On November 17, 2013, the deadliest and costliest November tornado outbreak in Illinois history took shape, becoming the fourth-largest for the state overall. With more than 30 tornadoes in Indiana, it was that state's largest tornado outbreak for the month of November, and the second largest outbreak recorded in Indiana. Associated with a strong trough in the upper levels of the atmosphere, the event resulted in 77 tornadoes tracking across regions of the Midwest United States and Ohio River Valley, impacting seven states. Severe weather during the tornado outbreak caused over 100 injuries and eleven fatalities, of which eight were tornado related. Two tornadoes—both in Illinois and rated EF4 on the Enhanced Fujita scale—were the strongest documented during the outbreak and combined for five deaths. In addition to tornadoes, the system associated with the outbreak produced sizeable hail peaking at in diameter in Bloomington, Illinois, as well as damaging winds estimated as strong as in three locations.
The development and progression of a severe weather event on November 17 had been well anticipated, and appeared in Storm Prediction Center products as early as November 12. The first storms associated with the event formed during the afternoon and evening hours of November 16 over the Great Plains, producing primarily hail and strong winds. However, tornadic activity was limited to November 17, as individual supercell thunderstorms tracked across the Midwest United States, at times producing long-tracked tornadoes. The first tornadoes formed over Illinois, while the final tornadoes developed over Tennessee. Towards the end of November 17, these individual systems had merged into an extensive squall line that tracked eastward across the Mid-Atlantic states, producing damaging wind before exiting into the Atlantic Ocean early on November 18.
With damage estimated at $1.6 billion, the tornado outbreak became the seventh weather event and fifth tornado outbreak in the U.S. to accrue over $1 billion in damage that year. Tornadoes alone were responsible for $1.067 billion in damage, with the EF4 tornado that struck Washington, Illinois accounting for $935 million of the total. In addition to structural damage, widespread power outages affected thousands of electricity customers across the same regions impacted by the tornado outbreak and subsequent squall line. Non-tornadic deaths and injured include one killed in Jackson County, Michigan, when a tree fell on a car, one killed by live wires in Shiawassee County, Michigan, one killed after touching a downed wire in Detroit, and two minor injuries in a home damaged by wind in Ohio.
Meteorological synopsis and forecasts
Based on computer models, the Storm Prediction Center (SPC) began to assess the possibility of a widespread severe weather event for November 17 as early as November 12, though too much uncertainty existed at the time to delineate the probability and scale of the event. As models gained a better consensus, the SPC later anticipated substantial increase in atmospheric instability along the periphery of a potential cold front, indicating the risk of damaging wind gusts and tornadoes. Additional confidence over the next 24 hours resulted in a substantial expanding of this risk area into the lower Great Lakes on November 14, valid for Day 4. The overlap of increasing moisture in the atmosphere, wind shear, and high levels of CAPE were expected to contribute to the potential severe weather. The development supercells and later squall lines was anticipated, and was also expected to impact the Mid-Atlantic states. At around midday on November 15, the SPC issued a slight risk for severe weather for the 16th in regions concentrated primarily around Iowa and Missouri, forecasting the possibility of storms particularly during the evening to overnight hours of November 16. The system associated with those storms was to track eastward and strengthen, and as a result the SPC also issued a Day 3 slight risk earlier on November 15 for a large swath of the Eastern United States in effect for November 17. The slight risk included a 30% chance of severe thunderstorm activity was forecast for an area concentrated on Indiana and Ohio.
