A mid-latitude cyclone, also known as an extratropical cyclone, is a huge low-pressure system of synoptic scale that is found in the middle latitudes. It has a cyclonic flow, but it is not a tropical storm or a hurricane. It causes windy and wet weather, and occurs most often in the United States during the winter season.
Mid-latitude cyclones have a definite life cycle. They originate as a frontal wave, grow to an occluded cyclone and finally die over a period of several days as cut-off cyclones. Their formation is associated with fronts, and they form easily in winter because of the large difference in the temperature between the north pole and the equator. A jet stream, which brings cool air from the north to the southern regions of United States, plays an important part in the location of these cyclones.
One system can cause extreme weather across the entire United States. Sometimes winds with strength similar to hurricane-force winds develop during a mid-latitude cyclone, causing severe damage along the coasts. If the cyclone slows down and persists for an extended period of time over an area, the enormous precipitation, mostly snow, damages trees, power lines, buildings and bridges. The "comma" appearance of a mid-latitude cyclone on satellite pictures makes its identification easy.