Storm track arrows are displayed when the movement of a storm cell is predictable. The number associated with the arrow represents the storm's top (in hundreds of feet). The arrow orientation represents the direction of the storm's movement over the course of fifteen minutes. All arrows are the same length, so the length of an arrow does not represent the speed of the storm's movement.
Storm tracks are updated every fifteen minutes. Radar imagery is updated every five minutes. Storm track arrows are not always expected to begin at the center of a storm.
Radar Storm Track Arrows