Mixed-Layer CAPE, or MLCAPE, is Convective Available Potential Energy calculated for a parcel whose temperature and moisture are averaged through a shallow near-surface layer, most commonly the lowest 100 millibars, before that averaged parcel is lifted. The area of positive buoyancy the averaged parcel accumulates through the environmental profile is reported in joules per kilogram, the same units as any other CAPE.
Why the averaging step exists
A well-mixed convective boundary layer is not uniform in temperature and moisture at the ground. Surface heating turns the lowest few hundred meters into an overturning turbulent layer, and the parcel that eventually feeds an updraft is a mix of air pulled up from the surface and drier, cooler air pulled down from the top of the mixed layer. The 100 mb averaging is a first-order representation of that stirring.
In practice the averaged parcel has a lower dewpoint than the surface parcel and often a slightly cooler temperature. Lifting that averaged parcel gives a smaller positive area, and MLCAPE is systematically lower than SBCAPE on a mixed afternoon boundary layer. That is not a bug. It is the point.
How forecasters use it
MLCAPE is the workhorse buoyancy input for most supercell and severe-storm parameters, including the Significant Tornado Parameter and the Supercell Composite. It is chosen for those uses because the parcel it represents better matches the inflow of an organized storm that has been ingesting mixed boundary-layer air for tens of minutes, rather than a fresh updraft rising off a very local hot spot.
A useful workflow is to look at SBCAPE and MLCAPE side by side. A boundary layer that is genuinely well mixed shows the two values close together. A large gap, with SBCAPE much higher than MLCAPE, points to a shallow moist layer at the surface that will not survive being pulled into a rising storm intact.
Where it misleads
The 100 mb averaging depth is a convention, not physics. Along the immediate Gulf coast or on a strongly capped morning, moisture and warmth are trapped in a shallow layer a few hundred meters deep, and averaging through 100 mb mixes in dry air the storm will never see. MLCAPE reads low, SBCAPE reads high, and the correct answer often lies between them.
The other place MLCAPE understates the environment is when strong warm-air advection is deepening the moist layer during the calculation window. A morning MLCAPE may be too low for a mid-afternoon storm environment because the mixed layer itself has grown between the two times. As always, treat any single CAPE value as one snapshot of a moving atmosphere and cross-check with the trend and with the parcel choice that best represents the storm.