For the second day in a row, severe thunderstorms pounded mainly eastern portions of South Central Nebraska, including many of the same areas affected on the 11th. The majority of severe weather focused near to 30 miles north of the Interstate 80 corridor in Hall, Merrick, Hamilton, Polk and York Counties. Brief, short-track tornadoes also occurred for the second day in a row, with three confirmed EF0 tornadoes. This included one tornado between Aurora and Giltner, and two others a few miles west of Stromsburg. No damage was reported from these brief touchdowns. In addition, the afternoon and early evening featured numerous reports of large hail, along with torrential rainfall and localized flooding.  The largest reported hail of 3 inches in diameter occurred just south of Exeter in Fillmore County. Even several hours after severe storms ended across the area, strong northwest winds developed behind the cold front in the evening as the pressure gradient increased, including a measured 58 MPH gust at the York airport.
This Thursday featured a fairly classic spring setup for severe weather, with a nearly vertically-stacked low pressure system slowly lumbering east near the Nebraska and Kansas border both at the surface and aloft.  A nearly stationary surface front arced northeast and east from the low roughly along the Interstate 80 corridor. Meanwhile, a modest dryline swept into the area from the southwest, and a cold front gradually worked southward in the wake of the slow-moving surface low. The combination of these multiple boundaries, along with considerable early afternoon sunshine and the close proximity of the mid-level low pressure system, effectively made this a cold core severe weather situation. While the first strong storm of the day fired up by mid-afternoon in Furnas County nearly directly underneath the mid-level low, the focus quickly turned northeast of the low, with the first severe storms developing in Sherman County around 330 PM. Over the course of the next several hours, severe storms then developed eastward down the Interstate 80 corridor, and southward along the Highway 81 corridor into Fillmore County before gradually slipping eastward out of the area.  By late afternoon, the mesoscale environment in the vicinity of the brief tornadoes in Polk County featured 1500 J/kg mixed-layer CAPE, and around 20 knots of low level 0-1km bulk shear.
Heavy rainfall and flooding were also an issue, especially across a narrow corridor north of Interstate 80. As the severe storms trained over this area, 24-hour rainfall totals of 1.5 to over 4 inches were reported across parts of southern Sherman, southern Howard, far northern Hall, Merrick, northern Hamilton, southwest Polk and northwest York Counties. Some of these areas had just received 1-2 inches of rain in severe storms the day before, thus pushing 48-hour rainfall totals into the 4-6 inch range. The town of Polk reported one of the highest 24-hour and 48-hour rainfall amounts, at 4.34 and 6.12 inches, respectively. This resulted in extensive lowland flooding of fields and small creeks, with water also covering numerous roadways. Even a few days after the rainfall, flooding continued along the Big Blue River in Polk County.

Hail in FURNAS, NEBRASKA
From 12-MAY-11 13:40:00 to 12-MAY-11 13:40:00 (CST-6)
Location: CAMBRIDGE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in SHERMAN, NEBRASKA
From 12-MAY-11 14:36:00 to 12-MAY-11 14:36:00 (CST-6)
Location: HAZARD
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Hail in SHERMAN, NEBRASKA
From 12-MAY-11 15:07:00 to 12-MAY-11 15:07:00 (CST-6)
Location: ROCKVILLE
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Hail in YORK, NEBRASKA
From 12-MAY-11 15:10:00 to 12-MAY-11 15:10:00 (CST-6)
Location: BENEDICT
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Hail in SHERMAN, NEBRASKA
From 12-MAY-11 15:24:00 to 12-MAY-11 15:24:00 (CST-6)
Location: ROCKVILLE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Hail in SHERMAN, NEBRASKA
From 12-MAY-11 15:28:00 to 12-MAY-11 15:28:00 (CST-6)
Location: ROCKVILLE
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Hail in BUFFALO, NEBRASKA
From 12-MAY-11 15:32:00 to 12-MAY-11 15:32:00 (CST-6)
Location: RAVENNA
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in HALL, NEBRASKA
From 12-MAY-11 15:51:00 to 12-MAY-11 15:51:00 (CST-6)
Location: GRAND IS
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in BUFFALO, NEBRASKA
From 12-MAY-11 15:52:00 to 12-MAY-11 15:52:00 (CST-6)
Location: RAVENNA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in BUFFALO, NEBRASKA
From 12-MAY-11 15:55:00 to 12-MAY-11 15:55:00 (CST-6)
Location: RAVENNA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in HOWARD, NEBRASKA
From 12-MAY-11 16:19:00 to 12-MAY-11 16:19:00 (CST-6)
Location: ST LIBORY
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Hail in POLK, NEBRASKA
From 12-MAY-11 16:20:00 to 12-MAY-11 16:20:00 (CST-6)
Location: POLK
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in MERRICK, NEBRASKA
From 12-MAY-11 16:37:00 to 12-MAY-11 16:37:00 (CST-6)
Location: CHAPMAN
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in HALL, NEBRASKA
From 12-MAY-11 16:43:00 to 12-MAY-11 16:43:00 (CST-6)
Location: GRAND IS
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in FILLMORE, NEBRASKA
From 12-MAY-11 18:12:00 to 12-MAY-11 18:12:00 (CST-6)
Location: FAIRMONT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in YORK, NEBRASKA
From 12-MAY-11 18:23:00 to 12-MAY-11 18:23:00 (CST-6)
Location: MC COOL JCT
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YORK, NEBRASKA
From 12-MAY-11 18:30:00 to 12-MAY-11 18:30:00 (CST-6)
Location: YORK
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in POLK, NEBRASKA
From 12-MAY-11 18:53:00 to 12-MAY-11 18:53:00 (CST-6)
Location: OSCEOLA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

