Tornadic supercell in SW Iowa on 8/26/04  (Video and storm chase by Jon Davies)

<Photo of F2 tornado     <Preliminary tornado tracks
^SE of Riverton, Iowa, looking W (click to enlarge).                                                              (click to enlarge)
See also video images further down.

   
^12 hr forecasts of RUC model parameters, valid Thursday evening, 00 UTC 8/27/04 (click to enlarge).  Courtesy Earl Barker.
(SW and southern Iowa is suggested to have possible good environment for tornadoes, with large 0-1 km EHI, good deep layer shear, and low LFC heights.)

 
^21 UTC surface map showing convergence and backed winds over SW Iowa and SE Nebraska.   Satellite photo shows developing supercell (click to enlarge).

Video captures and descriptions from storm chase:

Cloud base & lightning with developing storm just SW of Nebraska City around 5 pm CDT (view toward SW).

Storm becomes very wet with broad core of precipitation (view toward WSW from I-29 and Hwy 2 in Fremont Co., Iowa).

                            
Panoramic view toward SW through W from about 4 mi west of Riverton, Iowa. 
This supercell storm is still very wet, but precipitation has cleared enough to reveal a low wall cloud  to the west, shortly after 5:30 pm CDT.

The wall cloud above lowers... Is this a tornado? (time is roughly 5:35 pm CDT, looking WSW, same location)	Probably not... we're looking over a hill, which can be misleading. But occasional lowered cloud tags make me wonder.
Within a couple minutes, rain obscures this feature.	Moving east past Riverton, Iowa, and then south, 20 minutes later a tornado becomes visible from a position 8 mi S of Shenandoah, Iowa, looking W (at roughly 6:03 pm CDT).
A zoomed-in view shows condensation not always touching the ground.	But the tornado is clearly on the ground at other times (view toward W), probably located somewhere SE of Riverton, Iowa.
A closer view around 6:08 pm CDT.	The tornado begins to "rope" around 6:10 pm CDT

           
This panoramic view looking SW through WNW at about 6:12 pm CDT shows the tornado "roping out" under an occluded mesocyclone SE of Riverton.
Meanwhile, a new mesocyclone and wall cloud with strong rotation develops closer to my location, not far to my southwest.

This new mesocycone lasts about 10 minutes, then weakens, replaced by a larger area of rotation developing rapidly to my west (looking W from about 4 mi W of Coin, Iowa, in Page County).	As this new mesocyclone approaches from the west, rapid rotation is evident at cloud base.   A brief funnel forms, and condensation wisps are seen near ground, marking the location of a weak tornado at 6:30 pm CDT.
Retreating eastward through Coin, to a point about 3 mi N of College Springs, Iowa, another tornado begins to develop at 6:44 pm CDT from the same cloud circulation (view toward NW)	Around 6:45 pm CDT, small condensation vortices become visible near ground, rotating around each other.
Multiple vortices are visible here.	It had rained heavily prior to the tornado, but some ground-level debris soon becomes visible.
I was unaware of the farmhouse to the left of the debris cloud when I zoomed in...	But a later close-up inspection of video shows what looks like a piece of roof coming off the house, as the "invisible" tornado's rear inflow quadrant stirs up a stream of debris while damaging the farmstead.
The tornado moves east and slightly northeast, paralleling the E-W county road I'm on, and spinning up occasional condensation vortices at the ground. At no time did this tornado have a full condensation cloud touching ground.	Looking N between College Springs and Shambaugh, Iowa, a broad area of rapid rotation is visible in the clouds, with occasional ground-level vortices seen, like this one (arrow).
At U.S. Hwy 71, nothing but rain and rotating clouds are visible, looking N from 3 mi south of Shambaugh at 6:55 pm CDT.	At this point, eastbound paved road options disappeared, and the chase was ended.  The mesocyclone moving away to my northeast looked increasingly rain-wrapped and possibly undercut by outflow.  What was unnerving about the last tornado was that it was largely "invisible".  Rain preceding it had eliminated dust for debris tracers, and, beneath a rapidly rotating cloud base, there were only occasional brief condensation vortices visible near ground to mark its location, along with spotty debris from farmsteads.  Nevertheless, the tornado produced F2 damage.

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