Sully, Hyde, and Hand Tornado Outbreak of 24 August 2006

Introduction

During the late afternoon to early evening hours on Thursday August 24th, portions of South Dakota experienced a severe weather outbreak that is uncommon for late summer. An unseasonably strong upper level trough, sweeping across the northern U.S, influenced the weather across the state.  Concurrently, a strong upper level jet of 70 to 80 knots entered southeastern South Dakota from the southwest.  These two features induced a “double barrel” surface low pressure pattern across central South Dakota.

 


 

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Clear skies and southeast winds ahead of the surface lows allowed for warming surface temperatures at both Pierre and Mobridge (91 degrees F at 3:00 pm CDT) and an unusual westward extent of the tropical airmass (Mobridge and Pierre surface dewpoints of 68 degrees F at 3:00 pm CDT). A prefrontal trough of surface low pressure linked the two low pressure centers and proved necessary in the development of isolated convection.

 


 

Sully County Tornado

The first tornado of the day occurred about 5 miles northeast of Onida in Sully Co. from 4:40 to 4:52 pm CDT. The tornado was classified F-1.

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The storm intensified quickly as it traveled through northern Sully Co. By 4:40 pm CDT, the storm had organized into a supercell. Supercell thunderstorms typically produce severe weather, and are characterized by highly organized, persistent, rotating updrafts or mesocyclones. The organization of the storm’s updrafts and downdrafts can allow it to exist for several hours, as was seen in the Sully Hyde Hand storm. Some clues that indicated the Onida storm was highly organized at 4:40 pm CDT, are the area of higher reflectivity (strong updraft) and the “V-notch” or “Flying Eagle” (high altitude updraft) structure.
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A moderate mesocyclone was evident in the Storm Relative Motion velocity (SRM) data with 37 knots of rotational velocity at an approximate range of 75 nm. Additionally, a tornado vortex signature (TVS) was noted with the Maximum Delta V of 67 knots at approximately 15100 ft AGL. A TVS is defined as gate-to-gate azimuthal shear that is possibly associated with tornadic circulations. According to the National Severe Storms Laboratory, only 30% of all TVS signatures are associated with tornadoes. 

 

 


 

Sully County Tornado (continued) 

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Photo Courtesy of Andy Fischer

 


 

Miller Tornado

The fourth tornado of the day occurred approximately 4 miles west of Miller in Hand Co. from 6:07 to 6:20 pm CDT. The tornado was classified F-2.

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At 6:04 pm CDT, the supercell thunderstorm was located northwest of Miller with a highly organized structure. To its advantage, the thunderstorm used three boundaries (see above) and maintained severity. The mesocyclone not only propagated along the warm front but also anchored on the prefrontal trough. Although some reflectivity noted close to the prefrontal trough is indeed that of the three-body scatter spike (TBSS, indicative of large hail), looped analysis does support the location of the prefrontal trough. Warm, moist southeast flow was predominant in this southeast sector and fueled the storm’s updraft. Surface flow to the immediate northwest of the storm, the storms outflow, was relatively cool and out of the northwest. The strong prefrontal trough, as shown in the figures above, was aided by the strong surface inflow to the storm in holding off the rear-flank downdraft or outflow. Weaker thunderstorms typically do not take on this organization. Thus, allowing their outflow or downdrafts to choke the much needed warm humid airmass, or fuel, as the cell matures. 

 


 

 

Miller Tornado (continued) 

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The image directly above and just to the right depict data that was taken of the storm at approximately the same time the tornado was on the ground near Miller. 
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The storm appeared ready to tornado again close to Miller as all indications were present in the velocity data. The storm scale circulation was classified as a strong mesocyclone with approximately 52 knots of rotational velocity at about 62 nm out from the radar. A TVS was also present with a Low-Level Delta V of 77 knots in the lowest slice or 5600 ft agl.

 


 

Miller County Tornado (continued)

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Photo courtesy of Hand County Sheriff, Doug DeBoer of the tornado approximately 5 miles west of Miller.

 


 

Wessington/Majors Gulch Tornado

The fifth and final tornado to affect the Aberdeen county warning area was the Wessington tornado. While in Hand County, the tornado was rated an F-2. This tornado started approximately 5 miles west of Wessington at 6:28 PM in a locale known as Majors Gulch. The tornado tracked in Hand Co. for approximately 5 miles before entering Beadle Co., about 2 miles south of Wessington, at 6:37 PM. This was the same tornado that would track for roughly 24.5 miles and affect the Wolsey vicinity. More information on this tornado that affected Beadle Co. can be found on the NWS Sioux Falls website.

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By 6:33 pm CDT, the organization and intensity again increased rapidly. A well defined and impressive Bounded Weak Echo Region (BWER) was noted at roughly 12000 ft agl. A BWER is defined as a vertical channel of weak radar returns, surrounded on the sides and top by stronger returns. The BWER is a signature of a very intense storm and is located in the updraft region. The reason for the lower radar returns is the updraft carries air upward at an extremely fast rate. This doesn’t allow raindrops or hydrometeors enough time to grow.
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Another impressive feature of the storm at 6:33 pm CDT is the gate-to-gate signatures visible in the lowest 4 slices. The Low-Level Delta V, TVS, at this time was 102 knots at 5600 ft agl. Rotational velocity was incredible with greater than 50 knots from 5000 to 32000 ft agl.

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