I would like to start with what we know about tornadoes and epidemics and what is said to be the cause of such catastrophic events, such as the outbreak that occurred on November 17 in the Midwest, portion of the United States. Tornado outbreaks occur when there are multiple tornadoes said to be produced by the same weather system. The classification of an outbreak can vary depending on interpretation. It is said that in order to be classified as a “tornado epidemic” there must be a certain number of tornadoes that touch down. The United States is said to have the most tornadoes of any country. This can be attributed to the mid-latitude location. The Rocky Mountains have the ability to block humidity and "arch" the atmospheric flow; thus forcing a lot of dry air to the mid-tropospheric levels, due to downslope winds. While the Rocky Mountains are forming a “dry line,” the Gulf of Mexico on the other hand contributes a low level of humidity. This mixture of warm air and cold air is responsible for the large number of tornadoes that form in the United States. These events are most prevalent in the Midwestern and Southeastern United States, along with the Great Plains, also known as (tornado alley), although some of these systems have been known to move as far north as Canada. The tornado outbreak that occurred on November 17 was said to be one of the largest outbreaks to occur in the last eight years. This particular storm spawned a total of 72 tornadoes that crossed seven states, according to the National Weather Service. This epidemic is considered the fourth largest of its kind, especially this time of year. This epidemic covered a vast amount of territory; just to give an example there were t...... middle of paper ......t-tornado-outbreak/20074227 http://en.wikipedia.org/wiki/Tornado_outbreak Glickman, Todd S. (ed.) (2000). http://www.crh.noaa.gov/ind/?n=nov172013outbreak http://www.accuweather.com/en/weather-news/how-rare-was-midwest -tornado-outbreak/20074227 http://en.wikipedia.org/wiki/Tornado_outbreak Glickman, Todd S. ( ed.) (2000). Glossary of Meteorology (2nd ed.). American Meteorological Society. ISBN 978-1-878220-34-9. ^ Grazulis, Thomas P. (1993). Significant tornadoes 1680-1991: chronology and analysis of events. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 1-879362-03-1. Jump up ^ Galway, Joseph G. (1977). "Some climatological aspects of tornado outbreaks". Mon Weather Rev. 105(4):477–84. Bib code:1977MWRv..105..477G. doi:10.1175/1520-0493(1977)1052.0.CO;2. http://www.noaanews.noaa.gov/2011_tornado_information.html