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The parcel that yields the highest CAPE is found from the ensemble.   CAPE for this parcel is then re-computed using the model virtual potential temperatureand identified as the most unstable (MUCAPE) value.   MUCAPE (CAPE_θv) is computed using virtual potential temperature of the parcel (θ_vp), the virtual potential temperature of the environment (θ_ve), and the environmental saturated equivalent potential temperature (θ_esat). 

MUCAPE (CAPE_θv) has overall higher values than CAPE_θe (and indeed what forecasters would diagnose from vertical profiles of the atmosphere).

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• greater than 1000 J kg^-1 indicate potential for development of moderate thunderstorms.
• greater than 2000 J kg^-1 indicate a potential for severe thunderstorms.  
• 3000 to 4000 J kg^-1 or even higher usually signify a very volatile atmosphere that could produce severe storms if other environmental parameters are in place.

Note:

CAPE and MUCAPE can be a guide to the intensity of convection, but only if convection triggers.  

Processes such as entrainment, detrainment and precipitation load are not considered and so model MUCAPE values are likely to be overestimated.  MUCAPE is available in ecCharts etc.

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At each grid point the convection scheme inspects the temperature structure of the forecast atmosphere progressively from the surface to 350hPa.  At each level, if there exists a level of free convection (LFC), it evaluates the energy required for a rising parcel to overcome the inhibiting effect of the underlying temperature structure.

The minimum of these values are available in ecCharts etc.  

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MUCAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and MUCAPE.  It is used to identify areas of potentially extreme convection.  Vertical wind shear tends to promote thunderstorm organisation, although excessive wind shear can be detrimental to convective initiation by increasing entrainment of environmental air into the storm.  But if active convection is indeed established, then larger wind shear tends to be associated with higher organisation and severity of convection.

For example: supercells produce the majority of strong to violent tornadoes and very large hail (more than 5cm in diameter) and tend to occur in environments with strong wind shear (0-6 km shear > 20 ms^-1).  Supercells can be very long-lived (more than 6 hours in some cases).

For diagnostic purposes both MUCAPE and MUCAPE-shear should be used together, or alternatively one can examine MUCAPE and wind shear as separate parameters.

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