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.November 17, 2013 Severe Weather and Tornado Outbreak, National Weather Service Indianapolis, Indiana, accessed May 25, 2019 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 of November 16 over the Great Plains, primarily producing 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.

On November 16 at around 0700 UTC, the SPC upgraded their maximum convective outlook for the following day to a "moderate" for areas of the Ohio River Valley and the Great Lakes region on the basis of destabilizing atmospheric conditions, favorable wind shear, and an approaching shortwave trough. The outlook was maintained for the 1730 UTC Day 2 update on November 16, but was also expanded to the west and a mention of the risk for strong tornadoes was added to the discussion. Isolated storms began to form during the afternoon of November 16 as the overall weather system swept across the West North Central region and into the East North Central region of the United States. With development assisted by a dry line, the storms did not produce any tornadoes, but caused strong winds in hail as large as softballs in northeastern Kansas. The threat of additional severe activity that day was mitigated by cooling surface temperatures and marginal moisture and instability in the lower atmosphere. By the end of the day, convection had moved closer to the Mississippi River, with the focus of intensifying storm activity shifting southeastwards to areas in and around the Ozark Plateau.

November 17

With the overall upper-level system expected to track eastward across the United States High Plains on November 17, the SPC issued a slight risk for severe thunderstorm activity for an area surrounding the confluence of the Mississippi and Missouri rivers, valid for the early morning of November 17. Model data indicated the weakening of the capping inversion coinciding with increasing instability and wind shear, which would lead to the enhanced potential for storms and tornadoes as the night progressed and the low-level jet strengthened. This overnight severe weather largely failed to develop, leading to only a very few isolated reports. However, at 0600 UTC on November 17, the SPC upgraded to a high risk for severe weather on the 17th (the second latest date in the year a high risk has been issued since 2000, and the latest in the Midwest) for regions of Illinois, Indiana, southwestern Michigan, and western Ohio, in anticipation of conditions becoming increasingly favorable for a significant tornado outbreak; this area of high risk was expanded at 1300 UTC to encompass a roughly circular region of the United States Midwest containing nearly 19 million people. Those same regions were listed as having at least a 30% chance of tornadoes, coupled with a 45% chance for wind. The issuance of a high-risk zone reflected the anticipation of an intense upper-level trough and a strong mid-level jet stream producing highly conducive conditions for the development and prolonging of severe weather.

The first signs of convection associated with the broad weather system began to form around dawn on November 17 over portions of Illinois, Iowa, and Missouri, triggered by a propagating cold front. At 1440 UTC, the SPC issued a PDS tornado watch concentrated mostly around Illinois, citing the nearly certain likelihood for severe weather over the region. Throughout the morning, discrete supercell thunderstorms developed ahead of the cold front across the length of Illinois; their expected persistence eastward prompted the SPC to issue a second PDS tornado watch at 1621 UTC, this time for areas of Indiana, Michigan, and Ohio. One of these storms would maintain a mesocyclone over a path. The first tornado during the outbreak touched down 1652 UTC and tracked across Peoria and Tazewell counties in Illinois; the twister was later rated EF2. This was followed shortly after by a long-tracked EF4 tornado, the strongest and costliest of the day, which tracked through multiple Illinois counties.

By around 1730 UTC on November 17, the system associated with the tornado outbreak had become centered near Milwaukee, Wisconsin. As intense storm cells propagated across Illinois and Indiana, strong low-level wind shear in the vicinity of a warm front coupled with the weather system made areas as far north as Northern Michigan a viable environment for tornado development. Numerous tornadoes touched down as a result of the supercells, several of which were strong to violent. In addition to spreading northward and eastward, severe thunderstorms later spread southward into the Cairo, Illinois area and later Kentucky as a result of falling barometric pressures paving the way for the spread of moisture over the area. Additional significant tornadoes occurred in that area as well. At around the same time, preexisting storms over Indiana and Illinois had begun to consolidate into a somewhat disorganized albeit extensive squall line. As a result of the merging, the threat of tornadoes generally decreased slowly as the storms moved from Indiana into Ohio while the development of a major wind event seemed more apparent, particularly in Ohio. Despite the change in storm mode, additional strong tornadoes still occurred as far east as Western Ohio due to circulations embedded in the squall line. This trend in severe weather also took place in other areas as clusters of isolated supercells began to coalesce into singular lines of storms.

By the 0100 UTC update of the convective outlook on November 18, storms had cleared the high risk area from the previous outlooks. However, a moderate risk for severe weather was maintained across parts of central/eastern Ohio, western Pennsylvania, and western New York ahead of the rapidly moving convective system. In addition to a 45% probability of wind, up to a 10% probability of tornadoes was maintained, with the highest threat in central/eastern Ohio, although owing to extremely marginal surfaced based instability, only one or two further isolated tornadoes were reported in this area beyond 0100 UTC. By the late evening, most of the discrete storms that existed earlier in the day had merged into squall lines and bow echoes, with any remaining mesocyclones embedded within these bands rather than individual storms. Forced along the extent of a cold front, the primary squall line raced eastward across Ohio at approximately , reaching Pennsylvania by 0200 UTC. As a result of decreased atmospheric instability, the threat for tornadoes and hail had greatly diminished by this time, while the probability for damaging winds remained elevated to high; as a result, watches issued east of Ohio were severe thunderstorm watches rather than tornado watches. The final tornado associated with the outbreak was an EF0 that dissipated at 0340 UTC in Lincoln County, Tennessee (south of the area impacted by the main outbreak of tornadoes); based on Central Standard Time, this would have still occurred on November 17. By around 0800 UTC, the line of storms was quickly approaching the United States East Coast before entering the Atlantic Ocean.

Confirmed tornadoes

Washington, Illinois