On December 18–20, 1957, a significant tornado outbreak sequence affected the southern Midwest and the South of the contiguous United States.
The outbreak sequence began on the afternoon of December 18, when a low-pressure area approached the southern portions of Missouri and Illinois.
Supercells developed and proceeded eastward at horizontal speeds of 40 to 45 miles per hour (64 to 72 km/h), yielding what was considered the most severe tornado outbreak in Illinois on record so late in the calendar year.
CST (21:00 UTC), surface dew points reached the low 60s °F across portions of southeast Missouri and southern Illinois, including the St. Louis area.
Additionally, very cold temperatures following a surface cyclone raised the lifted index to −6 due to high adiabatic lapse rates.
[15][16][17] Similarly favorable conditions occurred a day later, as a warm and moist air mass spread northward from the Gulf of Mexico.
[78][80][76] The United States Weather Bureau documented "very heavy destruction", albeit in a small area, and referred to the Sunfield Y as having been "wiped out".
[78][77][82] The United States Weather Bureau documented that the tornado reached a maximum width of 200 yards (180 m) and traveled 5 miles (8.0 km).
CST (21:55 UTC) about 10 mi (16 km) west-southwest of downtown Mount Vernon, whence local police monitored its movement from patrol vehicles and disseminated reports via radio.
In Mount Vernon, the tornado damaged or destroyed about fifteen to twenty buildings, including the Block Grade School, where students left only half an hour beforehand.
On December 19, a dust storm with 50-mile-per-hour (80 km/h) wind gusts tossed three roofs onto vehicles and reduced visibility to just 1⁄2 mi (0.80 km) in Dallas, Texas.
[84][18][42][85][63][28] After severe weather left the Murphysboro area in Illinois, police officers, firefighters, deputies, and other assistance were called out to the worst-hit subdivisions, Country Heights and Crown View.