When shear is larger, multicell storms are likely to form. These are storms that consist of multiple convective updrafts and downdrafts. A multicell storm forms when new convective cells develop along the boundary of the cold pool originating from an older cell. Such a boundary is often called an outflow boundary or gust front. The initiation of new cells is most likely to happen on the downshear side of the convective complex. Downshear is the direction from which the low-level wind blows when moving with the average wind (the movement expected for individual storm cells).

Fig. 1.8. (left) Cross-section through a multicell storm. New cells are formed as a result of the interaction of the gust-front with a sheared environmental flow. (right) The direction of movement of a storm system and the cells within that system are usually not the same.

Using numerical models it has been shown that a strong upward directed flow can form along the edge of the boundary when the vorticity within the cold pool and that of the environmental flow balance each other. A resulting erect upward flow can bring new parcels of air quickly to their levels of free convection and new convective cells may result. When this process repeats itself a multicell cluster forms, that consists of convective cells in various stages of their life cycles. The direction of movement of the individual cells is typically not the same as the direction of propagation of the multicell complex (see fig. 1.8.).

Numerical simulations show that the required amount of shear to trigger new convective cells differs a lot between situations. When there is a large difference between the level of free convection and the lifted condensation level and this layer is dry, convective cells may have trouble to initiate. In that case a lot of entrainment takes places into initiating updrafts and 10-20 m/s of shear in the lowest 3 kilometres of the atmosphere may be required to trigger new cells. This value may be much lower, even below 5 m/s, when the lifted condensation level and LFC are collocated and the environmental air is moist. In those situations, the new cells will often form at different places along the cold-pool boundary, because they are triggered so easily.

In contrast, when stronger shear is needed and indeed present, the generation on new cells is preferred on one specific flank of the complex and a well-organised complex may result that has a higher threat of producing severe winds and hail.

Compared to single cell storms, multicell clusters have a higher probability of producing severe weather. This includes damaging winds, large hail and occasionally (mostly weak) tornadoes.