell ² supercell ²

² storms in which severe weather is likely

 

Tornado indicators

Index / Environment ** Value Interpretation
Energy/Helicity Index 0.00 to 0.34  (mean = 0.14)
0.20 to 1.46  (mean = 0.64)
0.42 to 2.87  (mean = 1.48)		 Ordinary cell without tornadoes
Supercell & risk weak tornadoes (F0, F1)
Supercell & risk strong tornadoes (= > F2)
Severe thunderstorm Index < 1.5 Tornadoes possible
Storm Relative Helicity 0-1 km 7 to 73       m²/s²  (mean = 35 m²/s²)
7 to 46       m²/s²  (mean = 27 m²/s²)
39 to 116   m²/s²  (mean = 80 m²/s²)
102 to 241 m²/s²  (mean = 198 m²/s²)		 Waterspouts possible
F0 tornadoes possible
F1 tornadoes possible
F2 tornadoes possible
Storm Relative Helicity 0-3 km ** 16 to 108   m²/s²  (mean = 54 m²/s²)
34 to 82     m²/s²  (mean = 59 m²/s²)
57 to 163   m²/s²  (mean = 105 m²/s²)
135 to 279 m²/s²  (mean = 210 m²/s²)		 Waterspouts possible
F0 tornadoes possible
F1 tornadoes possible
F2 tornadoes possible
Shear 0-1 km ** 7 to 19 kt    (mean = 11 kt)
5 to 13 kt    (mean = 9 kt)
14 to 24 kt  (mean = 17 kt)
25 to 43 kt  (mean = 40 kt)		 Waterspouts possible
F0 tornadoes possible
F1 tornadoes possible
F2 tornadoes possible
Shear 0-6 km ** 14 to 35 kt  (mean = 25 kt)
23 to 38 kt  (mean = 34 kt)
19 to 43 kt  (mean = 30 kt)
33 to 98 kt  (mean = 52 kt)		 Waterspouts possible
F0 tornadoes possible
F1 tornadoes possible
F2 tornadoes possible

Most indices can be calculated via (model) soundings. For Belgium use this extern link via this page.

Blue links refer to an intern calculator, with * a forecasted weather map. For info on environment **, please check the model soundings.

Important note: stability indices have no magic threshold numbers as these may vary at each location. They are only aids for forecasting thunderstorms and should not be used as the sole basis for making a thunderstorm forecast.

 

 

Thunderstorm severity chart

Cloud top temperature (°C) Extent above freezing level (1000 ft) Precipitation (mm) Visibility rain (km) Visibility snow (km) Hail (cm) peak gust (kt)
-10 to -15 5 to 7 <1 >8 3 to 6   <20
-15 to -20 7 to 9 1 to 2 6 to 8 1 to 3   20 to 25
-20 to -25 9 to 12 2 to 3 4 to 6 0.5 to 1   25 to 30
-25 to -35 12 to 17 3 to 5 3 to 4 <0.5   30 to 35
-35 to -45 17 to 22 5 to 10 2 to 3   <1 35 to 45
-45 to -55 22 to 27 10 to 30 1 to 2   1 to 4 45 to 60
-55 to -70 27 to 35 30 to 100 <1   4 to 10 60 to 100
 
This chart was created by the German Military Geophysical Office (GMSO) to have a quick view on thunderstorm potential.

Enter the table with cloud top temperature and/or extent of cloud above freezing level; read across to predict severity. Use less severe condition if the table parameters disagree