After a rather quiet start to the severe weather season in South Central Nebraska, things changed in a hurry during the early to mid afternoon hours on this Wednesday as a line of severe thunderstorms marched from southwest to northeast across much of the area, but primarily impacting counties along and east of Highway 281. Although widespread straight line wind damage was the main story, there were also three confirmed tornadoes embedded within the line, consisting of an EF0 in extreme southern Adams County, an EF1 several miles north of Hampton in Hamilton County, and another EF1 that actually touched down on the edge of Osceola in Polk County. These tornadoes were relatively short-track, but did cause some damage. In terms of straight line wind damage, one of the many reports included 22 vehicles with rear or side windows blown out by flying gravel at Sandy Creek School south of Clay Center.  There were only a handful of hail reports, none of which were larger than nickels. No injuries were reported from these storms.
The primary driving force for severe weather was a powerful, negatively tilted mid and upper level low pressure system centered over Colorado. At the surface, a fairly well-defined quasi-stationary front extended across the region from southeast Colorado to east-central Nebraska, with a few areas of low pressure along it. This surface front marked a sharp division in instability, with mid-day mixed-layer CAPE values ranging from negligible across western portions of South Central Nebraska, to between 1500-2500 J/kg in eastern counties. Deep layer vertical wind shear on the order of 30-50 knots was also present, making the environment conducive to fairly widespread severe storm development.
Focusing on storm mode and timing, the nearly solid line of severe storms lifted northeast across South Central Nebraska in a generally west-northwest to east-southeast orientation, first crossing into the state out of north central Kansas around 1 PM CDT, and eventually exiting the Polk and Nance County areas around 4 PM CDT. While strong to severe wind gusts of 50-70 MPH were common across several counties, the brief tornadoes were likely a result of the line of storms encountering enhanced low level shear and easterly low-level winds along and north of the quasi-stationary front.

Hail in THAYER, NEBRASKA
From 11-MAY-11 12:35:00 to 11-MAY-11 12:35:00 (CST-6)
Location: DESHLER
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in THAYER, NEBRASKA
From 11-MAY-11 12:43:00 to 11-MAY-11 12:43:00 (CST-6)
Location: HEBRON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Hail in THAYER, NEBRASKA
From 11-MAY-11 12:58:00 to 11-MAY-11 12:58:00 (CST-6)
Location: CARLETON
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in CLAY, NEBRASKA
From 11-MAY-11 13:00:00 to 11-MAY-11 13:02:00 (CST-6)
Location: FAIRFIELD
Magnitude: 52.00 EG
Property damage: 30.00K
Crop damage: 0.00K
Reported by: County Official

Approximately 20 cars suffered window damage at Sandy Creek School from thunderstorm wind gusts picking up gravel in the parking lot.

Thunderstorm Wind in YORK, NEBRASKA
From 11-MAY-11 13:32:00 to 11-MAY-11 13:35:00 (CST-6)
Location: HENDERSON
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Wind gusts were estimated at 60 mph.

Thunderstorm Wind in HOWARD, NEBRASKA
From 11-MAY-11 14:00:00 to 11-MAY-11 14:00:00 (CST-6)
Location: ST PAUL
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Wind gusts were estimated at 60 mph.

Thunderstorm Wind in POLK, NEBRASKA
From 11-MAY-11 14:03:00 to 11-MAY-11 14:03:00 (CST-6)
Location: OSCEOLA
Magnitude: 55.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in POLK, NEBRASKA
From 11-MAY-11 14:15:00 to 11-MAY-11 14:15:00 (CST-6)
Location: STROMSBURG
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Wind gusts were estimated at 60 mph.

Thunderstorm Wind in NANCE, NEBRASKA
From 11-MAY-11 14:32:00 to 11-MAY-11 14:32:00 (CST-6)
Location: GENOA
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Wind gusts were estimated at 70 mph at the airport.

Tornado in HAMILTON, NEBRASKA
From 11-MAY-11 13:50:00 to 11-MAY-11 13:53:00 (CST-6)
Location: KRONBORG
Tornado F-scale: EF1
Property damage: 150.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

The confirmed tornado touched down approximately 6 miles north of Hampton and tracked northward to approximately 5 miles south and 2 miles east of Hordville, where it then lifted.  In total, this tornado overturned 3 irrigation pivots, destroyed 2 grain bins and downed 7 power poles. The top wind speed estimated with this tornado was 95 mph.

Tornado in POLK, NEBRASKA
From 11-MAY-11 14:00:00 to 11-MAY-11 14:02:00 (CST-6)
Location: OSCEOLA
Tornado F-scale: EF1
Property damage: 100.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

This confirmed tornado was witnessed by a number of spotters, including the Emergency Manager and Fire Chief.  The initial touchdown was south of Osceola, but lifted as it approached the town.  It then touched down once again on the northeast edge of town, before lifting for the final time approximately 3/4 of a mile to the north.  Damage included 4 metal sheds destroyed south of town, with some of the debris ending up in the city park, a large tree uprooted in the city park, and porch and shed damage to a residence on the north side of town.  The top wind speed estimated with this tornado was 90 mph.

Tornado in ADAMS, NEBRASKA
From 11-MAY-11 12:40:00 to 11-MAY-11 12:41:00 (CST-6)
Location: AYR
Tornado F-scale: EF0
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Newspaper

Brief touchdown was reported southwest of Ayr, causing damage to a farmstead which included four sheds destroyed, fencing and the doors to an outbuilding damaged.

Thunderstorm Wind in POLK, NEBRASKA
From 11-MAY-11 14:02:00 to 11-MAY-11 14:02:00 (CST-6)
Location: SWEDEHOME
Magnitude: 61.00 EG
Property damage: 35.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Wind gusts were estimated at 70 mph, causing a roof of a shed to be partially torn off, minor tree limb damage, and portion of a pivot to be overturned.