The region of the United States from North Dakota eastward to Michigan has more incidences of cold core funnels than anywhere else in the country.
Cold core funnels are condensation funnels that form in and around weak convective showers. They are not tornadoes because they usually do not touch the ground and they usually do not produce violent wind. The updrafts within these showers are relatively weak compared to those found in supercell thunderstorms, so there is less energy available for rotation.
The term "cold core funnel" comes from the fact that these weak funnel clouds form under the base clouds of showers and thundershowers in a mild, low-pressure environment. The air is not as unstable as it is in severe weather cases, and there usually is not much temperature contrast across the region. On occasion, cold core funnels do touch the ground and they are capable of low-end damage similar to a weak tornado.