<500mb NAM/WRF fcst for 00 UTC 6/12/08 
<SPC 0-1 km EHI (CAPE/SRH) at 20 UTC 6/11/08Storm chase: 11 June 2008 Little Sioux Iowa tornadic supercell - Jon and Shawna Davies
(Additional posts about the Little Sioux event can be found on my blog at http://davieswx.blogspot.com/2008/06/little-sioux-iowa-tornado-kills-4-boy.html and http://davieswx.blogspot.com/2008/08/61108-little-sioux-moorhead-iowa.html.)
With this unusually intense June upper system coming out into the plains (see 500 mb forecast below), Shawna and I started the day in Salina after having chased a supercell the night before northwest of Hays. Although this day looked like a potential large tornado outbreak, we both questioned whether storms would fire in Kansas until after dark with what appeared to be a stout capping inversion in place. To maximize our daytime chances for observing storms, we decided to head to eastern Nebraska and the Omaha area where it appeared storms would fire around mid afternoon. In the Lincoln area, we looked at SPC parameters on the laptop around 3:30 pm CDT, and saw that the best CAPE/low-level shear combinations were suggested over west and northwest Iowa (see SPC EHI mesoanalysis graphic below), so we decided to head north of Omaha on I-29 east of the Missouri River to catch storms as they formed near the river.
<500mb NAM/WRF fcst for 00 UTC 6/12/08 <SPC 0-1 km EHI (CAPE/SRH) at 20 UTC 6/11/08
Storms began to develop rapidly ahead of the cold front between 4:00 pm and 5:00 pm CDT as we drove north on I-29, so we picked a cell moving across the river into Woodbury County, Iowa. Over the next 3 hours, we observed 5 different mesocyclones while chasing in frustratingly hilly terrain east of the Missouri River between Sioux City and Harlan, Iowa, dropping southeastward along the line find a new rain-wrapped mesocyclone with a different cell every 30 minutes or so. The radar images below show the locations of 4 of these mesocyclones in a broken, wet line of embedded supercell storms.
radar base reflectivity 6:00 pm to 7:12 pm CDT Omaha & Des
Moines
Each supercell's mesocyclone became rapidly embedded in rain (very frustrating for spotting), and so we saw no tornadoes. We did intercept the mesocyclone that produced the Little Sioux campground tornado (4 scouts killed, see radar location above) east of Moorhead, Iowa and northwest of Dunlap, Iowa, but could see little apart from rain-wrapped structure. At one point, we were driving toward the Little Sioux storm in rain with little visibility near Soldier, Iowa, and I suddenly noticed that the radar image on my laptop was not updating (Omaha's radar was struck by lightning after 6:42 pm CDT). Realizing that the mesocyclone and possible tornado was closer than I thought, I told Shawna to turn the car around and retreat northward. After seeing Kory Hartman and Kenny Allen's video encounter with the rain-wrapped tornado near Moorhead, I'm very glad we didn't attempt to drive into the wet mesocyclone. We ended the chase in Harlan, Iowa where a tornado-warned bow echo overtook us at a Pizza Hut. We were shaken when we heard about the deaths at the Boy Scout camp.
Below are video captures of 4 of the mesocyclones we saw indicated on the radar images above. Given the terrain and rain-wrapped storms, it was a very frustrating and difficult storm chase for spotting purposes.
Looking N, mesocyclone (meso 1) and wall cloud lowering near Climbing Hill, Iowa
Llooking NNE, same mesocyclone (meso 1) wrapping up in rain
Looking NW, hazy mesocyclone (meso 2) and lowering on next cell south near Mapleton, Iowa
Looking NW, this is the rain-wrapped and dangerous Little Sioux mesocyclone (meso 3)
northwest of Dunlap, Iowa
This is Chris Kartens' view of the same cell a few minutes earlier at Moorhead, Iowa, near
the end of the tornado
Looking SW, this is a new mesocyclone south of Dunlap, Iowa
The only storm in western Iowa that produced a long-track intense tornado was the Little Sioux storm (see my survey map below).
The environment as indicated by the RUC model analysis profile at Council Bluffs, Iowa (below) indicated excellent support for tornadoes with plenty of CAPE, large SRH, storng deep layer shear, and a very surface-based setting. But it is very difficult to account for why other storms in the line over western Iowa and eastern Nebraska produced only brief weak tornadoes or no tornadoes at all. Additionally, it is difficult to account for why all supercells in the line became rain-wrapped so quickly, a fact that made the Little SIoux tornado especially dangerous.
This case provides plenty of motivation for study, as well as a reminder why campgrounds and outdoor facilities need to provide a concrete or below-ground shelter for tornado warnings. Warning was recieved at the Little Sioux campground (Omaha issued a tornado warning out at least 15 minutes before the tornado hit), and the scouts were well-drilled about where to go, but bunk houses simply do not provide adequate shelter! Donations are being collected by the Mid America Boy Scout Council in Omaha to build a tornado shelter at the Little Sioux Campground.
Jon Davies 9/6/08