EF3 Tornado — Warren, Iowa

2019-08-20 · near Indianola Arpt, Warren, Iowa

Event narrative

The second of three tornadoes associated with a fast moving QLCS. The particularly strong tornado moved across rural Warren county producing mainly crop and tree damage. However, the tornado encountered the several large steel buildings with I-beam construction. Significant damage occurred to these buildings with some being totally destroyed producing the EF3 damage. The tornado continued to the southeast beyond this area remaining in rural crop and woodland areas.

Wider weather episode

Generally speaking, a bit of everything occurred during an overnight/early morning severe mesoscale convective system that rolled across Iowa. The predominant weather revolved around damaging winds and heavy rainfall, though a few instances of severe hail and low end tornadoes were experienced as well. Among the reports, numerous trees were downed, buildings damaged or destroyed from straight line winds or one of the weak tornadoes, and numerous instances of 2 to 4 plus inches of heavy rainfall.

On the setup side of the coin, a lot of factors were in line for the event to potentially be a sustained nocturnal severe weather event. The previous day, the 19th, saw surface high pressure begin to exit the region eastward and allowing southerly return flow to work into eastern Nebraska and western Iowa. Tied in with mid to upper 70 dew points and steep mid-level lapse rates, a large area of 4000 plus J/kg CAPE values developed across eastern Nebraska and western Iowa and slowly built eastward. Additionally, effect bulk shear was consistently in the 40 to 50 kt range and low level shear roughly oriented to expected storm motion. As the evening progressed and the low level jet finally began to ramp up, boosting warm air advection and positive moisture flux, storms began to develop and eventually congeal. Organization, at least with regards to strong to severe criteria, was not immediate, waiting until around 5am local time to produce its first severe report. Severe reports then continued as the system moved southeast over the next 4 plus hours.

View location on OpenStreetMap → (41.2582, -93.5239)

Source: NOAA Storm Events Database, event_id 850374. Narrative written by the NWS forecast office that issued the report.