Tornadoes and Flash Flooding in Iowa, June 22, 2007

A rather complex weather situation unfolded during the day on the 22nd. The situation was complicated by the convection that occurred during the previous night. An outflow boundary from the previous nights convection pushed south across the CWA, reaching the southern part by sunrise. This boundary combined with a quasi-stationary frontal boundary that was across the southern counties. The atmosphere became unstable quite rapidly during the late morning and early afternoon. Surface low pressure was located over southwest Iowa, with a warm front extending east South of the warm front temperatures warmed to around 90 F. with dew points in the upper 60s to low 70s. Surface based CAPE rose to 3000 to 4000 J/kg with the lifted index falling into the -8 to -10 C. range. The cap broke by around 1800 UTC as an MCS began to develop over central Iowa. The environment was fairly favorable with 30 to 40 kts of effective shear and precipitable water values around 1.5 inches. Available cape in the -10 to -30 C. layer of the atmosphere was in the 300 to 500 J/kg range. DCAPE values were between 600 and 800 J/kg. Two tornadoes touched down in Warren County. First indication of ground contact from the first tornado was adjacent to I-35 0.60 miles north of the Cumming, IA/G-14 interchange and approximately 1 mile NW of the town of Cumming. Ground contact evident to the southeast along a path of approximately 125 degrees. First structural damage (EF1) noted at a farm house at G14 and 20th Ave. Path width at this point estimated at 100 yards. Continuous ground contact southeast to farmstead at G14 and 25th Ave where north facing garage was destroyed (EF1) and outbuildings and trailer damaged. The tornado continued SE and reached widest point of approximately 200 yards striking outbuildings. Debris swath is noted to NE and also SE indicating the possibility of multi vortex circulation. A farmstead at 30th Ave and Clark street sustained EF1 damage to grain bin and outbuildings. Path width approximately 150 yards. Corn from grain bin was noted well downstream at end of track, some corn impacted side of Phillips home in SE Norwalk. Continuous ground contact ended and path narrowed considerably just north of Clarke St. near the Happy Apple Orchard. This resulted in a continuous tornado path of 2.2 miles. Intermittent damage path continued SE for 1.5 miles across fields and wooded areas with path of 30 to 50 yards. Next structural damage occurred to a light garage (EF0). The track turned E/NE Approximately 080 degrees for .6 mile, with intermittent damage consisting of snapped and mangled trees. Path width 30 to 50 yards. Mature 2' diameter trees along farm pond snapped at trunk and tops of trees deposited to the NE. The track turns SE for .75 mile before again turning E/NE. Narrow swath of tree damage noted. The track continues ENE (approximately 070 degrees) across Highway 28 where EF0 damage was noted. Ground scouring/swirl marks noted along 1.9 mile track ending at the Phillips home on the SE corner of Norwalk where EF2 damage was observed. Tornado appeared to quickly dissipate on the NE of the home. Maximum path width 200 yards along the over 7 mile long track. The second tornado was first verified over the Legacy Golf Course, just NW of the Norwalk Fire Station. It is interesting to note the tornado passed directly over the fire station, producing a circular pattern in the gravel on the roof, and blowing out two overhead doors. The tornado was narrow, with most damage consisting of tree damage up to EF1 and light structural damage as it moved to the SE. The path width ranges from 10 yards to a maximum of approximately 100-150 yards. This NW to SE tornado path crossed less than .25 miles of the path of the West to East moving Cumming to Norwalk Tornado. A farmstead on R57 south of Coolidge St. sustained tree damage from tornado #2. This is just south of the Phillips home that was damaged by the first tornado. Winds were estimated at 125 MPH in the first tornado, and 100 MPH in the second. Fortunately, there were no injuries during the tornado occurrences themselves, however one person was injured during recovery. LCL levels were lower than the previous day, in the 750 to 1000 meter range. As the complex evolved, the primary modes of severe weather were hail and high wind. The freezing level was lower than the previous day, at 13,900 feet and many of the storms contained at least some hail. The hail was not overly large, with most reports in the three quarter to one inch range. During the storms evolution, there were several bow echo segments, resulting in high winds. Winds of 80 MPH occurred southwest of Norwalk, in the rear flank downdraft of the supercell over the county in association with the tornadoes. Some of the other high winds included a wind gust of near 85 MPH in Madison County where a house had about one third of the shingles removed and a northeast facing overhang removed, 78 MPH in Tama, and 75 MPH at the Des Moines Airport, and 65 MPH in Carroll. A downburst occurred in Marion County south-southwest of Pleasantville, with winds estimated to 60 MPH. The storms were prolific rainfall producers. Rainfall of 2 to 4 inches was common in central, into north central and parts of northeast Iowa. Spotty reports of up to 6 inches in a 2 to 3 hour period were received. There were numerous reports of water flowing over highways as well as bridge and highway washouts.