Alert archive / Iowa / Severe Thunderstorm Watches in 2011

Severe Thunderstorm Watch 317

May 21, 201155 counties in Iowa; 21 counties in Nebraska

The Storm Prediction Center issued this severe thunderstorm watch at May 21, 2011, 2:25 PM CDT for 55 counties in Iowa; 21 counties in Nebraska. It was in force for 10 hr.

The area

The outline as issued.

The facts

Issued: May 21, 2011, 2:25 PM CDT (19:25 UTC)
Expired: May 22, 2011, 12:00 AM CDT
In force: 10 hr
Issued by: the Storm Prediction Center
Largest hail expected: 2 in
Strongest gust expected: 60 kt

All 76 counties

Adair County, IA, Adams County, IA, Appanoose County, IA, Audubon County, IA, Boone County, IA, Buena Vista County, IA, Calhoun County, IA, Carroll County, IA, Cass County, IA, Cherokee County, IA, Clarke County, IA, Clay County, IA, Crawford County, IA, Dallas County, IA, Davis County, IA, Decatur County, IA, Dickinson County, IA, Emmet County, IA, Fremont County, IA, Greene County, IA, Guthrie County, IA, Harrison County, IA, Humboldt County, IA, Ida County, IA, Jasper County, IA, Kossuth County, IA, Lucas County, IA, Lyon County, IA, Madison County, IA, Mahaska County, IA, Marion County, IA, Mills County, IA, Monona County, IA, Monroe County, IA, Montgomery County, IA, O'Brien County, IA, Osceola County, IA, Page County, IA, Palo Alto County, IA, Plymouth County, IA, Pocahontas County, IA, Polk County, IA, Pottawattamie County, IA, Poweshiek County, IA, Ringgold County, IA, Sac County, IA, Shelby County, IA, Sioux County, IA, Taylor County, IA, Union County, IA, Wapello County, IA, Warren County, IA, Wayne County, IA, Webster County, IA, Woodbury County, IA, Burt County, NE, Butler County, NE, Cass County, NE, Cuming County, NE, Dakota County, NE, Dodge County, NE, Douglas County, NE, Gage County, NE, Jefferson County, NE, Johnson County, NE, Lancaster County, NE, Nemaha County, NE, Otoe County, NE, Pawnee County, NE, Richardson County, NE, Saline County, NE, Sarpy County, NE, Saunders County, NE, Seward County, NE, Thurston County, NE, Washington County, NE

The radar

The Omaha radar (KOAX), 20 km from the middle of the area, began a volume at 19:29 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What SPC expected

The probabilities SPC published with the watch, for the watch area over its life.

Two or more tornadoes: 20%
One or more EF2 to EF5 tornadoes: 5%
Ten or more severe wind reports: 30%
One or more gusts of 65 knots or more: 30%
Ten or more severe hail reports: 40%
One or more hailstones of 2 inches or more: 50%
Six or more combined severe hail and wind reports: 70%

Warnings issued inside it

42 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.

