Derecho - May 31, 1998

During the early morning hours of May 31st, 1998, a special type of squall line called a "Derecho" hit southeast lower Michigan. Derecho is a Spanish word meaning right or right turn, which is given to this type of squall line because of how it is formed. A Derecho usually forms along a nearly stationary front that lies in some sort of west to east direction. That front usually separates a very warm, moist, and unstable air mass to the south, from a relatively cooler and drier air mass to the north. The Derecho usually moves along the front toward the east, and turns to the right (or south) into the warm, moist and unstable air mass.

The Derecho moved across southeast lower Michigan from around 6 a.m. until close to 9 a.m. The speed of the line was close to 70 mph, especially across the Saginaw Valley, Thumb and Flint areas. Unfortunately there were two deaths and two injuries as a result of the Derecho. The deaths and injuries were all a result of falling trees. One death occurred in Pinconning (Bay county) as a tree fell onto a house, and the other was in Huron county when a tree fell onto a tent. The two injuries, which happened in Holly (Oakland county), were also a result of a tree falling onto a tent. This squall line was also responsible for widespread damage across southeast lower Michigan. The damage was most concentrated from Detroit's northern suburbs north across the Flint, Saginaw Valley, and Thumb regions. Preliminary damage estimates from the seventeen counties that make up southeast lower Michigan stood at 5.47 million dollars. There was also one weak (F0) tornado near the Genesee and Shiawassee county line. However, significant tornado damage was not discernable from the straight-line wind damage. At the Saginaw (MBS) airport, a wind gust of 86 mph was recorded, and at the Essexville Coast Guard station a wind gust of 81 mph was measured.

A preliminary list of storm damage can be found in the Local Storm Report.  Final damage reports can be seen in the Storm Data publication for May 1998.

There were 87 distinct severe weather reports across southeast Michigan from the Derecho. Of these 87, 84 occurred in a two hour time span from 5:50 a.m. to 7:50 a.m.! There were a total of 25 Severe Thunderstorm or Tornado Warnings issued by the National Weather Service here at White Lake. 18 of those warnings were issued between 6 a.m. and 7 a.m.! Below is a listing of the warnings issued:

537 am..... Midland county..... Severe Thunderstorm Warning
546 am..... Bay, Saginaw and Shiawassee counties..... Severe Thunderstorm Warning
606 am..... Livingston, Washtenaw, and Lenawee counties..... Severe Thunderstorm Warning
613 am..... Tuscola and Genesee counties.... Severe Thunderstorm Warning
625 am..... Shiawassee and Genesee counties..... Tornado Warning
631 am..... Huron, Sanilac, Lapeer, and Oakland counties..... Severe Thunderstorm Warning
653 am..... St. Clair, Macomb, Wayne, and Monroe counties..... Severe Thunderstorm Warning
659 am..... Livingston, Washtenaw, and Lenawee counties..... Severe Thunderstorm Warning
733 am..... Wayne and Macomb counties..... Tornado Warning
846 am..... Monroe county..... Severe Thunderstorm Warning

The first Base Reflectivity image below is from 5:35 a.m., just before the first warning for Midland county was issued. The squall line is easy to see in this image. It is important to note that the most dangerous part of a squall line is the leading edge. This is where the strongest winds in the entire complex are usually located. While squall lines, including this one, can produce severe hail and weak tornadoes, the most dangerous part of the thunderstorm complex is the straight-line winds which can reach to over 100 mph. This Derecho did produce wind gusts over 100 mph on the west side of the state, closer to Grand Rapids, Holland, and Muskegon.

During the lifetime of this Derecho there were several instances of Line Echo Wave Patterns (LEWP), which are bow-shaped segments of convective cells. In a squall line, mesocyclones and weak tornadoes are most likely if and when these LEWP develop. In the 6:25 a.m. reflectivity image, you can see one in eastern Shiawassee county and another one to the southwest in Ingham county. At 6:25 a.m. a spotter reported a tornado on the ground at the Shiawassee/Genesee county border.

Below is the Storm Relative Velocity (SRM) image from 6:25 a.m. The SRM shows velocity with respect to the thunderstorm's movement.  The green area indicates motion toward the radar (inbound), and red indicates motion away from the radar (outbound).  The radar's location is toward the southeast or bottom right portion of the image, and labeled DTX.  This couplet of inbound and outbound motion over eastern Shiawassee county indicated strong cyclonic rotation in the lowest levels of the storm, about 6000 feet above the ground.

The next reflectivity image is from 6:45 a.m., just before the leading edge of the Derecho hit the National Weather Service office. Also included is a Base Velocity image from 6:45 a.m. This velocity image shows the speed and direction of precipitation targets without taking into account storm motion. Just to the northwest of the DTX office, the velocity image was indicating wind speeds of 50+ knots (or 58+ mph). At 6:55 a.m. a wind gust of 67 mph was recorded at the DTX office.

The last two reflectivity images show the progression of the Derecho through southeast lower Michigan. Notice how quick the line moves through the Thumb region, and how the storm slows through the Metro Detroit area. The first image is from 7:30 a.m. and the second image is from 8:05 a.m.

Lastly, damage photos are included. The first three are from Midland county courtesy of the Midland County Emergency Manager, Roger Garner. The last two are from a spotter, Larry Lewicki, and the pictures were taken in Bay City. Our thanks to them.




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