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The 31 May 1985 Tornado Outbreak
During the aftemoon of May 31, 1985 a powerful cold front moved through Southern Ontario triggering a series of very damaging tornadoes along a line of supercells. Twelve people were killed and 224 others injured as the storms moved across the southern part of the province. Property damage was estimated at well over $100 million.
Thirteen separate tornadoes occurred in Ontario as part of a larger outbreak of 40 tornadoes that also affected parts of Ohio, New York, and Pennsylvania. Figure 1 shows all of the reported tornadoes in the lower Great Lakes region for May 1985. All but two of those were part of the 31 May 1985 outbreak. Two of the Ontario tornadoes were killers. The F4 tornado just west of Lake Simcoe resulted in 8 deaths and 155 injuries in the city of Barrie, and the long-track F4 tornado (just southwest of Lake Simcoe resulted in 4 deaths and 69 injuries and caused damage along a 115 kilometre track parallel to and just north of highway 9. A closeup of the southwestern Ontario tornado tracks is shown in Figure 2.
A quick look at the analyses for 1200 UTC 31 May 1985 shows a typical 500 mb pattern for spring tornado outbreaks in the Great Lakes region (Figure 3). There is a progressive but intensifying upper low as evidenced by the strong 12-hour height falls and difluent flow over southern Ontario. An intense short-wave trough in the vorticity analysis is approaching the region from the west.
At the jet stream level (250 mb in this case) there are two jet streaks that play a role in causing the massive amounts of divergence over the Great Lakes region (Figure 4). The sub-tropical jet to the southwest has its left-exit region approaching lower Michigan. Note the strong divergence over Michigan approaching soutwestern Ontario. The polar jet to the north will also come into play as its right-entrance region crosses northeastern Ontario later in the day. This "difluent jet" pattern is also commonly observed in association with major tornado outbreaks.
Figure 5 shows a surface weather depiction from 1800 UTC 31 May 1985. This was about the time that the storms exploded into a line of supercells over southwestern Ontario just ahead of the cold front. Note the unseasonably deep low (984 mb) tracking across the upper Great Lakes just ahead of the 500 mb low. This is another commonly observed feature with major spring tornado outbreaks in the Great Lakes region. The airmass was moderately unstable with surface based lifted indices approaching -6 C north of Lake Erie (not shown).
The sequence of visible GOES satellite images at 1900Z, 2100Z, and 2300Z on 31 May 1985 (Figures 6, 7, and 8) show the explosive development and evolution of the storms from mid-afternoon until just before sunset. One point of interest is the effect of lake breeze fronts on thunderstorm development with southwesterly surface winds. The northward advancing lake breeze front from Lake Erie merges with the Lake Ontario front to inhibit the southward extent of the supercells over Ontario.
An interesting footnote: During the early evening, the line of storms in Pennsylvania and New York built northeastward across eastern Lake Erie. One of the supercells developed over eastern sections of the Niagara Peninsula. Although baseball sized hail was reported near Niagara-on-the-Lake, no tornadoes were reported.
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Figures.
Figure 1. Tracks of 33 of the 40 tornadoes on 31 May, 1985 in Ontario, Ohio, New York, and Pennsylvania. (The F1 tornado in southeast Michigan and the F1 in northwest Ohio are not part of the outbreak)
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Figure 2. Tracks of the three main tornadoes in southern Ontario on 31 May, 1985. (Diagram courtesy of Project Atmosphere.)
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Figure 3. 500 mb analysis at 1200Z 31 May 1985.
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Figure 4. 250 mb analysis at 1200Z 31 May 1985.
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Figure 5. Surface mean sea-level pressure and frontal positions at 1800Z 31 May 1985 (NCEP Reanalysis).
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Satellite Imagery
Figure 6. GOES EAST Visible Image at 1900 UTC 31 May, 1985.
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Figure 7. GOES EAST Visible Image at 2100 UTC 31 May, 1985.
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Figure 8. GOES EAST Visible Image at 2300 UTC 31 May, 1985.
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