High Wind in YORK, NEBRASKA
From 12-MAY-11 22:55:00 to 12-MAY-11 22:55:00 (CST-6)
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

North to northwest winds increased across the area behind the departing surface low pressure system during the late evening and overnight hours, with gusts of 40 to 50 mph reported at a number of locations.  The highest wind gust reported was 58 mph at York.

Tornado in HAMILTON, NEBRASKA
From 12-MAY-11 15:46:00 to 12-MAY-11 15:50:00 (CST-6)
Location: GILTNER
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

No damage was caused by this brief tornado touchdown.

Tornado in POLK, NEBRASKA
From 12-MAY-11 16:33:00 to 12-MAY-11 16:34:00 (CST-6)
Location: STROMSBURG
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

No damage was caused by this brief tornado touchdown.

Tornado in POLK, NEBRASKA
From 12-MAY-11 16:36:00 to 12-MAY-11 16:37:00 (CST-6)
Location: SWEDEHOME
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

No damage was caused by this brief tornado touchdown.

Flash Flood in HOWARD, NEBRASKA
From 12-MAY-11 16:45:00 to 12-MAY-11 22:30:00 (CST-6)
Location: ST LIBORY
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, and while the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.  A few county roads actually remained closed over the next day or so, having been damaged by the flood water.

Flash Flood in HALL, NEBRASKA
From 12-MAY-11 16:45:00 to 12-MAY-11 22:30:00 (CST-6)
Location: (GRI)GRAND IS RGNL A
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, and while the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.  A few county roads actually remained closed over the next day or so, having been damaged by the flood water.

Flash Flood in MERRICK, NEBRASKA
From 12-MAY-11 16:45:00 to 12-MAY-11 22:30:00 (CST-6)
Location: WORMS
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, and while the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.  A few county roads actually remained closed over the next day or so, having been damaged by the flood water.

Flash Flood in HAMILTON, NEBRASKA
From 12-MAY-11 17:55:00 to 12-MAY-11 23:30:00 (CST-6)
Location: HORDVILLE
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, as well as Highway 66 east of Hordville.  While the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.

Flash Flood in POLK, NEBRASKA
From 12-MAY-11 17:55:00 to 12-MAY-11 23:30:00 (CST-6)
Location: POLK
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, and while the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.

Flash Flood in YORK, NEBRASKA
From 12-MAY-11 17:55:00 to 12-MAY-11 23:30:00 (CST-6)
Location: ARBORVILLE
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 500.00K
Reported by: Law Enforcement

Water was reported over a number of county roads, and while the primary flooding threat ended during the late evening hours and water then began to recede, area creeks continued to run high and ditches were full well into the morning hours of May 13th.

Hail in POLK, NEBRASKA
From 12-MAY-11 17:30:00 to 12-MAY-11 17:45:00 (CST-6)
Location: STROMSBURG
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Quarter to golf ball size hail was reported.

Hail in FILLMORE, NEBRASKA
From 12-MAY-11 18:30:00 to 12-MAY-11 18:30:00 (CST-6)
Location: FAIRMONT
Magnitude: 1.50 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in FILLMORE, NEBRASKA
From 12-MAY-11 18:35:00 to 12-MAY-11 18:35:00 (CST-6)
Location: EXETER
Magnitude: 3.00 
Property damage: 100.00K
Crop damage: 0.00K
Reported by: County Official

Hail in FILLMORE, NEBRASKA
From 12-MAY-11 18:35:00 to 12-MAY-11 18:35:00 (CST-6)
Location: EXETER
Magnitude: 1.50 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YORK, NEBRASKA
From 12-MAY-11 18:44:00 to 12-MAY-11 18:44:00 (CST-6)
Location: BRADSHAW
Magnitude: 1.50 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter