Thunderstorms Forecast: Indices

  

Criteria and calculations from www.skystef.be

SB & MU Lifted Index Criteria:

 

0 to -2: WK potential, needs trigger

-3 to -5: MOD potential

< -5: strong potential

 

Note: Good correlation between LI values < -5 and severe weather events

SB & MU CAPE Criteria:

 

300 to 1000: WK convective potential

1000 to 2500: MOD convective potential

> 2500: strong convective potential

 

Note: No correlation between CAPE values and severe weather events

CIN Criteria:

 

> 100: precludes TS development without significant forcing

50: associated with derechoes

=< 25: associated with significant TN

Vertical Totals Index
  
Cross Totals Index
  

Criteria:

 

> 28: strong potential for TS

Criteria:

 

< 18: weak potential for TS

18 to 19: moderate potential for TS

20 to 21: strong potential for TS

22 to 23: weak potential for severe TS

24 to 25: moderate potential for severe TS

>25: strong potential for SEV TS (hvy shwr & TN >29)

S-Index
  
Showalter Index
  
Rackliff Index
  
Modified Jefferson Index
  

Criteria:

 

<39: no TS (89%)

41 to 45: TS possible (42%)

>46: TS likely (75%)

 

Note: T850 - T500 = VT

A = 0 when T850 - T500 > 25

A = 2 when T850 - T500 22-25

A = 6 when T850 - T500 < 22

Criteria:

 

> 2: no TS

=< 2: TS

 

Note: Parcel lifted from 850 hPa. Index doesn't work well if a frontal surface or inversion is present between 850 hPa and 500 hPa.

Criteria:

 

25: showers possible

25 to 29: showers with TS possible

> 30: TS likely

 

Note: Best used for air mass thunderstorms (no dynamical triggering mechanism).

Criteria:

 

> 27: TS likely if in polar air mass

> 30: TS if other air masses

 

Note: Best used for air mass thunderstorms (no dynamical triggering mechanism).

Bulk Richardson Number
  

Criteria:

 

< 10: low risk of severe TS

> 35: low risk for supercells, but risk for single cells or multicells

15 to 50: risk for supercells (upper teens optimal)

 

Note: Works best with CAPE between 1500 and 3500 J/kg.

ML50 CAPE = conservative estimate of CAPE, lifting an average parcel in the bottom 50 hPa of a sounding.

Severe Weather Threat Index
  
Thompson Index
  
Severe Thunderstorm Index
  

Criteria:

 

< 300: weak potential for severe TS

> 300: moderate potential for severe TS

> 500: high potential for severe TS

 

Note:

S value: sin(dd500 - dd850)

(dd = wind direction (at 500 or 850 hPa))

All members must have positive values, otherwise set it to 0.

Last member = 0, unless:

130° =< dd850 =< 250°

210° =< dd500 =< 310°

dd500-dd850 > 0°

V850 and V500 => 15kts

 

Poor correlation between high SWEAT values and SEV weather events.

Criteria:

 

25 to 34: potential of TS

35 to 39: potential of TS approaching SEV

=> 40: potential of severe TS

 

 

Criteria:

 

2.5 to 3.5: minor severe event

1.5 to 2.5: major severe event

< 1.5: tornadic event

 

 

EH Index Criteria:

 

0.00 to 0.34 (mean = 0.14): Ordinary cell without TN

0.20 to 1.46 (mean = 0.64): Supercell & WK risk of TN (F0, F1)

0.42 to 2.87 (mean = 1.48): Supercell & strong risk of TN (= > F2)

 

Note: No good correlation between EHI values and severe weather events.

0-1 km SREH Criteria:

 

7 to 73 m²/s² (mean = 35 m²/s²): Waterspouts possible

7 to 46 m²/s² (mean = 27 m²/s²): F0 TN possible

39 to 116 m²/s² (mean = 80 m²/s²): F1 TN possible

102 to 241 m²/s² (mean = 198 m²/s²): F2 TN possible

0-1 km Shear Criteria:

 

7 to 19 kt (mean = 11 kt): Waterspouts possible

5 to 13 kt (mean = 9 kt): F0 TN possible

14 to 24 kt (mean = 17 kt): F1 TN possible

25 to 43 kt (mean = 40 kt): F2 TN possible

0-3 km SREH Criteria:

 

16 to 108 m²/s² (mean = 54 m²/s²): Waterspouts possible

34 to 82 m²/s² (mean = 59 m²/s²): F0 TN possible

57 to 163 m²/s² (mean = 105 m²/s²): F1 TN possible

135 to 279 m²/s² (mean = 210 m²/s²): F2 TN possible

 

Note: Poor correlation between  SREH values and severe weather events

0-6 km Shear Criteria:

 

14 to 35 kt (mean = 25 kt): Waterspouts possible

23 to 38 kt (mean = 34 kt): F0 TN possible

19 to 43 kt (mean = 30 kt): F1 TN possible

33 to 98 kt (mean = 52 kt): F2 TN possible

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"The weather as forecasted"