The 49th Parallel severe rainstorm - An example of elevated thunderstorms and their impact, June 8 to 11, 2002

Abstract

Severe flooding and record high river flows occurred over southern portions of Northwestern Ontario, southeastern Manitoba, and northern Minnesota (MOM area), as the result of a series of mesoscale convective events that occurred from the evening of 8 June 2002 through the early morning hours of 11 June 2002. Synoptically, a west to east quasi-stationary surface frontal boundary from North Dakota across northern Minnesota and Wisconsin provided the focus for a series of elevated thunderstorms. Mesoscale forcing resulted in “training” of thunderstorm echoes over the Winnipeg River and Roseau River basins for two consecutive days. The highest rainfall rates occurred on 9 and 10 June associated with intense thunderstorms that were continuously generating and moving across the area from the Roseau River to just southwest of Upsala, Ontario. This training of thunderstorm cells, or “train echoes”, resulted in a swath of rainfall accumulations of 200-400 mm in less than 72 hours.

This study looked at the various forcing mechanisms and the scale interaction that caused the extreme flooding event. This study also used hydrometric measurements to determine the severity of the rainfall and the impact it had on the basins. Record one-day rainfalls were recorded at Mine Centre, ON (172.0 mm) and Atikokan, ON (176.4 mm) that greatly exceeded the previous one-day record rainfalls. The three days of severe rainfall resulted in a record net 7-day inflow for Rainy Lake. The measured peak discharges on the Turtle River and Atikokan River have estimated return periods of about 500 years. In Minnesota, the Roseau River recorded a peak discharge with a recurrence interval greater than 500 years, which is about 120 percent greater than the previous peak discharge. This series of MCS’s caused hundreds of millions of dollars of damage to portions of two provinces and two states.