The 30 June 2018 Des Moines Flash Flood Emergency Event

A significant flash flood event evolved over the Des Moines metro on June 30-July 1, 2018. By the evening of June 30th, storm development began along a frontal boundary slowly sagging south through central Iowa. Slow, northerly storm motions coupled with southerly “backbuilding” of storms along the frontal boundary led to training storms over the same areas for several hours. The environment was primed for heavy rainfall given precipitable water values near or exceeding 2” and deep warm cloud depths. Rainfall rates of up to 5” per hour were realized within the heaviest cores into the late evening hours and quickly dumped 4-8” of rain over much of the Des Moines metro area, including reports of up to 10” in the northern suburb of Ankeny.
Flash flooding rapidly developed as the exceptionally heavy rain fell within an urban area. The dangerous situation eventually warranted a Flash Flood Emergency as numerous high water rescues took place and significant water was reported in areas that do not typically see flooding. One fatality occurred as a man was swept area after leaving his vehicle stranded in swiftly moving water. Damage was reported at a considerable number of homes and businesses. The high water also prompted the closure of several urban roads and highways, including a stretch of Interstate 35. According to an Exceedance Probability Analysis (AEP), the average recurrence interval (ARI) for this event ranged from 5 years (20% chance of yearly recurrence) for a 30 minute rainfall period) to a 500 year event (0.2% chance of yearly recurrence) for a 3 hour rainfall period.
The presentation will provide an overview of the meteorology, near storm environment, and radar evolution of the event. It also takes a look at the warning decision challenges including the issuance of the Flash Flood Emergency, as well as unique challenges associated with urban flooding in the Des Moines metro area.

Cory Martin
NOAA/NWS Weather Forecast Office, Des Moines, Iowa

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