EF3 Tornado — Polk, Nebraska

2011-06-20 · near Durant, Polk, Nebraska

Event narrative

The second EF3 tornado of the day touched down approximately 4 miles east of Polk and continued north for nearly 10 miles before lifting approximately 6 miles west-northwest of Osceola. Damage was noted right at the start of the path, with a brick home on Highway 66 west of Polk suffering significant roof, window and tree damage, and a semi-truck and trailer were knocked over onto its side. About 2 miles north of Highway 66, a residence suffered minor tree damage and lost an outbuilding, and a combine was tossed about 100 yards into a field. As the tornado continued north, numerous power poles were snapped, irrigation pivots were overturned, and crops were stripped from the ground. The most significant damage occurred when a farmstead on Highway 92 approximately 6 miles west of Osceola took a direct hit. The home as well as surrounding outbuildings and trees were either destroyed or suffered significant damage. A car, tractor, and an ATV were thrown from their original locations into the field just north of the home. The tornado finally lifted approximately 2 miles north of the intersection of Highways 92 and 39, but not before striking another farmstead, destroying an outbuilding and garage, and damaging a machine shed and barn. The maximum wind speed of this tornado was estimated to be 140 MPH.

Wider weather episode

With 11 confirmed tornadoes, several of them significant, this Monday afternoon proved to be the most active tornado day in over three years within South Central Nebraska, since 29 May 2008. This severe weather outbreak primarily targeted western and northeast portions of the area, largely sparing locations south of the Highway 6 corridor between Holdrege and Geneva. The final tornado breakdown per the EF Scale included two EF3, four EF2, two EF1 and three EF0. Despite considerable damage to mainly rural residences, these tornadoes resulted in no fatalities and only one known non-life threatening injury when a semi truck was tipped over near Elm Creek in Buffalo County. This Elm Creek tornado was also the only one to directly strike a community, although there were several close calls within a mile or two of places such as Amherst, Hampton and Bradshaw. Interestingly, tornadic storms also spared the immediate Tri-Cities population centers of Grand Island, Hastings and Kearney, despite damaging events occurring nearby to both the west and the east. This was largely due to the fact that an expansive mid-level dry intrusion became anchored over central sections of South Central Nebraska, protecting this area from storm development. In addition to tornadoes, the afternoon also featured several reports of large hail up to baseball size, and damaging winds up to 80 MPH.

This outbreak focused between 3 and 8 PM CDT, as a seasonably powerful mid-level low pressure system lifted into western Kansas and Nebraska out of Colorado, deepening and closing off as it did so. Strong wind fields aloft were in place at all levels, with a 50-70 knot jet streak nosing into the region between the 500 and 300 millibar level, and an 850-millibar low-level jet accelerating to between 30 and 40 knots. At the surface, a well-defined 992-millibar low pressure center migrated from north central Kansas around the noon hour, to near Hastings by early evening. A synoptically classic array of surface boundaries evolved around this low pressure center, with a warm front lifting north across South Central Nebraska, while a cold front and dryline pushed in from the west and southwest. In addition, a well-defined inverted surface trough axis extended nearly due north from the surface low into north central Nebraska. The net combination synoptic and mesoscale features yielded a very favorable setup for tornadic supercells across the area. At mid-afternoon, shear and instability parameters included approximately 50 knots of deep-layer vertical wind shear, 15-25 knots of 0-1 kilometer low-level vertical wind shear, 150 to over 300 m2/s2 of surface to 1 kilometer storm-relative helicity and 2000-4000 J/kg mixed-layer CAPE.

In general, there were two distinct corridors of tornadic storms across the area, one focused west of Highway 281, and one mainly east of Highway 281 and along and north of Interstate 80. The first tornadic storm of the day entered southwestern Harlan County around 3 PM CDT, having produced several damaging tornadoes across northwest and north central Kansas since early afternoon. This storm spawned an EF1 tornado in the Stamford area, before weakening. Within minutes however, a newly developed tornadic storm started tracking north from Phelps County into southwestern Buffalo County. Between 330 and 445 PM CDT, this storm dropped several tornadoes from near Elm Creek, to north of Amherst, including the first EF3 of the day. This EF3 near Amherst intensified to at least 3/4 of a mile wide, and significantly damaged or destroyed at least 10 homes. A veteran storm chaser described this tornado as being one of the strongest he had ever witnessed as it tracked north of Amherst. An additional tornadic storm that initiated in Buffalo County, along with subsequent storm-scale interactions from further development, moved north and produced several more tornadoes between 430 and 545 PM CDT, including an EF2 southwest of Rockville, and an EF1 that struck the Davis Creek Reservoir campground south of North Loup.

While tornadoes were ongoing across western portions of the area, vigorous new supercell development took place in eastern portions of South Central Nebraska around 430 PM CDT, just east of the surface triple-point. Two storms in close proximity to one another both became tornadic around 530 PM CDT near the Hamilton and York County line. The first produced an EF2 that crossed Interstate 80 southwest of Hampton. As this tornado moved north of Hampton, two farm workers fashioned a narrow escape by sprinting nearly 100 yards to an underground windmill pit and jumping inside as the tornado passed overhead. Just to the east, the other storm also dropped an EF2 tornado, which also happened to be the longest continuous tornado track of the day, traveling 15 miles from southwest of Bradshaw to near the York-Polk County line. Within a few minutes of this tornado's demise, the cyclic supercell quickly put down the second EF3 tornado of the day, tracking over 9 miles from east of Polk to west-northwest of Osceola. This tornado grew to around 1/2-mile wide, and damaged several farms along the way, with some of the most significant damage along Highway 92. After this tornado dissipated around 620 PM CDT, the parent supercell and other severe storms farther west finally lifted north out of South Central Nebraska. Although this ended the main severe weather event for the evening, a few additional strong to severe storms affected primarily Fillmore and Furnas Counties between 630 and 830 PM CDT.

Taking a backseat to significant tornadoes on this day were hail and heavy rainfall. Some of the largest reported hailstones included baseball size in Buffalo County, and tennis ball size in York County. Rainfall-wise, although there were no widespread flooding impacts, localized rural flooding occurred as many locations received 1-3 inches in a rather short time. Some northwestern portions of South Central Nebraska, including parts of Valley, Greeley and Sherman Counties measured 48-hour rainfall totals into the 3-4 inch range.

View location on OpenStreetMap → (41.0764, -97.7156)

Source: NOAA Storm Events Database, event_id 326864. Narrative written by the NWS forecast office that issued the report.