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Surface-based CAPE (SBCAPE), or Surface-Based Convective Available Potential Energy, is a measure of atmospheric instability. It represents the potential energy available to an air parcel originating at the surface and rising through the atmosphere until it reaches its level of free convection. SBCAPE is calculated using the temperature and dew point at the surface, and it's a key indicator of the potential for strong or severe thunderstorms.
Mixed Layer Convective Available Potential Energy (MLCAPE) is a measure of atmospheric instability calculated using the average properties of the lowest 100 hPa (or 100-mb) layer of the atmosphere. It essentially represents the energy available for convection if a parcel of air with those average characteristics were lifted from the surface.
The "most unstable CAPE," often abbreviated as MUCAPE, refers to the highest value of convective available potential energy (CAPE) found within a given atmospheric sounding or model forecast. This indicates the greatest potential for upward air movement and thunderstorm development, but not necessarily the highest probability of convection. MUCAPE is calculated by identifying the most unstable parcel of air within a specific layer of the atmosphere (often the lowest 300mb) and then calculating the CAPE for that parcel.
The Energy Helicity Index (EHI) in meteorology is a parameter used to assess the potential for tornadogenesis in severe thunderstorms. It combines the effects of both helicity (specifically storm relative helicity or SRH) and convective available potential energy (CAPE) to provide a more comprehensive assessment of tornado risk. The EHI is calculated by multiplying the SRH by the CAPE and then dividing by a threshold CAPE value.
SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1km and 3km 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 0-3km SRH (greater than 250 m2 s-2) and 0-1km 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.