Supercell occlusion is the process by which a low-level mesocyclone gets wrapped in the storm's own rain-cooled outflow, becomes cut off from warm inflow, and weakens while a new mesocyclone often organizes farther downshear along the flanking line. The label comes from the way the storm's downdraft flow curls around behind the mesocyclone, similar in shape to an occluded front on a much larger scale.
How the wrap develops
A mature supercell has a rear-flank downdraft that surges around the back and southern sides of the low-level mesocyclone. Early in the cycle, that outflow reinforces convergence at the meso and helps concentrate rotation near the ground. As the outflow continues to accelerate, however, it eventually wraps completely around the circulation.
Once the wrap closes, the mesocyclone is sitting in air that has already been cooled by precipitation and downdraft, cut off from the warm, moist inflow that fed it. Vertical stretching weakens. Buoyancy in the column drops. The low-level rotation loses its energy source and spins down, even though the mid-level mesocyclone aloft may still look strong on radar.
The handoff to a new mesocyclone
Occlusion does not always end the storm. In a cyclic supercell, the outflow surge that killed one meso often sets up a new zone of convergence along the leading edge of the wrapped cold pool, downshear from the old circulation. Warm inflow lifted along that new boundary can build a fresh updraft, and the mid-level mesocyclone shifts with it.
A new low-level mesocyclone then organizes in the fresh updraft, often within 20 to 40 minutes of the previous one occluding. The result is a supercell that produces a series of low-level mesos in sequence, and often a family of tornadoes rather than a single long-tracked one. Adlerman, Droegemeier, and Davies-Jones documented this repeating occlusion behavior in numerical simulations, which established the modern picture of cyclic mesocyclogenesis.
How chasers and forecasters read it
Visually, occlusion is often marked by the wall cloud moving from below the main updraft base back into the precipitation, sometimes disappearing entirely behind a curtain of rain and hail. A rope-stage tornado shrinking to the north or northwest of the storm's forward motion is a common signature.
The next signal is a new lowering along the flanking line to the south or southeast. That new base often develops rotation over the next 10 to 20 minutes and can produce the next tornado in the family. On radar, the mid-level rotation may shift southeast along with the new updraft, and the hook echo can reorganize downshear from where the previous one wrapped up.
Where the label misleads
Not every occlusion produces a new mesocyclone. A supercell that ingests marginal inflow, moves into a cooler airmass, or fights a stronger cold pool can occlude and simply not recover. In that case the storm continues as a rotating rain producer without a strong low-level circulation.
The occluded stage can also be more dangerous than it looks, not less. A tornado in the wrapped area is often obscured by rain, and the visual cue that normally tells a chaser or spotter where to position is inside the precipitation core. In HP supercells especially, occluded circulations can hide a tornado from view while the new meso is still organizing.