Forecasters assessing convective potential frequently examine the field of Convective Available Potential Energy. When examining Eta model forecasts of CAPE, however, there are 3 options from which to choose. The Eta outputs a surface-based CAPE (actually lifting the parcel in the lowest 70 mb with the highest theta-e), a most-unstable CAPE (the 30 mb layer with the highest theta-e in the lowest 180 mb is lifted), and a mixed-layer CAPE (which lifts a parcel with mean thermodynamic properties over the lowest 90 mb). While the values are usually fairly similar, big differences can result when the model forecasts an extremely shallow boundary layer, such as in the following sounding.
The maps below compare sfc-based, most-unstable, and mixed-layer CAPEs for a particular Eta forecast. Huge differences (as large as 3000 J/kg) can be seen over the Ohio Valley and Gulf f Mexico.
So, what is the best way to go?
It is always difficult to see which forecast is "best." There is, however, some thought that the very shallow boundary structure as seen in the sounding above is probably not very often realistic. The shallow convective scheme in the Eta tends to produce this structure over warm waters and their coastlines, so model forecasts of high sfc-based CAPE are likely too high over the Gulf region and southeast U.S. Atlantic waters. The mixed-layer CAPE may be a superior representation of instability, particularly in cases with well-mixed boundary layers. Regardless, it is always wise to view the model forecast soundings in conjunction with CAPE forecasts.