Tornadoes and Hail in Iowa, April 10, 2008

An intense spring storm took shape over the central U.S. during the day on the 10th. The surface low formed over the southern Rockies the previous night and moved into Kansas during the day, then moved into southwest Iowa during the evening of the 10th. The upper level low pressure became nearly stacked by the morning of the 11th. For the most part, Iowa remained on the cold side of the system through the day. During the late afternoon, as the low lifted northeast, a warm front extending east out of the low pushed north into the state. South of the front, temperatures were in the 60s to low 70s with dew point temperatures in the upper 50s to low 60s. The atmosphere became unstable with CAPE in the 500 to 1000 J/kg range by evening across southern Iowa. A strong low level jet was in place with wind speeds around 60 kts. With the strong winds, both in the low levels and aloft, the effective shear was quite high in the 40 to 55 kt range. The LCL was quite low with the rainfall that continued through the day and was around 500 meters during the event. A fair share of the CAPE was in the -10 to -30 C. layer of the atmosphere with 300 J/kg available. This was a classic low top convection case with the upper low and dry line approaching from the southwest. The freezing level was very low across the CWA, sloping from around 6000 feet in the Omaha, NE area, to 9700 feet in the Davenport, IA area. Lifted indices were around +4, however on the warm side of the warm front they were in the -2 to -4 C. range. The warm front surged north rapidly into Iowa during the evening, pushing from the Iowa-Missouri border into central Iowa in about 4 hours. Thunderstorms initially formed in an arc from northeast Kansas into western Missouri. The storms lifted north-northeast at 50 to 60 MPH. Rotation was noted in nearly all of the storms with tornadoes produced by some. One of the stronger storms was a cyclic tornado producer and moved out of northern Missouri into Wayne and Appanoose Counties. It continued northeast across Monroe, into Mahaska, and then Poweshiek Counties. Two small tornadoes touched down in Wayne County. They were only on the ground briefly and caused no damage as they were in open country. Wind speeds on these tornadoes were estimated at 75 MPH. A tornado touched down in Appanoose County near Seymour and produced winds of 90 to 100 mph. Damage from the tornado started about 1 mile south of the intersection of highway J46 and 105th Avenue in far western Appanoose County. The tornado moved rapidly northeast producing primarily tree damage across rural countryside. The tornado hit a residence on the southern side of Highway J46, producing EF1 damage to the house and various outbuildings. In addition, a large utility pole was pulled 4 feet south before listing severely to the north. The tornado and debris continue for close to another mile before ending. The storm moved north into the Iconium area. It was observed on southern shore of Rathbun Reservoir before traversing the lake and entering into the Antler Acres Trailer park subdivision. The tornado produced much roof and soffit damage, as well as window damage to many of the homes. However, one mobile home was lifted and shifted several feet before tie downs held it in place. The worst damage was to a mobile home which was flipped on its side after the tie downs were ripped out of the ground. An additional outbuilding about one quarter of a mile to the northeast was badly damaged as well. The tornado moved nearly to the Monroe County border before lifting. It set down again in Mahaska County and moved northeast to the southwest of Oskaloosa. Scattered damage occurred along its path with damage done to fuel pumps at a gas station in the Oskaloosa area as the parent storm passed. There was also considerable wind damage in the area caused by the RFD. The same storm moved into Poweshiek County. The tornado redeveloped just northeast of Barnes City and moved northeast. Several power lines were downed along with structural and shingle damage to houses. In addition, storm spotters reported some grain bin damage, and road signs were pulled out of the ground. One mobile home was ripped from its foundation in the 1900 block of Highway 85. Another brief touchdown occurred a short time later in Poweshiek County. In addition to the tornadic storms, several of the storms produced high winds and hail. The hail was generally no larger than an inch in diameter, with most of it around nickel size. The largest hail was west of Centerville in Appanoose County where one inch diameter hail fell. There were several reports of high winds, though much of the damage was relatively minor. High winds in Oskaloosa caused some roof damage and took down power poles. Damage was also reported to fuel pumps in Oskaloosa. In Poweshiek County, two barns were blown down and portions of a roof were peeled off at a farmstead east of Brooklyn. In addition, siding was torn off of the house and windows were blown out. These were some of the more substantial reports. Late in the event, additional thunderstorms formed over northern Missouri. They produced 58 MPH winds in the Lamoni area.