EF2 Tornado — Union, Iowa

2012-04-14 · near Cromwell, Union, Iowa

Event narrative

This tornado continued in from Adams county. NWS storm survey confirmed a strong EF2 tornado. It entered Union county just southwest of Cromwell and tracked east-northeast through the far northwest portion of Creston. The Greater Regional Medical Center and Southwest Community College sustained EF2 damage with the worst occurring to the Green Hills Education Center. The tornado continued tracking northeast and damaged a few farmsteads northeast of Creston.

Wider weather episode

The first significant severe weather outbreak of the season took place during the afternoon and evening of the 14th with all modes of severe weather taking place. An upper level low pressure lifted northeast out of the southwest U.S. The upper level center, as well as the surface low, passed northwest of Iowa, across southeast South Dakota into Minnesota. Severe weather developed north of the warm front as it lifted north into Iowa. A cluster of severe storms formed over northeast Kansas and southeast Nebraska. These storms moved east-northeast along the warm front into Iowa, causing a considerable amount of severe weather. Severe weather parameters were fairly strong, however the dynamics of a 100 kt mid level jet and 60 kt low level jet made up the difference. Just prior to the storm development, CAPE values were around 4000 J/kg south of the development. Helicity was high, in the 300 to 500 range. There was a large amount of effective shear with 50 to 65 kts available. CAPE in the -10 to -30 layer of the atmosphere was in the 200 to 400 J/kg range, while downdraft CAPE was 600 to 800 J/kg. Plenty of low level moisture was drawn into the state on the strong low level jet. Precipitable water vales rose to 1.25 inches quite rapidly. The LCL level was relatively high, but given the dynmics was relatively easy to overcome. The LCL was just under 1000 meters at the beginning of the event. There were three tornadoes reported during the event. One was northwest of the Creston area in Union County. The most significant tornado caused high end EF2 damage to the far northwest corner of Creston. This tornado formed southwest of Cromwell at 1752 CST in Adams county and quickly tracked into northwest Creston around 1800 CST. EF2 damage occurred to the Greater Regional Medical Center and the Southwestern Community College campus. The tornado continued tracking to the northeast and damaged several farmsteads northeast of Creston before dissipating about 10 miles northeast of town at approximately 1815 CST. The wind strength was estimated to be 130 MPH. Significant damage occurred with uninsured damage alone near $2,000,000. A tornado touched down in Warren County east of New Virginia. The tornado caused damage to farm buildings and campers with a considerable amount of sheet metal left lying around. The EF1 tornado produced winds around 100 MPH. Many of the thunderstorms brought with them high winds and hail. The final tornado touched down in Mahaska County. The tornado was an EF1 tornado with a broad and intense rear flank downdraft south of the track. The AWOS site at the Oskaloosa airport in Mahaska County recorded a 97 MPH wind gust from the rear flank downdraft of the storm. Reports from the Emergency Manager indicated that damage at the airport was limited to two hangar doors and some damage to airport buildings. There were no planes on the field at the time. Numerous reports of hail larger than quarter size were received. Some of the larger hail included baseball size hail in Clarke County at Murray and numerous reports of golf ball size hail along the track of the severe cluster from Union County to Mahaska County. The same cluster of storms also produced very strong winds along its path. Numerous reports of 60 to 70 MPH winds were received all along the path. Numerous reports of tree and power line damage were received. The Des Moines metro area was hard hit with winds of up to 65 MPH causing considerable tree damage. At one time, 20,000 customers were reported without power. In Marion County a garage was destroyed and a house significantly damaged by the wind east of Knoxville. The storms also produced very heavy rainfall. Rainfall of 2 to 4 inches was reported at several locations. At the Des Moines International Airport, 1.10 inches of rain fell between 1630 and 1637 CST. This amount of rainfall in that short of period of time was a 100 year event. Fortunately it had been quite dry across the state, limiting the flash flood potential. Flash flooding was reported in Polk, Cass, and Madison Counties. Water was reported flowing one to two feet deep. Numerous reports were also received of street flooding and significant ponding.

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Source: NOAA Storm Events Database, event_id 365281. Narrative written by the NWS forecast office that issued the report.