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December 1957 tornado outbreak sequence
The December 1957 tornado outbreak sequence was a significant tornado outbreak sequence that affected the southern Midwest and the South of the contiguous United States on December 18–19, 1957. The outbreak began on the afternoon of December 18, when from the west a low pressure area approached the southern portions of Missouri and Illinois.
==Meteorological synopsis==
At 6:00 a.m. CST/12 UTC on December 18, 1957, a vigorous shortwave trough entered the Great Plains with a cold front moving east across Oklahoma and Kansas.〔(【引用サイトリンク】url=http://docs.lib.noaa.gov/rescue/dwm/data_rescue_daily_weather_maps.html )〕 A dissipating stationary front over Oklahoma underwent frontolysis〔 and later redeveloped as a warm front which extended across central Illinois. By 3:00 p.m. CST, surface dew points reached the low 60s °F across portions of southeast Missouri and southern Illinois, including the St. Louis area. Although most areas were then recording overcast weather conditions,〔 a strong upper-level jet stream helped impart synoptic-scale lifting,〔 a factor that favors updrafts, and little vertical mixing occurred, so instability remained favorable for thunderstorm development. Additionally, very cold temperatures〔 following a surface cyclone〔 raised the lifted index to -6 due to high adiabatic lapse rates.〔 Wind speeds at the middle level of the atmosphere, just under from the ground, were close to as well.〔 Conditions were therefore very conducive to a large tornado outbreak on the afternoon of December 18.
Similarly favorable conditions occurred a day later, as a warm and moist air mass spread northward from the Gulf of Mexico. In fact, temperatures in the Mississippi Valley and the upper Midwest approached record highs for December. St. Louis and Detroit, Michigan, recorded afternoon highs of 57 °F, while Chicago, Illinois, measured 56 °F, only eight degrees lower than the local record high for December 19. Local residents and meteorologists described temperatures as being "springlike" for the time of year, even though meteorological winter was due to begin on December 23. Farther south, temperatures along the Gulf Coast reached the low 70s °F. Just as on December 18, a second tornado outbreak occurred in a broad warm sector from Arkansas to Illinois and south to Alabama.
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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