June 20, 2011: Tornado Outbreak - Including 2 EF3 and 4 EF2 Tornadoes

Storm damage survey and track of the Polk/Osceola tornado.

KUEX Reflevtivity from approximately 6:17 p.m. CDT. Tornado would have been on the ground on/near Highway 92.

Tornado near Farwell.
Photo courtesy of Benjamin Tillotson.

With 12 confirmed tornadoes, several of them significant, this Monday afternoon proved to be the most active tornado day in over three years within the Hastings County Warning Area, since May 29, 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 EF-3, four EF-2, three EF-1 and three EF-0. 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-City 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 and North Central Kansas, "protecting" this area from storm development.

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 moved into the area shortly before 3 p.m. CDT, as an EF-1 tornado crossed out of Norton County, Kansas, 5 miles west of the community of Long Island, and moved northeast. During its short stay in Phillips County, this tornado damaged a few farm buildings before the parent supercell storm went on to spawn another tornado in Harlan County, Nebraska. This supercell moved into southwestern Harlan County around 3 p.m., spawning an EF-1 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 3:30 and 4:45 p.m., this storm dropped several tornadoes from near Elm Creek, to north of Amherst, including the first EF-3 of the day. This EF-3 near Amherst intensified to at least 3/4 of a mile wide, and significantly damaged or destroyed at least 10 homes.  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 4:30 and 5:45 p.m., including an EF-2 southwest of Rockville, and an EF-1 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 4:30 p.m., just east of the surface triple-point. Two storms in close proximity to one another both became tornadic around 5:30 p.m. near the Hamilton and York County line. The first produced an EF-2 that crossed Interstate 80 southwest of Hampton. As this tornado moved north of Hampton, two farm workers made 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 EF-2 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 EF-3 tornado of the day, tracking over 9 miles from east of Polk to west-northwest of Osceola. This tornado grew to around 1/2 of a mile wide, and damaged several farms along the way, with some of the most significant damage along Highway 92. After this tornado dissipated around 6:20 p.m., 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 6:30 and 8:30 p.m.

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.

The set up responsible for this outbreak was a seasonably powerful mid-level low pressure system lifting 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.
 

Central Plains surface weather map valid at 4 PM June 20, 2011, depicting low pressure system and fronts

 500 millibar analysis valid at 4 PM June 20th, depicting strong mid level low pressure system.

 Analysis of derived "Significant Tornado Parameter" valid 5 PM June 20th

 Mixed-layer Convective Available Potential Energy (MLCAPE) valid at 4 PM June 20th, 2011

 0-6 km bulk shear magnitude valid 5 PM, June 20th

0-1 km bulk shear magnitude valid 5 PM, June 20th