Just before 1:00 AM on June 22nd severe weather struck Ottawa County producing an EF0 tornado east of Zeeland and a swath of EF0 rated straightline wind damage from Lamont to Marne. The severe winds continued east of Marne into Alpine Township in Kent County. Below are the damage surveys conducted by the NWS of the EF0 tornado (left graphic) and the straightline wind swath (right graphic). Both the tornado and the swath of damaging winds produced wind gusts of 70 to 85 mph or EF0 on the Enhanced Fujita Scale.
The severe weather was associated with bowing segements of a squall line (see animation below). Such bowing segments are called "bow echoes" because of their bow appearance on radar. The squall line originated over southern Wisconsin where it had already produced large hail, damaging winds and tornadoes earlier in the evening. The squall line moved out over Lake Michigan around 11:00 PM Monday night and began to bow over the middle of Lake Michigan before it moved ashore in Muskegon, Ottawa and Allegan Counties around 12:40 AM. A second cluster of thunderstorms developed offshore of Muskegon and Northern Ottawa Counties ahead of the approaching bowing squall line around midnight. The new cluster quickly evolved into a new bow echo itself over central Ottawa County as it merged with the original and larger bow echo between 12:45 and 1:00 AM. The consolidated bow echo squall line then raced across West Central Lower Michigan pushing into the Grand Rapids area at 1:15 AM and then Lansing at 2:30 AM. The squalls produced continuous lightning, briefly torrential rains, and widespread wind gusts of 50 to 60 mph.
Radar Loop from about Midnight through 300 AM
The tornado occurred at the intersection of the two bowing segments over south central Ottawa County east of Zeeland. Merging winds from the two squalls created shear that was picked up and stretched by the squall itself spinning up the tornado. The tornado moved to the east-southeast as the two squalls zipped together. These kind of tornadoes are called "spin-ups" becuase they grow upwards into the thunderstorm rather than downwards as does the classic Oklahoma tornado. The fact that they spin-up and are often small in scale makes them very difficult to detect using doppler radar alone.
The wind damage swath, located to the north of the tornado touchdown and beginning just west of Lamont, was associated with a rapidly developing circulation (technically a mesocyclone) associated with the new bowing squall line segment. Such circulations occurring at altitudes of 1 to 2 miles above the ground are common with bowing squall lines. However, sometimes they produce wind gusts of 75 to 100 mph at the surface as they initially develop. The winds can be tornadic, straightline, or a combination of both. The straightline wind occurrances usually result in a swath of damage several miles in width and tens of miles in length, whereas bow echo tornadoes tend to produce more concentrated damage over relatively shorter paths.
The image below is a Base Velocity and Reflectivity side by side. The bow echo was moving across Ottawa County at this time (around 100 am).
Click on either map to enlarge
Bow Echoes are a radar echo that is linear, but bent outward in a bow shape. Damaging straight line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation. Below is a typical progression of a bow echo. Often times the most significant damage occurs along the dashed line in the image, near the apex of the bow. Strong damaging winds are likely near the location of "c" also, which is the cyclonically rotating northern end of the line. If the rotation becomes tight enough tornadoes are possible. The northern portion of the line in Ottawa County began to exhibit rotation and a Tornado Warning was issued. The survey conducted did not find any tornadic damage however.
Idealized development of a bowing squall line segment or "bow echo".