| Episode Narrative |
A very dynamic weather system moved into Iowa during the afternoon of the 30th. The system was more reminiscent of what one would expect in April or May. A close upper level low pressure, negatively tilted, lifted northeast through the central U.S. The structure was very classic in appearance with a well defined comma cloudy, dry slot, and warm conveyor in place. By the early evening, the surface low became stacked with the upper low and was located over eastern Nebraska. A defined dry line extended south-southeast out of the low. Surface temperatures warmed into the mid 70s to mid 80s ahead of the low, with dew points in the mid to upper 60s. Dew points behind the dry line dropped into the upper 30s to mid 40s. The airmass became unstable by the late afternoon with CAPE values around 1000 J/kg and lifted indices in the -2 to -4 C. range. Strong dynamics were in place with a 90 kt mid level jet in place, a low level jet of 60 to 70 kts, and an effective shear of 45 to 55 kts. The freezing level was quite high ahead of the approaching low and was between 13,000 and 14,000 feet. The high freezing level, combined with the limited CAPE of 100 to 200 J/kg in the -10 to -30 C. layer of the atmosphere, limited hail production. The downdraft CAPE was in the 600 to 1000 J/kg range, with an LCL of about 1250 meters. Thunderstorms formed in two locations. The first was along the east edge of the dry slot across Kansas. These storms became severe and lifted northeast quite rapidly into southern and central Iowa. Spotty wind damage was reported and one of the storms dropped one inch diameter hail in Marion County. The storms became tornadic as the moved into central Iowa. During the evening of Sunday 30 September 2007 two tornadoes struck portions of Marion, Jasper, Mahaska, and Poweshiek Counties. The first tornado produced EF0 to EF2 damage along its track and was rated EF2 on the Enhanced Fujita Scale with maximum winds of 125 mph. The track was 22 miles in length from 4 miles northeast of Pella to 1 mile north of Interstate 80, 2 miles west of Malcom. Much of the track was three to five tenths of a mile wide, but as wide as seven tenths of a mile at times. The second, shorter and weaker tornado was 5 miles in length and rated EF1 on the Enhanced Fujita Scale. The track began just south of Highway 6, 2.75 miles northwest of Malcom with a maximum width of 150 yards narrowing to 25-50 yards. The storm hit a farmstead just east of 110th street producing EF1 damage with speeds just under 100 mph. The tornadoes caused extensive damage on a farmstead near Malcom with three barns totally destroyed. The barns were 20 by 30 feet, 20 by 40 feet, and 60 by 100 feet. In addition, numerous other damage reports came in from along the track. Fortunately, there were no deaths or injuries from the tornadoes. Thunderstorms also formed close to the upper low center on the nose of the dry punch. There were several reports funnel clouds, along with spotty reports of high winds and hail. A storm chaser confirmed a tornado touchdown 2 NNW of Lytton in Sac county and was on the ground until 4 W of Jolley in Calhoun county. The tornado was in open country and was a small EF0 tornado. One inch diameter hail fell in Sac County, with numerous reports of pea to marble size hail. |
