The upper air pattern was conducive for a major severe weather event in the Great Lakes that Friday, May 31.
An unseasonably deep low pressure system at 984 hPa crossed out of the Midwestern U.S. through the day, and then into the Upper Peninsula of Michigan.
Very warm air surged in ahead of this system.
Temperatures reached the upper twenties in Celsius (approximately 80-85 Fahrenheit) across much of southern Ontario, in addition to high dew point levels.
An unstable atmosphere (surface based lifted indices around minus 6) was the byproduct of this.
Directional wind shear was also present in the warm sector of the storm, in addition to high helicity values and a vorticity maximum approaching the lower lakes.

The situation was worsened by the presence of copious amounts of moisture, which would allow any storms that could form to become severe rather quickly.
Also, this was supportive of the HP (high-precipitation) counterpart of the supercell thunderstorm (Verkaik, 1997).
All of this added up to the distinct possibility of severe rotating storms that were messy, hard to see, and extremely dangerous.
What was needed now was a trigger, and that came in the form of a trailing cold front behind the low.
Severe thunderstorms and isolated tornadoes had already raked parts of the Midwest U.S. (particularly in Iowa and Wisconsin) the day before on May 30, associated with this same cold front (Grazulis, 1990).

The day started off on an active note with the warm front moving northwards.
A possible tornado was reported near Leamington, accompanied by golfball size hail from widespread severe thunderstorm activity in southwestern Ontario.
Following the warm frontal passage, skies cleared rapidly and temperatures quickly began to rise.
The cold front began crossing Lake Huron towards the noon hour, and with it several thunderstorms developed shortly after 1:30pm EDT, with the northernmost cell soon becoming most dominant.
Environment Canada issued a severe thunderstorm warning at 2:25pm for Bruce County (complementing the special weather statement issued early that morning).
At around 2:50pm, an F2 tornado touched down briefly in the Lion's Head area (north of Wiarton) before moving out over Georgian Bay and dissipating.

At daybreak on May 31, 1985, a strong area of low pressure was centered near Duluth, Minnesota.
A cold front extended south from the low across the western Great Lakes and then through Illinois and Missouri.
The low tracked across the northern Great Lakes during the afternoon, while the cold front progressed eastward across Indiana and western Ohio.
By late afternoon, temperatures had reached 87 degrees at Cleveland, 82 degrees at Youngstown, and 85 degrees at Erie, Pennsylvania.
At the same time, conditions in the upper atmosphere continued to become more favorable for an outbreak of severe weather.
By early afternoon, thunderstorms developed in Ontario, Canada just ahead of the cold front.

Despite a forecast for severe thunderstorms, though, the sun shone relentlessly for most of that Friday because of a fourth element: a stable air mass at about 2,000 feet, which served as a "lid" on the brew beneath.
Then, at 2:50 p.m., the "lid" moved, and huge cumulonimbus clouds—anvil-topped thunderheads—seemingly appeared out of nowhere all along the Ohio-Pennsylvania border.
The storms quickly grew into powerful tornadic supercells.
The upper air pattern was conducive for a major severe weather event in the Great Lakes that Friday, May 31. An unseasonably deep low pressure system at 984 hPa crossed out of the Midwestern U.S. through the day, and then into the Upper Peninsula of Michigan. Very warm air surged in ahead of this system. Temperatures reached the upper twenties in Celsius (approximately 80-85 Fahrenheit) across much of southern Ontario, in addition to high dew point levels. An unstable atmosphere (surface based lifted indices around minus 6) was the byproduct of this. Directional wind shear was also present in the warm sector of the storm, in addition to high helicity values and a vorticity maximum approaching the lower lakes.
The situation was worsened by the presence of copious amounts of moisture, which would allow any storms that could form to become severe rather quickly. Also, this was supportive of the HP (high-precipitation) counterpart of the supercell thunderstorm (Verkaik, 1997). All of this added up to the distinct possibility of severe rotating storms that were messy, hard to see, and extremely dangerous. What was needed now was a trigger, and that came in the form of a trailing cold front behind the low. Severe thunderstorms and isolated tornadoes had already raked parts of the Midwest U.S. (particularly in Iowa and Wisconsin) the day before on May 30, associated with this same cold front (Grazulis, 1990).
The day started off on an active note with the warm front moving northwards. A possible tornado was reported near Leamington, accompanied by golfball size hail from widespread severe thunderstorm activity in southwestern Ontario. Following the warm frontal passage, skies cleared rapidly and temperatures quickly began to rise. The cold front began crossing Lake Huron towards the noon hour, and with it several thunderstorms developed shortly after 1:30pm EDT, with the northernmost cell soon becoming most dominant. Environment Canada issued a severe thunderstorm warning at 2:25pm for Bruce County (complementing the special weather statement issued early that morning). At around 2:50pm, an F2 tornado touched down briefly in the Lion's Head area (north of Wiarton) before moving out over Georgian Bay and dissipating.
At daybreak on May 31, 1985, a strong area of low pressure was centered near Duluth, Minnesota. A cold front extended south from the low across the western Great Lakes and then through Illinois and Missouri. The low tracked across the northern Great Lakes during the afternoon, while the cold front progressed eastward across Indiana and western Ohio. By late afternoon, temperatures had reached 87 degrees at Cleveland, 82 degrees at Youngstown, and 85 degrees at Erie, Pennsylvania. At the same time, conditions in the upper atmosphere continued to become more favorable for an outbreak of severe weather. By early afternoon, thunderstorms developed in Ontario, Canada just ahead of the cold front.
Despite a forecast for severe thunderstorms, though, the sun shone relentlessly for most of that Friday because of a fourth element: a stable air mass at about 2,000 feet, which served as a "lid" on the brew beneath. Then, at 2:50 p.m., the "lid" moved, and huge cumulonimbus clouds—anvil-topped thunderheads—seemingly appeared out of nowhere all along the Ohio-Pennsylvania border. The storms quickly grew into powerful tornadic supercells.
1985 United States–Canada tornado outbreak