Northwest Iowa Tornadoes and
Large Hail in Southeast South Dakota
May 1, 2008

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.


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