Powerful Storm System Moves East, Loses Momentum

As active severe weather season continues, a setup for another severe event on May 21, 2024 similar to that of April 26 was forecast a couple of days in advance by the Storm Prediction Center (SPC). An updated outlook released at 12:54 am on Tuesday, April 26 enlarged the Moderate Risk (Level 4) area that had been in place over eastern Iowa the day before (above). 

Also issued is this large area of 15% hatched tornado risk.

Severe Thunderstorm Watch 275 is issued at 6:15 am for the majority of central and southern Iowa, and valid until 1:00 pm.

1:10 pm. Tornado Watch 277 is issued for much of Iowa, a small portion of northwest Illinois and southeast Minnesota.

3:00 pm. Updated convective outlook from the SPC as seen from Pivotal Weather's website, showing categorical, tornado, hail and wind outlooks.

3:20 pm. In short order comes Mesoscale Discussion 875, outlining an area of southwest Iowa that is favorable for strong tornadoes.

3:21 pm. Radarscope map of Iowa where an intense line of storms with multiple tornado and severe warned polygons are seen all up and down its length. The white circled area shows the location of Greenfield, Iowa, which would soon be ravaged by an EF4 tornado. Storms were tracking ENE.

4:17 pm. Radarscope showing the storm system as it continued east. It has now arrived over the Des Moines metropolitan area. Behind and west of the storm is a large polygon denoting flash flooding. Now inexplicably, the southern portion of the system began to lose energy despite a very favorable environment, perplexing some storm chasers located there.

About 4:40 pm I make the decision to drive northwest on Interstate 380 from Cedar Rapids to a position where I can intercept a leading severe-warned cell south of Waterloo. My reasoning was that if this situation did not pan out, I could always turn back south and make a play on other cells that were still farther west. I arrived at my first spotting location, the northbound I-380 weigh station, north of Brandon and south of La Porte City, Iowa. Above is a look at my spotting gear (5:33 pm) which includes cell phone (radar), amateur radio, CB/weather radio, Davis Vantage Vue weather data console, and my Nikon Z6ii camera.

5:42 pm. This is a look west at the incoming severe-warned storm cell from my I-380 weigh station spotting location. Very soon I would make the decision to leave this spot and head back south, as a I saw a discrete tornado-warned cell headed for the Cedar Rapids area. (I probably should have stayed here because as it turned out this was the more active cell).

Radar image for 5:42 pm. The approaching severe-warned storm can be seen at upper left. The target icon is my location.

6:31 pm. Now having arrived at my new spotting location at the BP Mart on County Home Road near the Toddville, Iowa exit of I-380. An inflow cloud can be seen along the northwestern horizon, feeding into the approaching storm (toward left).

6:37 pm. Radar image showing my former storm (upper left) now being tornado warned, and my new approaching storm having its tornado warning dropped... (of course!).

6:38 pm. Panorama of approaching severe-warned storm. Storm is about 27 miles distant. 

6:43 pm. Storm showing an interesting and vigorous updraft feature (just above horizon). A Robins, Iowa fire department vehicle is parked in the foreground.

6:48 pm. Radarscope dual pane mode, showing reflectivity (top) and velocity at bottom. The area shown with a white arrow was where a tornado might have resided.  

6:53 pm. A photogenic inflow cloud looking northwest.

6:55 pm. Storm, now about 10 miles distant, looking more ominous.

6:57 pm. Looking WNW. Possible forming funnel.

6:58 pm. Looking less like a funnel. The storm was now becoming more outflow dominant, with the winds blowing toward me.

7:01 pm. Storm forming a bit of a shelf cloud and is now about 5 miles distant.

7:02 pm. Spotters and truckers watch the incoming storm, which is minutes away. The two truck drivers at right decided to park their rig and wait out the storm before resuming north to Waterloo. 

7:02 pm. Inflow cloud panorama feeding into the storm (left).

7:04 pm. Severe-warned storm cell is almost upon my location and is showing a bit of a hook echo feature between the towns of Marengo and Fairfax. 

7:26 pm. Heading back home on County Home Road after most of the storm had passed. This image looks northeast at the departing storm and its strong rain shaft. The storm system in eastern Iowa weakened considerably compared to when it was in western Iowa. Heavy rain fell here, some elevated winds and pea size hail. Minor damage occurred. Nikon Z6ii camera.

The town of Greenfield, Iowa caught the brunt of the severe weather event, with four fatalities and much of the town destroyed. The tornado was recently upgraded from an EF3 to an EF4. It reached a maximum of 1,000 yards wide and traveled 44 miles. Thoughts and prayers go out to all those who were affected by this deadly tornado. This is why reports from ground-truth storm spotters is so important!