The Tri-State Tornado of 1925 occurred due to the confluence of multiple meteorological conditions. An eastward moving low pressure system pushed a warm front northward, while a cold front simultaneously approached from the southwest. This created a large draw of warm moist air from the Gulf of Mexico into the tri-state region of Missouri, Illinois and Indiana. These conditions coupled with an accelerated jet stream triggered the event.
The Tri-State Tornado is formally classified as part of a tornado outbreak. A total of 12 other tornadoes were confirmed in the same area and on the same day. The Tri-State Tornado was the largest, measuring over a mile wide, and it traveled in a northeastern direction from Missouri, across southern Illinois and into Indiana, carving a path 219 miles long. It is estimated that it packed winds of up to 300 mph and travelled at speeds approximating 70 mph. From formation until dissipation it lasted for 3.5 hours. In total, 747 people were killed by the outbreak, 649 by the Tri-State Tornado alone. As of June 2014, it still ranks as the deadliest tornado in U.S. history.
At the time of the event, the U.S. Weather Bureau was charged with the weather forecasting for the country. It was part of that agency's policy not to publicly acknowledge tornadoes in its weather forecasts. Tornado formation was seen as too unpredictable, and efforts to forecast them were thought to provoke needless public panic.