| Episode Narrative |
An extremely unstable airmass remained in place over Iowa on the afternoon of the 26th. Low pressure was over southeast South Dakota, with a warm front extending east-southeast into east central Iowa and a cold front extending southwest into northwest Kansas. A 30 t0 40 kt low level jet feed moisture north ahead of the approaching cold front. Surface temperatures warmed into the low 90s with dew points in the low to mid 70s across much of the state. CAPE rose to 5000 to 7000 J/kg ahead of the front with lifted indices of -12 to -15 C. Strong severe weather parameters were present with downdraft CAPE of 1600-1900 J/kg in the prestorm environment and available CAPE in the -10 to -30 C layer of the atmosphere of 1000-1300 J/kg. The freezing level was again very high in the 14,500 to 15,000 foot range. Precipitable water values were somewhat lower than the previous day, though quite high at 1.5 to 1.8 inches. The LCL was a relatively high 1000-1500 meters. The environment had fairly strong shear at 50 kts in northern Iowa, and a weaker 30 kts over the south. A strong cap at 700 mb was present in western Iowa during the mid to late afternoon hours with temperatures above +15 C. To the west of the cap, along the temperature gradient, strong convection developed by mid to late afternoon over southwest Minnesota, southeast South Dakota, into northeast Nebraska. Initially the storms lifted northeast ahead of the cold front. The storms formed into an arc line of storms and progressed east into Iowa. They weakened and became more multi-cellular as they progressed east. Several of the storms produced gusty winds and small hail. A few reports of winds of 60 to 65 MPH were received. Damage was relatively minor for the most part, with it restricted to tree and large tree limbs. One storm caused structural damage on a farm in Winnebago County with roof damage to a shed and 4 grain bins destroyed. Heavy rainfall resulted in some flash flooding. The storms weakened during the early morning hours. Shortly before sunrise, the storms strengthened again as the area became cold pool dominated. Around that time, a wind gust of 70 MPH was recorded at an Iowa RWIS site north of Des Moines, with another gust to 70 MPH measured at an RWIS site in Tama. High winds caused damage to grain bins and buildings on a farmstead in Marion County. In Kossuth County, water was flowing over roads southeast of the airport. In Palo Alto County, flowing water was reported over roads north of Ruthven in a campsite there. Significant flash flooding took place in the Pomeroy area of Calhoun County. Major flash flooding took place in Webster County as well with many roads flooded, some by several feet of water, in the city of Fort Dodge. Thunderstorms moving through central Iowa caused minor urban flooding in Polk County in the Des Moines metro area with a couple of inches of water in intersections and ponding up around a foot deep over roads in low areas. |
