Extratropical cyclones arise through a process called cyclogenesis. While cyclogenesis occurs during every month of the year, formation into significant extratropical storms almost always occurs during the late fall, winter, or early spring. These systems are, therefore, often referred to simply as "winter storms".
The most widely recognized manifestation of cyclogenesis is an area of relatively low surface pressure. Falling pressure at the earth’s surface occurs in response to air flowing away from the atmospheric column above the surface faster than it can be replaced. As long as this low-pressure area is maintained, air from higher-pressure regions will flow toward its center. This movement of air is what buildings (and people) at the surface experience as wind.
The primary energy source for extratropical cyclones, or winter storms, is the degree of contrast in temperature between the poles and the equator. This contrast is strongest, and thus the conditions for strong cyclogenesis are most favorable between 30 and 60 degrees north latitude in the region near the east coasts of Asia and North America. The winter storms affecting Europe typically originate in the Atlantic Ocean to the east of North America and subsequently move eastward toward and across the European continent. Winter storms affecting North America often originate in the Pacific Ocean and the Gulf of Mexico and subsequently move eastward. Pacific storms often develop as they approach the U.S. West Coast and sometimes redevelop as they pass over the Rocky Mountains. Storms originating in the Gulf of Mexico, like the 1993 “Storm of the Century”, often redevelop off the Mid-Atlantic coastline and intensify into blizzards which have come to be known as Nor'easters. Blizzards are intense extratropical cyclones characterized by strong coastal winds, heavy frozen precipitation, and extremely cold temperatures.
Extratropical cyclone systems typically comprise multiple areas of relatively low and high pressure, the locations of which can change quickly and frequently. This is in sharp contrast to the homogeneous, symmetric structure that is observed for tropical systems in low latitudes. As an extratropical system evolves, the interaction of these low and high areas creates changes in the horizontal pressure gradient field. The evolving pressure gradient field drives the response of the associated windfield. Extratropical cyclones typically do not achieve the wind speeds of the most intense tropical cyclones. However, at a single time, an extratropical cyclone system’s damaging windfield can affect an area of tens of thousands of square kilometers. Individual locations can be subjected to periods of high winds for up to several days. Examples include the winter storms Lothar and Martin in late December 1999 in Europe and the Storm of the Century in the Eastern U.S. in March of 1993.