NSSL SWAT Case Study - 03 July 1999
Northern Michigan



A supercell formed over the Northern Lower Peninsula of Michigan on the evening of July 3, 1999. The storm moved east-southeast bringing heavy rains and strong winds as well two reported tornadoes. The original report listed the second and third tornadoes as a continuations from the previous county, thus the original reports listed 1 tornado, crossing 3 counties with, a total path of 29 miles. Since tornado segments are recorded in STORM DATA by counties, it is not unusual to see one tornado that crossed county borders reported as more than one tornado. So we adjusted the path of the tornado slightly to reflect a single tornado that crossed into three counties. When taking a further look at the reported path of the tornado and the WSR-88D radar it appeared as if there were two tornadoes that had occured. This is a case of a cyclic supercell and clearly shows an occluding mesocyclone with the new mesocyclone forming out ahead.

Presented here are various WSR-88D radar images from the Gaylord, MI radar (KAPX). Included in some of the WSR-88D images, there is output from NSSL's Mesocyclone Detection Algorithm (MDA) and Tornado Detection Algorithm. A yellow circle depicts a mesocyclone. The red-in-yellow circle depicts a mesocyclone whose base is at the lowest elevation scan (where it is a more likely tornado threat). A red triangle indicates the location of a tornado as detected by the TDA.

Overview of the day
  • Reflectivity loop (21:00-00:23).
  • A Closer Look
  • Reflectivity loop (23:21-00:03).
  • Storm Relative Velocity (SRV) loop (23:21-00:03).
  • DESCRIPTION: The reflectivity loop was constructed using the base reflectivity at half hour intervals. It shows a supercell developing over the Gaylord, MI area and moving east-southeast. As the storm intensifies a F1 tornado develops near the Montmorency/Oscoda County line around 7:21 pm EST (23:21 UTC). The tornado strengthened to an F2 and passed through the city of Commins, MI ,dissipating around 7:48 pm EST (23:48 UTC). A second tornado occurred beginning near the Huron State Forest and the Alcona County (the county to the east of Oscoda) line from approximately 7:51 pm EST (23:51 UTC) to 7:58 pm EST (23:58 UTC).

    A closer look at the base reflectivity and storm relative velocity while the storm is tornadic are shown. The loops are approximately 5 minute intervals at the 0.5 degree elevation angle. One thing to note in these loops is the border of the Huron National Forest, which can be seen as the white horizontal line that separates the Oscoda County name from the Mio City name on the images.

    The Occlusion
  • SRV at 23:43 UTC
  • SRV at 23:51 UTC
  • DESCRIPTION: The WSR-88D radar was able to capture the cyclic nature of the supercell as seen in the above images. At 23:41 UTC the first tornado and associated mesocyclone can be seen while to the east a larger area of rotation has developed. At 23:51 UTC the mesocyclone associated with the first tornado has begun to weaken at lower levels while the new mesocyclone begins to dominate.

    Various Images
  • Cross Section at 23:21 UTC.
  • SRV 4 panel at 23:31 UTC.
  • Reflectivity at 23:56 UTC
  • SRV at 23:56 UTC
  • DESCRIPTION: A cross section taken at 23:21 UTC shows the vertical structure of the supercell from the southwest to the northeast shown by the white line. The 4 panel image of SRV taken at 23:31 UTC shows a well defined TVS that is tilted to the northeast with height. A small white circle is present as a reference point for the viewer. On the base reflectivity and storm relative velocity images at 23:56 UTC the TDA algorithm and the associated TVS tracks (previous track shown in white and future tracks shown in purple) are shown. The TDA algorithm identifies two tornadoes, e and l, along approximately the same track as the observed damage path. Notice how close to two paths are just north of the state forest. The damage survey team may not have noticed a break in the damage and associated the damage to one tornado. The TDA algorithm is also in agreement that there may have been two separate tornadoes on the evening of July 3, 1999.


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