Surface-Based CAPE
SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered.
100-mb Mixed Layer CAPE
MLCAPE (Mixed Layer Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the mean potential energy conditions available to parcels of air located in the lowest 100-mb when lifted to the level of free convection (LFC). No parcel entrainment is considered.
Most Unstable CAPE & Convective Inhibition
MUCAPE (Most Unstable Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to the most unstable parcel of air found within the lowest 300-mb of the atmosphere while being lifted to its level of free convection (LFC). No parcel entrainment is considered. CIN (Convective INhibition) represents the "negative" area on a sounding that must be overcome before storm initiation can occur.
0-6-km Vertical Shear Vector
The 0 through 6-km above ground level shear vector denotes the change in wind throughout this height. Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth.
Storm Relative Helicity
SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 3-km layer 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 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear "boundaries" between non-tornadic and significant tornadic supercells.
Corfidi Vector
The "Corfidi Upshear" vector is an estimate of net storm motion for a "backbuilding" MCS, where the low-level storm inflow is subtracted from the mean wind. The "Corfidi Downshear" vector is an estimate of net storm motion for a "forward propagating" MCS where the low-level storm inflow is added to the mean wind.
Lifting Condensation Level
The LCL (Lifting Condensation Level) is the level at which a parcel becomes saturated. It is a reasonable estimate of cloud base height when parcels experience forced ascent.
Non-Supercell Tornado Parameter
The non-supercell tornado parameter (NST) is the normalized product of the following terms:
(0-1 km lapse rate/9 C/km) * (0-3 km MLCAPE/100 J/kg) * ((225 - MLCIN/200) * ((18 - 0-6 km bulk wind difference)/5 m/s) * (surface relative vorticity/8**10-5/s)
This normalized parameter is meant to highlight areas where steep low-level lapse rates correspond with low-level instability, little convective inhibition, weak deep-layer vertical shear, and large cyclonic surface vorticity. Values > 1 suggest an enhanced potential for non-mesocyclone tornadoes.
Critical Thickness Value
Thickness is the vertical distance between two isobaric surfaces, which is often proportional to the temperature of that layer. As such, empirical studies have shown that certain values of differing thickness have shown some skill in differentiating between rain or snow.
Snow Squall Parameter
A non-dimensional composite parameter that combines 0-2 km AGL relative humidity, 0-2 km AGL potential instability (theta-e decreases with height), and 0-2 km AGL mean wind speed (m/s). The intent of the parameter is to identify areas with low-level potential instability, sufficient moisture, and strong winds to support snow squall development. Surface potential temperatures (theta) and MSL pressure are also plotted to identify strong baroclinic zones which often provide the focused low-level ascent in cases of narrow snow bands.
The index is formulated as follows:
Snow Squall = ((0-2km mean RH - 60%) / 15%) * (( 4 - 2km_delta_theta-e) / 4) * (0-2km mean wind / 9 m s-1)
The 2km_delta_theta-e term is the change in theta-e (K) from the surface to 2km AGL, where negative values represent potential instability.
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