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May 2003 tornado outbreak sequence
The May 2003 tornado outbreak sequence in the United States was a series of tornado outbreaks that occurred from May 3 to May 11, 2003. Tornadoes began occurring over the affected area on April 30, but the most prolific continuous period was the seven-day period of May 4–10. There were 401 tornado reports in 19 states and 1 Canadian province, 1,587 reports of large hail, and 740 reports of wind damage. More severe weather broke out this week alone than any other week in U.S. history.〔 The old record for most tornado reports in one week was 171 during the week of the May 1995 tornado outbreak sequence (May 12 to May 18, 1995).
==Meteorological synopsis==
The main meteorological factor for this series of tornado outbreaks was the presence of a persistent 500 mb trough over the western United States, coupled with a series of shortwave disturbances which propagated through the central and eastern United States. These shortwaves provided a mechanism for the deepening of surface low pressure areas, which followed the upper level flow from southwest to northeast. The cyclones induced a strong north to southeasterly flow in the low levels of the atmosphere (1000 mb, 850 mb) off the Gulf of Mexico. This persistent flow provided an abundance of warm, moist maritime tropical air in the central and eastern US.
The interaction between warm, dry air from the western United States and the moist Gulf airmass resulted in a boundary known as the dry line. This boundary, along with other factors provided a source of lift promoting thunderstorm development. CAPE describes the instability in the atmosphere and the tendency for it to rise; high CAPE values are usually associated with severe weather. CAPE values during the period of April 30–May 11 were extremely high in the affected areas. A strong, persistent southeast to northwest upper level flow contributed to wind shear, and induced strong rotation in many of the thunderstorms that developed. These rotating thunderstorms, or supercells, are capable of spawning tornadoes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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