The 24 August 2016 "Surprise" Tornado Outbreak: Part 1
Todd Holsten, National Weather Service Northern Indiana, Syracuse, IN
Jeff Logsdon, National Weather Service Northern Indiana
Amos Dodson, National Weather Service Northern Indiana

During the afternoon and evening of 24 August 2016 an outbreak of tornadoes occurred from central Indiana through northwestern Ohio.  This outbreak produced 23 tornadoes, including 11 in Indiana, 10 in Ohio, and 2 in southeastern Ontario.  This was the largest single tornado event ever to occur in August for both Indiana and Ohio.

An existing line of showers and embedded thunderstorms, downstream of a mesoscale convective vortex over Illinois during the morning of the 24th, devolved into isolated cyclic supercells by afternoon across central and eastern Indiana. While this event was poorly forecast- stemming from a lack of coherent numerical model guidance, near-term forecasts and trends from high-resolution numerical guidance allowed forecasters to rapidly shift focus and switch to an effective severe weather operations mode.

Even though 14 tornadoes occurred in the National Weather Service (NWS) Northern Indiana (IWX) county warning area on August 24th, no fatalities and only one minor injury occurred. Communication between emergency managers, trained spotters, media, public, and the NWS was crucial to preventing a loss of life. A local volunteer served as a virtual operations support team coordinator, monitoring reports from chasers in the field and relaying relevant information to the office through NWSChat (i.e., collaboration software). His timely relay of accurate storm information to the office, along with pictures and video, allowed for real-time verification and aided in effective warning decisions.

This presentation will focus on a local threat assessment using numerical model guidance from the National Centers for Environmental Prediction including near term trends of the High- Resolution Rapid Refresh model.  Mesoscale processes and storm-scale evolution that occurred during this event will also be examined. Finally, IWX warning operations will be reviewed, including radar interrogation and warning decision making considerations that lead to timely impact-based warnings.