Continuing an active few days of strong to severe thunderstorms and localized flooding, this Saturday afternoon and early evening featured a few fairly isolated severe storms across northeast portions of South Central Nebraska. Large hail was the main story with these storms. Although hail to at least quarter size was reported in Greeley and Polk Counties, the storm of the day pounded York County late in the afternoon. Hail of 1 to as much as 3 inches in diameter fell for several minutes in the city of York itself as this supercell storm drifted northeast. Widespread heavy rainfall was not an issue with these rather isolated storms, although a small corridor from the east side of York to near Gresham picked up around 1 inch.
Aloft, the primary instigator for severe storm development was a lobe of mid-level energy swinging across the Central Plains along the southern periphery of a powerful, closed mid-level low pressure system centered over western South Dakota. At the surface, a rather subtle, north-south oriented trough axis pushed into South Central Nebraska during the afternoon, serving as the low-level focus. While the initial development of isolated strong storms took place by mid-afternoon west of Highway 281, the more isolated supercell development that focused across York County developed a short time later well ahead of the initial activity. The mesoscale environment in the York County area during this time featured around 2000 J/kg mixed-layer CAPE, and roughly 30 knots of deep-layer wind shear.

Hail in GREELEY, NEBRASKA
From 21-MAY-11 15:45:00 to 21-MAY-11 15:55:00 (CST-6)
Location: SCOTIA
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in YORK, NEBRASKA
From 21-MAY-11 15:55:00 to 21-MAY-11 15:55:00 (CST-6)
Location: GRESHAM
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in YORK, NEBRASKA
From 21-MAY-11 15:56:00 to 21-MAY-11 15:56:00 (CST-6)
Location: YORK
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in GREELEY, NEBRASKA
From 21-MAY-11 16:10:00 to 21-MAY-11 16:10:00 (CST-6)
Location: GREELEY
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in POLK, NEBRASKA
From 21-MAY-11 17:39:00 to 21-MAY-11 17:39:00 (CST-6)
Location: OSCEOLA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in YORK, NEBRASKA
From 21-MAY-11 16:10:00 to 21-MAY-11 16:10:00 (CST-6)
Location: YORK
Magnitude: 1.75 
Property damage: 100.00K
Crop damage: 250.00K
Reported by: Trained Spotter

Hail in YORK, NEBRASKA
From 21-MAY-11 16:20:00 to 21-MAY-11 16:20:00 (CST-6)
Location: YORK
Magnitude: 1.50 
Property damage: 25.00K
Crop damage: 250.00K
Reported by: Trained Spotter

Hail in YORK, NEBRASKA
From 21-MAY-11 17:01:00 to 21-MAY-11 17:01:00 (CST-6)
Location: GRESHAM
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 250.00K
Reported by: Public

Hail in YORK, NEBRASKA
From 21-MAY-11 16:10:00 to 21-MAY-11 16:10:00 (CST-6)
Location: YORK
Magnitude: 3.00 
Property damage: 1.00M
Crop damage: 250.00K
Reported by: Trained Spotter

Hail up to 3 inches in diameter fell in York for several minutes. The roof of a city elementary school was totaled by the hail.