Thunderstorm Wind — Poweshiek, Iowa

2010-04-06 · near Grinnell, Poweshiek, Iowa

Event narrative

On the north edge of Grinnell, cars were reported off the road and flipped. Power lines were down and roofs were blown off homes.

Wider weather episode

The frontal boundary from the previous night lifted north as a warm front as low pressure over Kansas pushed northeast into Iowa. The low eventually tracked from southwest Iowa in the afternoon to east central Iowa by early evening. To the southeast of the front, temperatures warmed into the upper 70s to mid 80s, with dew point temperatures in the upper 50s to mid 60s. The atmosphere became quite unstable with lifted indices falling to -5 to -8 Celsius and CAPE rising to 2000-3000 J/kg. The situation was very dynamic with a synoptic scale upper level trough to the west and broad southwest flow over the central U.S. The jet at 300 mb was around 100 kts with 500 mb winds southwest at 75+ kts. This aided in producing deep layer shear of 75 to 100 kts over southeast Iowa by late afternoon. Ample inflow was present with moisture transport at the 850 mb level produced by a 35 to 40 kt flow. This increased precipitable water values to between 1 and 1.2 inches. Thunderstorms fired along the frontal boundary during the mid to late afternoon as the front became a cold front. With 300-400 J/kg of CAPE in the -10 to -30 layer of the atmosphere, and 700-1000 J/kg of downdraft CAPE available, the atmosphere was loaded for the production of wind and hail. In addition, the freezing level was a relatively low 12,000 feet. The line of thunderstorms that formed ahead of the advancing cold front became severe quickly, producing large hail. Much of the hail was in the 1 to 1.5 inch diameter range, however there were several reports of golf ball to tennis ball size hail. The largest hail was over northeast Iowa with golf ball size reported in Grundy County, and tennis ball size from the northwest edge of Waterloo toward Cedar Falls in Black Hawk County. Also in Black Hawk County, a few stones were measured to 3.5 inches in diameter in Cedar Falls. As the system unfolded, the storms began to produce high winds. Wind gusts of 75 to 100 MPH occurred with the stronger storms. Multiple homes had siding damage due to hail blown through the siding, and windows were knocked out in Marion County east of Monroe. High winds snapped power poles in Jasper and Poweshiek Counties. A camper was blown off of Interstate 80 at mile post 178, resulting in a multiple car accident. In Poweshiek County, cars were blown off of the road and flipped, power poles snapped, and roofs were taken off of houses near Grinnell. Power was knocked out to several thousand customers in the Grinnell area. Six members of the Grinnell College track team were caught in the storm and sustained injuries from the hail. They were bruised, taken to the hospital, treated and released. Officials in the city of Grinnell estimated damage due to the high winds from this storm at $10,000,000.

View location on OpenStreetMap → (41.7400, -92.7200)

Source: NOAA Storm Events Database, event_id 214639. Narrative written by the NWS forecast office that issued the report.