An examination of environmental parameters and radar evolution associated with tornadic supercells in south-central Nebraska on 15 June and 17 June 2009

After a relatively quiet start to the 2009 severe weather season, things quickly changed by mid-June as a significant trough developed across the western United States, resulting in the first tornadic activity across south-central Nebraska since early April. On the evening of 15 June, a supercell produced an EF-0 tornado near the village of Assumption, Nebraska, approximately 11 miles southwest of Hastings. This tornado was only on the ground for a few minutes and had a damage path of around one-half mile, but achieved a maximum width of 250 yards. Although the synoptic and mesoscale environment did not uniquely stand apart from many severe weather days, this tornadic storm exhibited some unique tendencies. An examination of WSR-88D radar imagery revealed that tornadogenesis was coincident with an RFD occlusion, with the tornado actually moving opposite to the mean storm motion.

Less than 48 hours later, during the late afternoon of 17 June, a more prolific tornadic supercell developed along a warm front in northern Buffalo County Nebraska. Over the course of the next three hours, this storm tracked nearly due east for 60 miles, producing several tornadoes along the way. The largest and longest-track tornado was rated EF-2, and nearly paralleled U.S. Highway 34 for five miles in Hamilton County, lifting approximately three miles due west of Aurora, Nebraska. This tornado reached a maximum width of one quarter to one half mile. Similar to the event two days prior, the synoptic and mesoscale environment associated with this event was supportive of supercells, as well as strong long track tornadoes. Several aspects of this environment are examined here, including a well-defined capping inversion that limited the southward extent of convection across the region. In addition, an examination of WSR-88D radar imagery, along with side by side corresponding storm photos and video clips, will highlight the evolution of this cyclic tornadic supercell.