In the midday and afternoon hours of Wednesday, March 18, 1925, the deadliest tornado in United States history and second-deadliest worldwide moved through Eastern Missouri, Southern Illinois and Southern Indiana, killing 695 people and injuring 2,027 more in what is sometimes known as the Tri-State tornado. The tornado touched down as part of a larger severe weather outbreak that hit the United States on the same day, and produced catastrophic damage across numerous villages and towns in all three states. Despite not being officially rated, it is widely accepted to have been equivalent to an F5 on the Fujita scale.
The tornado touched down in Shannon County, Missouri, moving to the northeast. The tornado immediately began to produce heavy damage to structures before directly impacting Annapolis, destroying ninety percent of the town and killing two people. The tornado then moved through Bollinger County, where it hit two schools and injured several children who were taking shelter. Deep ground scouring was observed as the tornado moved past Sedgewickville, and debris from the town was found almost 50 mi (80 km) away. It hit several other small communities, including Brazeau and Frohna, before crossing state lines into Southern Illinois and directly impacting Gorham, in which over half of the town's population was killed. To the northeast, it would hit the northern portions of Murphysboro, where 234 residents would die as the tornado barreled through the town at an estimated forward speed of 62 mph (100 km/h).
Further east, the tornado crossed into Franklin County, narrowly missing the towns of Royalton and Zeigler but devastating rural areas before heading towards the large mining town of West Frankfort. The tornado struck the northwest side of town, where in a manner similar to what was seen at Murphysboro, a number of densely populated neighborhoods, businesses and mining operations fell victim to the tornado. At the Peabody Mine 18 in Caldwell, a large 80-foot coal tipple weighing several hundred tons was blown over and rolled by the tornado. The tornado proceeded to devastate additional rural areas across Hamilton and White counties, between the two counties claiming 45 lives and injuring 140, 20 of whom later died. The tornado then crossed the Wabash River into Gibson County, Indiana. The town of Griffin was completely destroyed and the large town of Princeton ravaged before the storm passed into Pike County, where it finally dissipated, over three hours after touching down.
The tornado killed at least twenty farm owners in southeastern Illinois and southwestern Indiana, more than the combined total of the next four deadliest tornadoes in the recorded history of the United States. The tornado killed a combined total of 695 people, the majority of which occurred in Illinois.
The extratropical cyclone that set the synoptic stage for the outbreak was centered over northwestern Montana at 7:00 a.m. CST (13:00 UTC) on March 17. Meanwhile, a diffuse area of surface low pressure was centered near Denver, Colorado, in association with a lee trough. Occluded fronts extended from Hudson Bay southwestward into the northern Plains states and into the lee trough. The synoptic cyclone moved south-southeastward across the mountain states to eastern Colorado. A warm front stretched along the Gulf Coast, separating warm, moist air from cool, showery weather with areas of fog that extended from Texas to the Carolinas. A well-mixed early-season continental tropical (cT) air mass existed over West Texas and northern New Mexico. To the east, hot, dry, buoyant maritime tropical (mT) air was advecting from the Gulf of Mexico. Simultaneously, a mid- to upper-level shortwave trough likely approached the northwest coast of the U.S. and moved rapidly through the persistent ridge then digging southeastward across the Great Basin and central Rocky Mountains and emerging in the Plains over Colorado. This initiated a Colorado low cyclogenesis.
At 7:00 a.m. CST on March 18, the surface low-pressure area, at approximately 1,003 hPa (29.6 inHg), moved to far northeastern Oklahoma while the warm front shot north into the circulation where the front then extended eastward. A maritime Polar (mP) cold front draped southwestward across eastern Texas with a dry line forming directly to the south of the low. The open shortwave, likely somewhat negatively tilted, was continuing to approach from the northwest and an apparent outflow boundary moved just to the south of the warm front over northeastern Arkansas and northwestern Tennessee. Several weak pressure troughs were traversing the cool sector over the north-central U.S.. Surface temperatures in the warm sector near the dry line and warm front ranged from 60–70 °F (16–21 °C), and the dew point was 55–65 °F (13–18 °C), with higher values farther south and increasing over time as the deepening low-pressure area continued to pull up air from the Gulf of Mexico. This resulted in unstable air and lower cloud bases, or low LCL heights, which is favorable to tornadogenesis. From southeastern Kansas to Kentucky and Indiana, early morning showers and thunderstorms north of the low and warm front cooled and stabilized that air, retarding northward advancement of the front, and led to a sharp contrast in temperature from north to south. Such baroclinic zones are also associated with tornadic storms. Ahead of the surface dry line, which are uncommon as far east as the Mississippi River, an apparent "dry punch" of air aloft served to further increase instability. Concurrently, a capping inversion likely suppressed storms throughout the warm sector, leaving the Tri-State supercell undisturbed by nearby convection.
By 12:00 p.m. CST (18:00 UTC), the deepening surface low was centered over south-central Missouri, the shortwave axis was moving easterly and oriented over eastern Oklahoma, and the dry line was rapidly advancing eastward directly south of the low as the warm front, situated due east of the low, slowly shifted northward. Morning clouds cleared by midday across much of the Tri-State tornado's eventual path. A pronounced pressure trough extended northeast of the low and signaled its future track as a prefrontal trough formed southeast of the low ahead of the dry line. A bulge in the dry line may also have been forming slightly south of the low, and southerly to southeasterly surface winds were backing and increasing with time throughout the warm sector. The tri-state supercell formed in a highly favorable area just ahead of the triple point where the cold front, warm front, and dry line met. The supercell initiated very near the surface low and moved east-northeastward, faster than the low, such that the storm gradually deviated east of the low's track.
By 2:00 p.m. CST (20:00 UTC), the low was centered slightly south-southwest of St. Louis, Missouri, as the tri-state supercell neared the Mississippi River. Other storms in the warm sector, removed from the supercell, were initiating around 3:00 p.m. CST (21:00 UTC). Around 4:00 p.m. CST (22:00 UTC), the low's central pressure lowered to around 998 hPa (29.5 inHg), centered over south-central Illinois, as the supercell was moving into Indiana. This pressure is not particularly low compared to many other outbreak setups, but the pressure gradient was strong, which induced strong gradient winds and significant advection in the warm sector. A very strong low level jet was also in place just above the surface as winds veered with height, resulting in low-level curvature and long hodographs. Strong wind shear thus existed, with pronounced directional shear likely in the vicinity of the warm front, with winds at the 700 hPa height level west-southwesterly around 70 mph (110 km/h) and winds at the 500 hPa level about 90–110 mph (140–180 km/h). Theoretical hodographs returned estimated storm relative environmental helicity (SREH) values of 340 m2 s−2 in the vicinity of the tri-state supercell track. Strong thunderstorms were now scattered throughout the warm sector and a line of severe thunderstorms was occurring near the dry line. The supercell appeared to still be discrete and isolated, with a severe storm north of Cairo, Illinois, placed well to its south.