Grampian Weather Services
"The weather ... as forecast"
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