SPC Severe Analysis

Data Courtesy Storm Prediction Center

Supercell Composite Parameter

A multiple ingredient, composite index that includes effective storm-relative helicity (ESRH, based on Bunkers right supercell motion), most unstable parcel CAPE (muCAPE) and convective inhibition (muCIN), and effective bulk wind difference (EBWD). Each ingredient is normalized to supercell "threshold" values, and larger values of SCP denote greater "overlap" in the three supercell ingredients. Only positive values of SCP are displayed, which correspond to environments favoring right-moving (cyclonic) supercells.

Violent Tornado Parameter

A multiple ingredient, composite index that includes effective bulk wind difference (EBWD), effective storm-relative helicity (ESRH), 100-mb mean parcel CAPE (mlCAPE), 100-mb mean parcel CIN (mlCIN), and 100-mb mean parcel LCL height (mlLCL), 0-3 km mean parcel CAPE and 0-3 km lapse rates.

Research using observed soundings found that 0-3 km CAPE and 0-3 km lapse rate were notable discriminators of violent tornado environments (verses weak and/or significant tornado environments). These parameters were combined into the effective layer version of the Significant Tornado Parameter (STP) to create the Violent Tornado Parameter (VTP).

SFC-Significant Tornado Parameter (effective layer)

A multiple ingredient, composite index that includes effective bulk wind difference (EBWD), effective storm-relative helicity (ESRH), 100-mb mean parcel CAPE (mlCAPE), 100-mb mean parcel CIN (mlCIN), and 100-mb mean parcel LCL height (mlLCL).

A majority of significant tornadoes (F2 or greater damage) have been associated with STP values greater than 1 within an hour of tornado occurrence, while most non-tornadic supercells have been associated with values less than 1 in a large sample of RAP analysis proximity soundings.

Significant Hail Parameter

The Sig. Hail Parameter (SHIP) was developed using a large database of surface-modified, observed severe hail proximity soundings. It is based on 5 parameters, and is meant to delineate between SIG (>=2" diameter) and NON-SIG (greater than 2" diameter) hail environments.

It is important to note that SHIP is NOT a forecast hail size.

Since SHIP is based on the RAP depiction of MUCAPE - unrepresentative MUCAPE "bullseyes" may cause a similar increase in SHIP values. This typically occurs when bad surface observations get into the RAP model.

Developed in the same vein as the STP and SCP parameters, values of SHIP greater than 1.00 indicate a favorable environment for SIG hail. Values greater than 4 are considered very high. In practice, maximum contour values of 1.5-2.0 or higher will typically be present when SIG hail is going to be reported.

Effective Storm Relative Helicity (m 2 s-2)

Most Unstable CAPE/Effective Bulk Shear

SRH (Storm Relative Helicity) in the lowest 500 m AGL has been found by Coffer et al. (2019), October issue of Weather and Forecasting, to be a better discriminator than effective SRH between significant tornadoes and nontornadic supercells. This calculation of 0-500 m SRH is limited to within the effective inflow layer, as long as the inflow base is at the ground.

3-km CAPE (J/kg) & Surface Vorticity

SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m2 s-2) and 0-1-km SRH (greater than 100 m2 s-2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear thresholds between non-tornadic and significant tornadic supercells.

Bulk Shear - Surface-3km | Surface-3km MLCAPE