The most frustrating forecasts are the ones where every ingredient for severe weather is present and nothing happens. The usual explanation is a layer of warm air aloft that never gave way.
What the cap is
Air temperature normally falls with height, and a rising parcel that stays warmer than its surroundings continues to rise on its own.
A cap is a shallow layer where temperature instead increases with height, so a parcel rising into it becomes colder than its surroundings and stops.
Until something removes or breaks through that layer, deep convection cannot form regardless of how unstable the air below it is.
Where the warm layer comes from
Caps often originate as air that has descended from higher levels and warmed by compression on the way down.
Warm dry air transported from elevated terrain can also spread out over cooler, moister air at lower levels, producing the same layered structure.
The result in either case is a stable lid sitting above a reservoir of warm humid air that would otherwise be rising freely.
Why a cap can make the day worse
While the lid holds, the air beneath it continues to heat and moisten through the day without releasing any of that energy.
Instability accumulates instead of being spent in ordinary afternoon showers, so by the time anything does break through, the available energy is far greater.
Storms that form late under a strong cap tend to be isolated and intense rather than numerous and weak, which is the configuration most associated with large hail and tornadoes.
What finally breaks it
Continued surface heating can eventually warm the low levels enough for parcels to punch through, though this often fails to arrive before the sun weakens.
Large-scale lifting associated with an approaching disturbance can raise and cool the capping layer until it no longer blocks ascent.
Terrain, converging surface winds and outflow from distant storms provide localized lifting strong enough to breach a cap in one spot while it holds everywhere else.
Why this is the hardest part of the forecast
The difference between a cap that holds and one that breaks can be a fraction of a degree in a shallow layer, which is at the edge of what models resolve.
The two outcomes are not close in consequence: nothing at all, or a small number of very dangerous storms in the same environment.
This is why outlooks sometimes carry a high risk that verifies as a quiet evening, and why a busted forecast in this situation reflects genuine physical uncertainty rather than carelessness.