Directwx.com

Summary: Nice supercell near Yankton, SD which produced several funnels and softball sized hail.

Targeted the warm front, dryline, cold front triple point near Yankton, SD. Left Omaha with Christy, Craig, Anton and CJ at 3:45. Had to leave 2 others behind due to lack of vehicles. Environment consisted of temps around 80F with dewpoints of 55-60 in the warm sector. Upper level winds were a little weak for good supercells with speeds of only 40-50 knots from 500-250 mb. In addition there was very little shear from 850-500 mb. However the low levels near the warm front exhibited strong backing with surface winds out of the East at 20 knots and from the South at 30 knots at 850 mb. This resulted in sufficient storm relative helicity of 200-400 m2/s2 in the target area.

Got to Sioux City around 5:15 pm and could see the first few cu going up. Decided to keep going North on I-29 to get closer. A nice cell with a decent radar echo was located near Yankton and we could see this visually so we turned West on hwy 50. The cell had a nice rain-free base on south side so we got in behind it and followed.

These cells became severe warned at 5:20 pm. We decided to try and get the cell farthest to the East where it would have unobstructed inflow. Radar indicates this storm continued to slowly organize and at 6:30 became tornado warned. We were traveling North on Hwy 19 North of Vermillion at this time. We could see the cell had wrapped around with a poorly defined wall cloud. Radar showed a nice hook right about this time.

We observed a brief funnel however the system was somewhat high based with bases around 3500 ft. We continued to follow this cell for the next hour observing strong rotation of the storm and 3 more funnels. The surface had cooled however to temps in the mid-60s and perhaps it was this cooling that prevented the system from producing any true tornadoes. We probably should have left the cell a little sooner to move back into the warm sector however it seemed each time we considered that idea it would start to wrap up again and look like it was going to produce. There was confirmed hail with this storm of at least baseball size but we did not see any hail and actually barely any rain.

Additional storms formed to the East closer to the warm sector which was pinching off as the dryline rotated Northeast. While the storms looked unimpressive on radar they did produce an apparently large tornado near Rock Valley, IA seen in the video here

Here is an image of the severe reports from the NWS Sioux Falls