The 1985 United States–Canada tornado outbreak, referred to as the Barrie tornado outbreak in Canada, was a major tornado outbreak that occurred in Ohio, Pennsylvania, New York, and Ontario, on May 31, 1985. In all, 43 tornadoes were counted including 14 in Ontario, Canada. Ninety people were killed, with 14 deaths occurring in Canada, and 76 occurring in the United States. It remains the largest and most intense tornado outbreak ever to hit this region, and the worst tornado outbreak in Pennsylvania history in terms of deaths and destruction.
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 °F (27–29 °C)) 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 which would be 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 Midwestern United States (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 °F (31 °C) at Cleveland, 82 degrees at Youngstown, and 85 °F (29 °C) 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 above ground level, 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, appeared seemingly out of nowhere all along the Ohio-Pennsylvania border. The storms quickly grew into powerful tornadic supercells.
Grand Valley–Tottenham, Ontario
A rotating storm system, which had initially developed east of Clinton, Ontario, moved quickly to the northeast. Approaching the northern outskirts of Arthur, the storm system produced the longest-tracked tornado in Canadian history at approximately 4:15 p.m. Initially, many power lines and hydro towers were destroyed by tornado, including those used to deliver electricity from the Bruce Nuclear Generating Station to the Greater Toronto Area. Paralleling Ontario County Road 109 and destroying several properties, the tornado quickly widened, intensified, and reached violent proportions by the time it reached the small crossroads community of Grand Valley, just before 4:30 p.m. By this point, the tornado's damage path had increased to approximately 200 metres (660 ft) wide.
The tornado caused severe damage in the small town, where two people were killed. An elderly woman visiting from Scotland died as the home she was sheltering in was destroyed, and a man was killed in his pickup truck on a nearby farm. The worst damage was found along Amaranth Street (running west to east, parallel to the tornado's path) where the Grand Valley Public Library, three churches, including the Grand Valley Church of Christ, and many other masonry-style homes were severely damaged or destroyed. In Grand Valley, approximately sixty structures in total sustained damage. The most severe was on the north side of Amaranth Street, where some homes and buildings exhibited what was described by surveyors as "classic F4 damage". The roof of the Grand Valley Public Library was found 200 m (220 yd) away, deposited onto a nearby house.
Continuing eastward through open country, the tornado brushed the northern outskirts of Orangeville (more specifically the community of Mono) about fifteen minutes later. Here the southern portion of the Mono Shopping Plaza was near-completely swept, injuring 67 people — one of them seriously. The tornado then caused extensive damage to approximately fifty buildings about 2 km (1.2 mi) south of the town of Tottenham at around 5:00 p.m.; many of these buildings were only recently built. Two more people died here, as an elderly man was crushed under an equipment shed on his farm and a woman was killed in her home. The tornado continued to the east-northeast, crossing Highway 400 into the York region. Passing to the between the cities of Newmarket and Bradford, the tornado lifted west of Mount Albert at 5:25 p.m.
With a path length of around 115 km (71 mi), the Grand Valley–Tottenham tornado was officially determined to be the longest-tracked tornado in Canadian history; a record that still stands today. When the earliest tornado track maps were published in 1986, the official path of the tornado extended almost twice as far towards the Peterborough area. In more recent years, this theory has been disproven; the supercell that produced this tornado was a cyclic one, and it dropped two separate F2 tornadoes following the dissipation of the Grand Valley tornado.
At about the same time the Lion's Head tornado dissipated, two very severe thunderstorms had developed (that likely owed their inception, at least partially, to lake breeze convergence): one to the east of Clinton and another farther to the north, in the Walkerton area. These two developing supercells would grow into a pair of devastating storms within the next hour—likely the most prolific tornado producers in Canadian history to date.
The second tornado of the day touched down south of Hopeville around 3:45pm, causing some localized F3 damage along its track. This tornado lifted after a 17-kilometre (11 mi) path, but another tornado quickly formed just north of Corbetton, in northern Dufferin County, at about 4:15pm. It stayed over rural areas for most of its 40-kilometre (25 mi) path, however a few homes (especially in the Terra Nova and Mansfield area) sustained F3 damage. It has been disputed whether this path was of two separate tornadoes or just one. Shortly after this tornado dissipated, there were hints of another brief touchdown near Angus in the Blackdown Park training area of Canadian Forces Base Borden (north of Alliston). The next tornado was the last of this storm, but was the most infamous one. It formed in southern Simcoe County (Essa Township), less than 10 kilometres (6.2 mi) southwest of Highway 400 and the Barrie city limits.