Surveying Devastation - A Look into the Post-Event Assessment and Rating of the Greenfield, Iowa 21 May 2024 EF4 Tornado

On 21 May 2024, the community of Greenfield, Iowa was struck by a long-track, catastrophic tornado that carved a nearly continuous damage path across southwest Iowa for over 40 miles. The tornado was part of an outbreak that produced 13 tornadoes in Iowa, killing 5 people and devastating hundreds of properties, many of which were in Greenfield.

To assess the magnitude and scope of damage, a ground-based survey was conducted by multiple teams of meteorologists from the Des Moines National Weather Service (NWS) on the following two days, 22-23 May 2024. The immense debris field left by the Greenfield tornado presented challenges for the survey team, requiring the in-town ground-based survey to be conducted entirely on foot. Surveyors leveraged the Enhanced Fujita (EF) Damage Assessment Toolkit to collect damage information, estimate wind speeds, and determine a tornado rating. The Enhanced Fujita Scale utilizes damage indicators to determine an estimated three-second wind gust. The three-second wind gust can vary significantly from instantaneous measurements. In Greenfield, the damage indicators for the EF-3+ damage consisted mostly of one or two family residences of varying age and construction. In the following days, additional damage information from satellite, security camera footage, chaser videos, and Civil Air Patrol overflight was integrated into refining the tornado track and filling in any gaps left from the ground-based survey. Per NWS directives, a suspected EF-4/EF-5 tornado requires a Quick Response Team (QRT) that consists of wind experts to provide input for the rating. Collaborative efforts between the QRT and NWS Des Moines officials resulted in an EF-4 rating for the tornado with estimated wind speeds of up to 185 mph (82 m/s).

Damage survey objectives go beyond data collection and analysis and have become an important part of an event. NWS Des Moines surveyors talked with storm victims, asked about the storm event sequence, recollections, and warning reception details. NWS Des Moines meteorologists engaged directly with community partners, including the governor of Iowa, state and county emergency management, state highway patrol, and congressional representatives. Weather briefings were conducted with county emergency management routinely both in-person and virtually for over a month following the disaster. This presentation will discuss post-storm activities, including the unique survey details of the 21 May 2024 Greenfield, Iowa tornado.