Issued	Product	Office	Counties	Lasted
May 21, 2:34 PM CDT	Tornado Warning	OAX	Butler, Colfax, Dodge, Saunders	17 min
May 21, 2:56 PM CDT	Severe Thunderstorm Warning	OAX	Burt, Cuming, Thurston	25 min
May 21, 4:55 PM CDT	Severe Thunderstorm Warning	OAX	Butler	35 min
May 21, 5:21 PM CDT	Severe Thunderstorm Warning	OAX	Burt, Cuming, Dodge	39 min
May 21, 5:27 PM CDT	Severe Thunderstorm Warning	OAX	Butler, Colfax, Platte	48 min
May 21, 5:59 PM CDT	Severe Thunderstorm Warning	DMX	Emmet	46 min
May 21, 6:04 PM CDT	Severe Thunderstorm Warning	OAX	Butler	26 min
May 21, 6:20 PM CDT	Severe Thunderstorm Warning	OAX	Cuming, Thurston	40 min
May 21, 6:21 PM CDT	Severe Thunderstorm Warning	OAX	Nemaha	24 min
May 21, 6:31 PM CDT	Tornado Warning	OAX	Butler	29 min
May 21, 6:45 PM CDT	Severe Thunderstorm Warning	OAX	Cuming, Stanton, Wayne	45 min
May 21, 6:47 PM CDT	Severe Thunderstorm Warning	FSD	Plymouth	28 min
May 21, 6:48 PM CDT	Severe Thunderstorm Warning	OAX	Fremont, Mills, Montgomery, Page	42 min
May 21, 6:50 PM CDT	Severe Thunderstorm Warning	OAX	Cass, Sarpy	40 min
May 21, 6:51 PM CDT	Tornado Warning	OAX	Butler, Saunders	24 min
May 21, 6:57 PM CDT	Flash Flood Warning	OAX	Butler	2 hr
May 21, 7:00 PM CDT	Severe Thunderstorm Warning	OAX	Butler, Colfax, Platte	45 min
May 21, 7:00 PM CDT	Severe Thunderstorm Warning	OAX	Thurston	45 min
May 21, 7:07 PM CDT	Severe Thunderstorm Warning	OAX	Mills, Montgomery, Pottawattamie	38 min
May 21, 7:11 PM CDT	Severe Thunderstorm Warning	OAX	Butler, Dodge, Saunders	49 min
May 21, 7:21 PM CDT	Severe Thunderstorm Warning	OAX	Cass, Douglas, Sarpy	39 min
May 21, 7:28 PM CDT	Severe Thunderstorm Warning	DMX	Emmet	32 min
May 21, 7:34 PM CDT	Severe Thunderstorm Warning	OAX	Cass, Sarpy	41 min
May 21, 7:40 PM CDT	Severe Thunderstorm Warning	OAX	Pottawattamie, Shelby	50 min
May 21, 7:46 PM CDT	Severe Thunderstorm Warning	OAX	Harrison, Pottawattamie	44 min
May 21, 7:49 PM CDT	Severe Thunderstorm Warning	FSD	Lyon	26 min
May 21, 7:51 PM CDT	Severe Thunderstorm Warning	DMX	Adair	39 min
May 21, 7:57 PM CDT	Severe Thunderstorm Warning	DMX	Emmet	33 min
May 21, 8:12 PM CDT	Severe Thunderstorm Warning	OAX	Cuming, Stanton	25 min
May 21, 8:14 PM CDT	Severe Thunderstorm Warning	OAX	Pottawattamie	31 min
May 21, 8:16 PM CDT	Severe Thunderstorm Warning	FSD	Lyon	29 min
May 21, 8:16 PM CDT	Severe Thunderstorm Warning	OAX	Harrison	44 min
May 21, 8:21 PM CDT	Severe Thunderstorm Warning	DMX	Guthrie	24 min
May 21, 8:32 PM CDT	Severe Thunderstorm Warning	OAX	Harrison, Pottawattamie, Shelby	43 min
May 21, 8:39 PM CDT	Severe Thunderstorm Warning	DMX	Dallas, Madison	36 min
May 21, 8:41 PM CDT	Severe Thunderstorm Warning	DMX	Crawford	34 min
May 21, 8:57 PM CDT	Severe Thunderstorm Warning	OAX	Mills, Pottawattamie	33 min
May 21, 8:59 PM CDT	Severe Thunderstorm Warning	DMX	Clarke, Decatur, Lucas, Warren, Wayne	31 min
May 21, 9:03 PM CDT	Severe Thunderstorm Warning	DMX	Madison, Warren	27 min
May 21, 9:44 PM CDT	Severe Thunderstorm Warning	DMX	Appanoose, Lucas, Monroe, Wayne	31 min
May 21, 9:51 PM CDT	Severe Thunderstorm Warning	OAX	Monona	39 min
May 21, 10:19 PM CDT	Severe Thunderstorm Warning	DMX	Mahaska, Poweshiek	26 min

Discussions around it

SPC mesoscale discussions over the same counties from four hours before the watch to its end.

The nearest sounding before it

Omaha, NE (KOAX), launched 12:00Z, 7.4 hours before the alert went out and 21 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 1247 J/kg
Mixed-layer CIN: -16 J/kg
Mixed-layer LCL: 607 m above ground
0-6 km bulk shear: 50 kt
0-1 km storm-relative helicity: -6 m²/s²
0-3 km storm-relative helicity: -20 m²/s²
Significant tornado parameter: -0.1
Supercell composite: -0.4

Open this sounding in Storm Lab

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
May 21, 12:51 PM CDT	Mesoscale Discussion	SPC	Adair, Adams, Audubon, Boone, Buena Vista, Calhoun and 31 more	114 min
May 21, 5:25 PM CDT	Mesoscale Discussion	SPC	Adams, Appanoose, Clarke, Davis, Decatur, Fremont and 9 more	65 min
May 21, 6:07 PM CDT	Mesoscale Discussion	SPC	Adair, Adams, Audubon, Boone, Buena Vista, Calhoun and 42 more	53 min
May 21, 2:33 PM CDT	Mesoscale Discussion	SPC	Andrew, Atchison, Buchanan, Clay, Clinton, DeKalb and 4 more	87 min
May 21, 5:34 PM CDT	Severe Thunderstorm Watch	SPC	Adair, Andrew, Atchison, Buchanan, Caldwell, Carroll and 26 more	7 hr
May 21, 3:41 PM CDT	Severe Thunderstorm Watch	SPC	Adams, Clay, Fillmore, Franklin, Greeley, Hall and 10 more	9 hr

Nearby

Severe Thunderstorm Watches in Iowa, 2011 Severe Thunderstorm Watches in Nebraska, 2011 Every severe thunderstorm watch in 2011

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.