This is serious. It means that National Weather Service radar or a storm spotter has detected or seen a powerful storm. These types of storms are capable of causing serious damage, either from hail that is larger than an inch or winds that are in excess of 58 mph. These are targeted to just the area in the storm’s path, unlike a watch, which covers many counties. If you are outdoors and in the path of the storm, seek shelter or try to pull off the road to avoid driving in the storm. These storms can generate tornadoes with little warning.