December 1957 tornado outbreak sequence
| Type | Tornado outbreak |
|---|---|
| Duration | December 18–19, 1957 |
| Tornadoes confirmed | 37 |
| Max rating1 | F5 tornado |
| Duration of tornado outbreak2 | ~1½ day |
| Highest winds |
|
| Largest hail | .75 inches (1.9 cm) in diameter in St. Francois County, Missouri on December 18 |
| Casualties | 19 fatalities, unknown injuries |
|
1Most severe tornado damage; see Fujita scale 2Time from first tornado to last tornado |
|
1Most severe tornado damage; see Fujita scale
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.
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. 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 6 kilometres (3.7 mi) from the ground, were close to 70 miles per hour (110 km/h) as well. Conditions were therefore very conducive to a large tornado outbreak on the afternoon of December 18.
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