Flash Flood — Polk, Iowa

2018-06-30 · near Elkhart, Polk, Iowa

Event narrative

Emergency manager reported multiple water rescues across Ankeny, Johnston, Des Moines and Urbandale. Vehicles were stalled out in street flooding with water half way up the doors of some cars.

Wider weather episode

A frontal boundary was draped across areas just west of Iowa during the morning hours of June 30th, and slowly moved eastward into the state throughout the day. By the afternoon, it had made its way into central portions of the state, draping itself in a southwest to northeast fashion. Ahead of the front, the convective environment was ripe, with MUCAPE values in excess of 4000 J/kg and effective shear in the 30 to 40 kt range, with best values near the front. While there was reason for concern with regards to severe weather, such has hail and damaging winds, there was also strong concern for an area of west-central, north central or central Iowa to receive heavy rainfall with the boundary placement, potential outflow boundary locations, low level jet initiation, and a very moist environment. There was expectation for storms to be slow moving or training initially along the frontal boundary with propagation vectors opposing mean flow.

Initially storms fired up in the early afternoon across parts of northwest and north central Iowa in the very unstable environment, producing severe hail and wind damage. Fortunately, given their progressive nature, no water issues were seen with that round of convection. As the afternoon progressed, convection increased across northern areas, but remained quiet further south into central and southern Iowa. The initial northern convection pushed out an outflow boundary that made its way into the I-80 corridor, acting as another area for convective initiation. By the late evening, convection had fired up along the parent frontal boundary as well as portions of the aforementioned outflow boundary, producing marginally severe storms in most cases. By the time the low level jet kicked in, areas around Des Moines had already received a couple inches of rain, and now found the area in line for much more. Overall motion of storms was northeastward, but with nearly stationary frontal and outflow boundaries, backbuilding was induced by the low level jet, which resulted in copious amounts of heavy rainfall right on top of the Des Moines metro and surrounding areas. While some damaging wind was still ongoing, the primary concern was for heavy rainfall, flash flooding, and eventually a flash flood emergency. The Des Moines metro area took on widespread amounts of 6 or more inches, including a few reports of nearly 10 inches. The main impacts revolved around flash flooding and the flash flood emergency with sewer systems and local streams absolutely overwhelmed. Dozens of vehicles were flooded or stranded, with numerous water rescues performed. Many streams were out of their banks, and an individual was swept away and killed during the flash flooding. One home had flooding and a gas leak that lead to an explosion of the home. Luckily all occupants had already been evacuated. Two streams with NWS stream gauge observations, Four Mile Creek and Walnut Creek, set all-time crest records. Countless basements and businesses were flooded across the metro area and dozens of homes were completely flooded out on the southeast side where Four Mile Creek flooding was the worst. Two businesses had roof collapses from the heavy rain as well. The flash flooding and stream flooding continued into the overnight hours on July 1, so another entry is made in July 2018 for this event as well.

View location on OpenStreetMap → (41.7668, -93.5074)

Source: NOAA Storm Events Database, event_id 764991. Narrative written by the NWS forecast office that issued the report.