50 years ago today, the upper Midwest was devastated by an outbreak of severe weather. A similar storm today would result in what might be one of the largest-ever severe thunderstorm losses to the insurance industry. For four consecutive days in May 1965, severe weather, including 37 significant tornadoes, of which at least nine were major tornadoes, affected much of the central United States. The most devastating part of the outbreak occurred when six tornadoes swept across the western and northern portions of Minneapolis/St. Paul. Of the six tornadoes that swept the seven-county region, four were rated F4, one was rated F3, and the other produced F2 damage.

ChanhassenTornado_Milligan

Shown is a photo of a tornado crossing to the west of the junction of highways 7 and 101 on May 6, 1965 (taken by Minnetonka resident. H. B. Milligan). It is believed that this was the tornado that touched down in Chanhassen at 6:27 p.m. and dissipated in Deephaven at 6:43 pm.

The outbreak was significant in several ways. First, it occurred just three weeks after the historic Palm Sunday tornado outbrea, which no doubt left local residents feeling a heightened sense of alarm about storms. Second, it was the first time in Minneapolis/St. Paul history that civil defense sirens, now used nation-wide, were used to warn people about severe weather. The U.S. Weather Bureau issued quick and successful warnings that were widely communicated by local radio and television stations. These significant new communication methods likely saved many lives, despite the fact that the storms killed 13 and injured 683. Finally, the tornadoes served as yet another example that helped dispel the myth that tornadoes won’t hit a city, a lake or a river.

Historical accounts have focused mostly on the tornadoes during the outbreak because of their remarkable pattern: at a single point in time three tornadoes were on the ground, and two tornadoes hit the same location just hours apart. Some of the hardest-hit cities included Chanhassen, Deephaven, Fridley, Mounds View and the Spring Lake Park suburbs, all of which have undergone tremendous growth since 1965. It is estimated that 600 homes were destroyed during the series of storms, and over a thousand more were damaged. Today, the affected suburbs have expanded from what was farmland 50 years ago, to what is now primarily residential.

The increased number of homes and the increased value of homes in Minneapolis/St. Paul suburbs also increase the potential for catastrophic damage from a similar future event. In some cities, the type of housing exposed to storm perils affects the potential loss severity. Two of the F4 tornadoes touched down in the west suburbs, and these suburbs have seen explosive growth since 1965. For example, one of the 1965 F4 tornadoes reportedly impacted 35 homes on Lotus Lake and 50 homes on Christmas Lake. This area now hosts many multi-million dollar lake homes. Home values in other nearby neighborhoods have grown significantly as well, including multi-million dollar homes that line the expansive shores of Lake Minnetonka. Fridley, Mounds View and Spring Lake Park were impacted by two F4 tornadoes that touched down just hours apart and crossed paths. Those cities may not include as many high-value homes, but the number of residential properties has grown, and now includes not only more residential properties, but high-value commercial properties as well. For example, one of the 1965 tornadoes tracked less than a mile from what is now the Medtronic world headquarters campus.

Despite the loss of life and property damage caused by the May, 1965 storms, it’s important to note that the historical data includes only limited reports of hail and damaging winds. These perils most likely also impacted the area 50 years ago, and arguably, including damage from these lesser-reported perils would increase the financial impact of the storms.

Although there are large uncertainties on exact track, width, and strength of all the tornadoes that occurred 50 years ago across the seven-county metro region, BMS Analytics digitized these historic tracks to create a deterministic scenario in an attempt to understand the potential impact of a similar storm today. The scenario suggests an insurance industry loss between $10 – $14 billion dollars. This is equivalent to a return period of 2,500 – 7,500 years according to various catastrophe models.

1965_TornadoTrack_Analysis

BMS Analytics digitized deterministic tornado scenarios with four perturbation to account for track uncertainty

Recently we have seen images of single tornadoes hitting cities like Tuscaloosa, AL; Joplin, MO; and Moore, OK and causing billions of dollars worth of insured damages ($6 Billion in total insured loss). Recalling the events of early May, 1965, is a reminder that several major tornadoes can hit a large metropolitan area such as Minneapolis/St. Paul on the same day. Although unlikely, a similar event would be extremely damaging due to the ongoing population and wealth growth described above. And if such an event occurred, it is likely a similar outbreak would have a large impact on the insurance industry.