Incredible cyclical tornadic supercell over western and northern Iowa Saturday afternoon and evening...we're still reviewing video to determine how many tornadoes we actually saw. We know for sure that we saw at least 10, with possibly more than that.
To set the stage...it became increasingly obvious throughout the day on Saturday that there was a significant threat of tornadoes - and that any tornadoes that did develop could be strong. Mike Brady, Roman Totten, and I left Peoria at 6 AM, targeting Onawa, IA.
A low pressure center over western NE was forecast to move northeast, and deepen throughout the day. Backed SE surface winds intensified ahead of the low, and brought unseasonably warm, moist air north into IA/NE. Record to near-record temperatures reached the upper 80's, with dewpoints just below 70 by 4 PM. This created a very unstable convective environment, with surface based CAPE reaching well in excess of 3000 j/kg by 21Z (4 PM CDT). The wind fields near the warm front/triple point in western Iowa were extremely favorable for tornadic development, with Surface-3 KM Storm Relative Helicity of 391 m2/s2 (per OAX 21Z sounding). Hodographs were large, and curved clockwise, as clearly shown by the aforementioned sounding.
A few blips started to appear on radar before 5 PM, and one started to become dominant. It started moving northeast, but as the mesocyclone's rotation increased, it began to turn right, and slowing down. By the time the storm crossed the river, it was moving ENE at 35 MPH.
We moved east of Onawa, and parked up under the meso. After getting hit with golf-ball sized hail for a bit, and seeing several funnels, the rotation and motion under the meso started to ramp up rapidly. A small, non-condensated tornado touched down briefly about 200 feet down the road, and dissipated after about 30-45 seconds.
We raced east after the tornado had lifted, and tried to navigate some terrible terrain/soupy dirt roads as fast as we could. We missed the 3/4 mile wide tornado that struck the town of Mapleton (video from my friends Skip Talbot and Mike Boik), and entered the town minutes after the tornado went through. After a quick stop to make sure that people were ok, we got through, and continued after the beastly supercell.
The inflow into the storm at this point was incredible - easily 50 MPH, backed to the SE. This continued for about the next 5 hours...when inflow into a storm is that strong, it's not a matter of if it'll put a tor down. but when.
As we got east of Mapleton, we noticed that a new wall cloud was developing outside the rain curtain. Then, the fun began...
Tornadoes just started dropping. One, after another, after another. Beautiful stovepipes. A multiple-vortex. Several ropes. Power flashes. We found tornadoes in our video that we didn't even notice that night because we were focused on bigger tornadoes elsewhere. And, we were on what could be the largest tornado in recorded history - a final number hasn't come in yet, but the path width was measured between 2 and 3 miles wide. Here's some of our video still captures...
Most of these tornadoes were between a quarter and half mile wide at some point in their life cycle, and EF-2 to EF-3 in strength - they would drop, strengthen, and then either become rain-wrapped, or get ingested by the 3 mile monster.
After about 5-6 hours of dropping tornadoes, the storm weakened, and we drove to Mason City, IA for the night. An absolutely incredible storm, and an absolutely unreal night.
#1 lesson learned...backed SE winds can do anything.
Here's our video from damage in Mapleton, and some more stills from the tornadoes...