December 1957 tornado outbreak sequence

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. 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.

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.

Mt. Vernon, Illinois

The second deadly tornado to develop on December 18 became one of two tornadoes to strike Mt. Vernon in Jefferson County, Illinois. The first tornado was the strongest and formed at 3:55 p.m. CST about 10 miles (16 km) west-southwest of downtown Mt. Vernon, whence local police patrol vehicles and radio reports monitored its movement. As the tornado approached downtown Mt. Vernon, it completely leveled small, "prefabricated," ranch-style homes in southwest Mt. Vernon; damage in this area was later rated F4 by meteorologists. Thereafter, the tornado weakened as it neared downtown Mt. Vernon and may have even dissipated before reforming as a separate tornado to the north. It then continued on a skipping path and caused less severe damage to older homesites northeast of Mt. Vernon. In Mt. Vernon, the tornado damaged or destroyed about 15 to 20 buildings, including the Block Grade School, where students left only 30 minutes beforehand. The funnel was described as being "swirling black clouds," filled with debris, that vanished northwest of downtown Mt. Vernon.

Gorham–Murphysboro–DeSoto, Illinois

The deadliest tornado of the entire tornado outbreak sequence closely followed part of the March 18, 1925, Tri-State Tornado path, and indeed affected the same towns of Gorham, Murphysboro, and De Soto that were hit in 1925. It may have touched down in eastern Missouri but was first observed at 4:45 p.m. CST (some sources say 4:30 p.m. CST) in Gorham, destroying or damaging 40 homes there. As it neared and passed through the southeast side of Murphysboro, the tornado paralleled the Big Muddy River, moving east-northeast. It produced the worst damage, posthumously rated F4, in this area, destroying old buildings; however, the most intense damage only affected a small section of southeast Murphysboro, where 10 of the 11 deaths occurred. Afterward, the tornado continued on to damage parts of De Soto, Hurst, and Bush. Murphysboro was powerless for almost three days as most utilities were in the worst-hit area. The tornado injured 200 people along its path.

Non-tornadic effects

Severe thunderstorms in connection with the outbreak on December 18 produced hail up to .75 inches (1.9 cm) in diameter in St. Francois County, Missouri. Severe winds estimated at up to 60 miles per hour (97 km/h) also affected the Hannibal area in that state, downing power lines and wires. In addition to the six known tornadoes in Missouri, unconfirmed reports of tornadoes occurred in Jefferson County, along with many reports of funnel clouds elsewhere in the state. Other unconfirmed tornadoes were reported in other states, including an alleged tornado that hit Rockville, Indiana. 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 0.50 miles (0.80 km) in Dallas, Texas. In addition to the three confirmed tornadoes that hit the state that day, severe winds in Tennessee, reportedly unrelated to tornado activity, destroyed farm buildings, tore off roofs, and downed trees and electrical wires; though these may have been due to tornadoes, none was confirmed. In addition to two confirmed tornadoes, unconfirmed reports of tornado damage arrived from Royalton and Elkville, Illinois; though attributed to thunderstorm winds, these damages may have been due to tornadoes. Additionally, severe thunderstorm activity on December 18–19 contributed to severe flood conditions across parts of southern Illinois and in Missouri.

Aftermath

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. Then-Illinois Director of Public Health Dr. Roland Cross also sent for the hard-hit Mt. Vernon area. Then-Illinois Governor William G. Stratton directed Illinois state police to the affected areas of southern Illinois and also readied the Illinois National Guard for possible deployment to the region.

Oddities/records

According to official Storm Data records from the Storm Prediction Center, the outbreak sequence was the most intense December severe weather event in U.S. history based on the number of intense tornadoes. The outbreak on December 18, in terms of F2–F5 tornadoes, was the most intense ever to affect the U.S. state of Illinois—and also, with 13 deaths in the state, the deadliest to affect Illinois in meteorological winter—even though the strongest tornadoes affected a relatively limited area of the state. The entire tornado outbreak sequence produced 37 recorded tornadoes, and fully 78.4% of them were rated as significant, F2–F5 events based upon damage estimates by the Fujita scale. Of these, 20 occurred in Illinois on December 18 alone, the largest such number of F2–F5 tornadoes in that state by a single outbreak. The second-highest total was 10 F2–F5 events in the outbreak of April 19, 1996, and the third-highest was seven such events on June 5, 2006. The tornado outbreak sequence was also responsible for the largest number of F2–F5 tornadoes in a December outbreak nationwide, including the highest number of violent, F4–F5 tornadoes by a tornado outbreak in December—both records having been achieved by midnight CST/0600 UTC on December 18. The outbreak remains only the second of two to produce an F5 tornado in December, the other being the December 5, 1953, tornado outbreak in Vicksburg, Mississippi.