Small Supercell and Tornado near Forest City, Iowa

All images and text
© copyright Gene Moore

A small but intense rotating thunderstorm produced a classic tornado just east of the small community of Forest City, located in north-central Iowa. The original plan was to work the warm front along highway IA 9 in the northwestern part of the state. Storms started early that day and most appeared to be heavy rain storms. Water was standing in the fields from previous rains and flooding was a definite threat. After chasing a storm early in the afternoon and fighting heavy rain, I decided to deviate from the forecast and drop a little south to catch some of the dryline, hoping the storms would be easier to navigate. The first cell I came upon was severe with large hail and showed signs of rotation. After working my way around to the southwest side of the storm, I found a well developed mesocyclone. I passed through rural areas and back roads until I had good position. A rough looking tail cloud was developing to my west.

developing storm west first circulation passes over (north)

Life became interesting as I was waiting on the rain and small hail to pass over. A new storm cell, with light rain falling from it, came in from the southwest. It looked like a spoiler and I would have to wait it out. It was also trashing the mesocyclone on the current storm by feeding cold air into it. Bad news, or so it seemed. While I was waiting at the end of this road, I suddenly noticed strong winds from the east. Since winds are typically from a westward direction, easterly winds can be indicative of tornadic storms. When a sudden shift of winds happens, the typical chaser response will be to quickly check west for the presence of inflow. But, all I could see was rain from this very small cell. I could see right through the rain core. Not much of a storm I thought, but this cell was perfectly concentric.....hummmm. The wind continued to increase and the vehicle begin to rock. Rain was coming down at enough of an angle that I could open the west window and not get wet. The rain curtain was now just a quarter mile away and was moving rapidly north. I strained to see the rain behind it; yep, going south, and very fast. Time to bail out!

two funnels tornado down behind trees

I shifted the vehicle into gear and headed south. I could see a bulbous laminar lowing in the middle of the rain. I remained close to the second cell and rode it out as I did not want to lose sight of the developing funnel. I was amazed all this action could come from such a small storm. The two storms merged overhead and strengthened.

The heavy rain passed northeast and two funnels were visible to my north and northeast. The north one was from the first storm. I could still see the remains of the tail cloud buried in the rain (not pictured). The top of the funnel had a big bend in it and the bottom was headed for the ground, soon to be a tornado. The curtain of rain around the second developing tornado was diminishing and the funnel was vertical which meant it would become more stable and probably last longer. I chose to chase the south funnel. The cells took off to the east as if a push had hit them from behind. I had to play catch up to get in position. I wanted to stay southwest of the tornado for the best lightning and to keep out of the rain.

The tornado came down while I was moving and I drove about one more mile to get in position. In the first image the tornado is in progress just to my northeast and moving rapidly away. All the storms had accelerated in the last few minutes. Another cell directly to my east received a tornado warning just before this storm and is also blasting off to the northeast. The anticipated stronger wind in the jet stream appears to have arrived. I could hear the tornado siren form a near by town (a couple miles away). The siren must have been quite loud.

In this image the debris cloud obscures the funnel a third of the way up from the ground. The swirl at the bottom is made up of mostly water that was deep in the fields from the last storm. The circulation is obvious, but the camera has problems in these situations. Small spinning water droplets have a flat grey or sometimes white appearance that's generally low contrast. It absorbs the light while the laminar funnel reflects it and appears white.

tornado in progress  

tornado 1 mile northeast tornado northeast thin rope stays on ground
The tornado stayed down for only about seven minutes and traveled four miles through plowed fields. On one occasion I did see pieces of debris fly into the air. It might have hit a storage shed or small building for farm equipment, but I didn't see it threaten any of the houses in the area. The second two shots show the funnel lifting and thinning at the bottom. In the last image of this set, a very thin rope persists on the ground with a swirl of water around it. The tornado was not very strong and probably would not have severely damaged a house had it hit one.


  storms moving into MN  
 

The storms were moving fast and I was pushing to keep up. To the north, cells were piling up on the warm front. A definite setup for more tornadoes. At the border I came upon a big sign "bridge out." It just takes one mistake like this to kill a chase and this one looked bad. There were no exits to the east or west. What a trap. I would have to back track. While fleeing from the blockade another vehicle approached at a fast clip from the south. As it whizzed by I recognized a face. Storm chaser Jim Leonard was about to find out about the bridge. After reaching the Interstate the storms were speeding for the horizon. I decided to wait for more storms to come up from the southwest. Soon, a rapidly moving dryline pushing the moisture out of the area and ending the activity over Iowa. To my north storms exploded as they hit the warm front. Tornado warnings were flying fast and furious for Minnesota. This chase was over.

 

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