Thunderstorms Forecast: Indices

  
Instability
Boyden Index

Criteria:

 

> 94: TS possible

 

Best used for frontal or trough thunderstorms during winter season

Likelyhood
Vertical Totals Index
  

Criteria:

 

> 28: strong potential for TS

Cross Totals Index
  

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
  

Criteria:

 

<39: no TS (89%)

41 to 45: TS possible (42%)

>46: TS likely (75%)

Showalter Index
  

Criteria:

 

> 2: no TS

=< 2: TS

 

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

Bradbury Index
  

Criteria:

 

< -2: TS likely in summer

< +3: TS likely in winter

< +3: TS likely in all seasons with cyclonic 500 hPa pattern

Rackliff Index
  

Criteria:

 

25: showers possible

25 to 29: showers with TS possible

> 30: TS likely

 

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

Modified Jefferson Index
  

Criteria:

 

> 27: TS likely if in polar air mass

> 30: TS if other air masses

 

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

Type

Bulk Richardson Number (BRN): proportional to CAPE and inversely proportional to shear

 

Very high BRN values: the shear is too weak to stop the outflow pool of cold (downdraught) air moving quickly away from the parent updraught: new cells may form as the gravity current propagates downstream, but usually well away from the parent - and distinct from it. Also, as the updraught is quasi-zero sheared vertically, the rain shaft falls into a saturated environment (little or no evaporation possible), and there is no potential for a substantial downdraught.

 

Mderate BRN values: the shear element (especially speed shear) now plays a crucial part in skewing the updraught, tilting the growing cloudy environment, allowing some or all of the precipitation shaft to fall into unsaturated air - evaporative cooling (plus precipitation drag) will generate a cold downdraught - the drier the air, the greater the potential for accelerating cold/gravity current flow. As the downdraught hits the surface, it splays out, meeting environmental inflow and generating new cells away from (but close by) the parent cell - this is the basis of the multi-cell thunderstorm.

 

Low BRN values (but NOT quasi-zero numbers .. see below): Supercell storms may occur (other factors being right). The shear is now strong (and composed of both speed and significant directional components - this latter is most important), and the new/growing cell effectively forms alongside or even within the environment of the 'parent', the whole forming a steady-state system. The situation is complex though and rotation of the developing storm must be present - this is thought to be due to advection (and significant distortion / stretching) of low-level horizontal vorticity into the updraught of the storm environment, but knowledge here is still incomplete, though growing. (It is here that Helicity comes into play .... see elsewhere).

 

Very low BRN values: the shear is too strong against very weak CAPE: the developing convective (cloudy) towers are ripped apart and are generally too ill-organised for persistent self-sustaining Cb development. However, note that within the environment of a tropical storm, a low BRN may be associated with organised convective 'streets'.

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)

 

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

Coverage & Severity
Severe Weather Threat Index
  

Criteria:

 

< 300: weak potential for severe TS

> 300: moderate potential for severe TS

> 500: high potential for severe TS

 

Add way to calculate S.

Thompson Index
  

Criteria:

 

25 to 34: potential of TS

35 to 39: potential of TS approaching SEV

=> 40: potential of severe TS

 

 

Severe Thunderstorm Index
  

Criteria:

 

2.5 to 3.5: minor severe event

1.5 to 2.5: major severe event

<1.5: tornadic event

 

 

Tornadoes