In the upper atmosphere, an unusually potent upper-level disturbance (shown above) was forecast to approach the Iowa-Minnesota region early in the evening. The spreading apart or “fanning” of winds (as noted in the figure) was particularly conducive to concentrating a pocket of strong upward motion over the region. The intense wind shear and strong dynamics associated with the storm system was forecast by models to create not only supercells but also bow-echo-like complexes with embedded tornadoes and downbursts.

Source: Washington Post December 17, 2021 00:41 UTC