Mixed-Layer CIN, or MLCIN, is Convective Inhibition 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 to its level of free convection. The negative area between the averaged-parcel path and the environmental profile is the MLCIN value, reported in joules per kilogram as a negative number.
Why the mixed-layer parcel matters here
Once a summer boundary layer has been stirred for a few hours, the parcel that actually reaches the base of an organized updraft is not the hot, humid parcel at the sensor. It is a mixed parcel with a lower dewpoint and, in many cases, slightly cooler temperature after being blended with drier air aloft. That averaged parcel encounters more resistance in the layer just above the boundary layer than the surface parcel does.
MLCIN is therefore typically larger, in absolute value, than SBCIN. That is not a pessimism bias. It is a more realistic representation of what a storm's inflow will actually be doing when the boundary layer has mixed.
How forecasters use it
MLCIN is the CIN of record for supercell and severe-storm composite parameters, including the Significant Tornado Parameter and Supercell Composite. When those parameters read low because MLCIN is large, the message is that any storm the environment produces has to work through a real cap, and either heating, upward forcing, or moistening has to relax that cap for storms to become surface-based.
The workflow question with MLCIN is not just how large the value is, but how quickly it is trending toward zero. On a capped severe-weather day, MLCIN often shrinks steadily through mid-afternoon as surface heating deepens the mixed layer into the base of the cap. The hour at which MLCIN drops to a value the available forcing can overcome is often close to the hour of first storm initiation.
Where it misleads
The 100 mb averaging depth is a convention. When moisture is trapped in a shallow layer at the surface, averaging over 100 mb pulls in drier air a real inflow parcel would not include, which makes the mixed parcel cooler and drier and inflates MLCIN. In that case the true cap the storm will face lies between MLCIN and SBCIN, and a look at both is worth more than either alone.
Like every CIN metric, MLCIN describes a barrier, not a probability. A moderate MLCIN with a strong forcing feature approaching often produces storms. A modest MLCIN with no lift mechanism does not. Read it in tandem with the forcing picture, not as a standalone initiation forecast.