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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />activities. Trails and paths near lakes and rivers are often flooded, preventing bicycling, jogging, <br />and walking. Recreational departments will require extra labor hours to return recreational <br />resources to proper working conditions. <br />4.3.6.4. Potential for cascading effects <br />Most cascading effects associated with extreme rainfall are identical to those associated with flash - <br />flooding and urban flooding. <br />Extreme rainfall hazards can easily be compounded by other pre-existing hazards, as well as hazards that <br />develop after an event. In many cases, extreme rainfall --especially of shorter durations --occurs with severe <br />supercell thunderstorms, squall lines, and mesoscale convective systems. Almost by definition, these <br />systems are multi -hazard events. Thus, straight-line downburst winds, large hail, tornadoes, and frequent <br />lightning are often associated with the same storms that produce extreme rainfall rates. Power may be <br />out, which complicates efforts to remove water using sump pumps. This was the case in June of 2013, <br />following a major wind event in the Twin Cities. The July 23-24 super storm produced record -setting and <br />basement -inundating rainfall from storms that also produced heavy damage from tornadoes. There were <br />instances during the evening in which tornado warnings and Flash -Flood warnings were in effect for the <br />same area simultaneously. Seeking shelter in a basement posed flood -related risks. <br />Extreme rainfall also can play a role in tree mortality, and associated damages to public sidewalks, personal <br />property, and electrical systems. On June 21, 2013, a major tree fall event that was also the largest <br />weather -related power outage in state history, resulted notjust from the prolonged downburst winds, but <br />also from intense rains that fell both earlier in the day, and during the storm. Though the winds were 50- <br />60 mph with some higher gusts for over 10 minutes in many places, they produced far more damage than <br />would be expected at those speeds. The severity of tree damage likely resulted from the saturated soils, <br />which provided less resistance than normal, allowing trees to become "loose" and eventually topple. <br />Whether short or prolonged in duration, extreme rainfall is often associated with summerlike air masses. <br />Thus, extreme rainfall may occur before, during, or after an extreme heat event. Similarly, extreme rainfall <br />can occur during drought conditions, as was the case in 1987. <br />Additional specific cases of high -impact multi -hazard extreme rainfall events will be outlined in the <br />Historical (statistical) data/previous occurrence section. <br />4.3.6.5. Geographic scope of hazard Blc <br />Extreme rainfall rates may cover between 50 and 1500 square miles at a time. After accounting for <br />movement, the total area affected by rainfall more than 3 inches may cover thousands of square miles, <br />with hundreds of square miles receiving over six inches of rain. In exceptionally rare cases, 6-inch rainfall <br />totals may cover an area greater than 1,000 square miles --approximately the size of two Twin Cities area <br />counties. The Minnesota State Climatology Office has documented 12 of these "mega" rainfall events in <br />Minnesota since the mid-1800s. These events are always associated with catastrophic damage and often <br />loss of life. <br />119 <br />