Northwest Iowa Tornadoes and Large Hail in Southeast South Dakota

On the evening of May 1, 2008, a line of supercell thunderstorms developed across southeast South Dakota and northwest Iowa. These storms produced five tornadoes across northwest Iowa (see detailed track map), as well as hail up to softball size in southeast South Dakota. Below is a map showing where the severe weather occurred as well as a brief description of what occurred.

Tornado Reports

Time (UTC)	Location	County	State	Lat	Lon	Comments
2359	SIOUX CENTER	SIOUX	IA	43.08	-96.17	JUST NORTH OF SIOUX CENTER (FSD)
0007	ROCK VALLEY	SIOUX	IA	43.2	-96.3	(FSD)
0008	2 SE INWOOD	LYON	IA	43.27	-96.41	(FSD)
0025	INWOOD	LYON	IA	43.29	-96.44	(FSD)
0030	1 E DOON	LYON	IA	43.28	-96.21	(FSD)
0044	2 W GEORGE	LYON	IA	43.34	-96.04	TORNADO MOVING NORTH. (FSD)

Hail Reports

Time (UTC)	Size (inches)	Location	County	State	Lat	Lon	Comments
2149	1.00	4 NE TYNDALL	BON HOMME	SD	43.03	-97.81	(FSD)
2217	0.75	7 W YANKTON	YANKTON	SD	42.89	-97.53	(FSD)
2222	0.75	1 WSW SIOUX FALLS	MINNEHAHA	SD	43.54	-96.75	FROM CENTER OF SIOUX FALLS (FSD)
2223	0.88	4 W LESTERVILLE	BON HOMME	SD	43.04	-97.67	FEW NICKEL. (FSD)
2237	0.88	5 NE UTICA	YANKTON	SD	43.03	-97.43	(FSD)
2256	0.75	7 ENE ARMOUR	DOUGLAS	SD	43.36	-98.22	(FSD)
2308	1.75	5 NW IRENE	YANKTON	SD	43.13	-97.23	(FSD)
2310	1.00	MENNO	HUTCHINSON	SD	43.24	-97.58	(FSD)
2321	1.00	9 S MOUNT VERNON	DAVISON	SD	43.58	-98.26	(FSD)
2328	1.00	DIMOCK	HUTCHINSON	SD	43.48	-97.99	(FSD)
2339	4.25	12 S MARION	TURNER	SD	43.25	-97.26	AT TURKEY RIDGE (FSD)
2345	1.00	3 ESE SIOUX FALLS	MINNEHAHA	SD	43.53	-96.68	3 ESE FROM CENTER OF SIOUX FALLS (FSD)
0016	0.88	2 SW WALL LAKE	MINNEHAHA	SD	43.51	-96.99	(FSD)
0017	1.50	4 S SIOUX FALLS	LINCOLN	SD	43.48	-96.74	(FSD)
0022	0.75	1 WSW SIOUX FALLS	MINNEHAHA	SD	43.54	-96.75	FROM CENTER OF SIOUX FALLS (FSD)
0105	1.00	4 SSW WENTWORTH	LAKE	SD	43.94	-96.99	(FSD)
0118	0.75	ARCO	LINCOLN	MN	44.38	-96.18	(FSD)
0125	0.88	8 SSE HURON	BEADLE	SD	44.26	-98.16	(FSD)
0150	0.75	WOLSEY	BEADLE	SD	44.41	-98.47	(FSD)

Visible satellite picture at 4:15 pm May 1, 2008

Thunderstorms began to develop along a boundary that separated relatively dry air from moist air. This boundary lifted north out of northeast Nebraska into southeast South Dakota and northwest Iowa into a relatively unstable atmosphere. Initially, storms developed along the Missouri River Valley in far northeast Nebraska moving north around 30 mph. These storms formed in a location that was covered in clouds much of the day, and as a result was slightly more stable than the storms that developed in northwest Iowa. As a result, the primary threat with these storms was large hail as the rotation in the storms was confined to the middle portions of the storm.

Visible satellite picture from 6:32 pm CDT, May 1, 2008

Thunderstorms that developed near the Missouri River Valley continued to work north producing softball sized hail near Turkey Ridge. At the same time, thunderstorms rapidly developed along the boundary across northwest Iowa. The atmosphere over northwest Iowa had become extremely unstable due to the partial sunshine in the afternoon hours. Winds in the atmosphere rapidly changed direction and speed with height, and as these storms developed they immediately began rotating, producing tornadoes shortly after their initiation.

Radar image at 6:55 pm cdt, May 1, 2008

The first tornado developed shortly before 7 PM near Sioux Center, Iowa. The tornado moved north-northwest as the circulation wrapped back into the storm. The velocity image above on the left shows the winds blowing towards (shown in green) and away (shown in red) from the radar. The tight circulation is noted where the green colors are immediately adjacent to the red colors. The corresponding radar image is shown to the right.

Radar image of the Rock Valley, Iowa tornado at 7:08 pm CDT

Additional tornadoes rapidly developed across northwest Iowa. The tornado near Rock Valley, IA developed a rather large debris circulation. Trained spotters reported a debris cloud that at times reached a quarter mile wide.

Radar image from the Lyon County Iowa tornadoes at 7:25 pm CDT

Several tornadic circulations continued to develop along the boundary across northwest Iowa. Two additional tornadic circulations are noted in the above image near Inwood and Doon, Iowa.