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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.2.1.8. Future Trends Ble <br />Urban flooding is a naturally occurring hazard that affects cities and regions around the world, and is <br />expected to become even more problematic in the future. Damages from floods are also increasing as are <br />the number of people who are affected by them. <br />Human -induced land cover change and climate change are important factors in urban flooding. Rapid <br />population growth and increasing migration from rural areas to cities lead to intense urbanization, which <br />often increases flood risk. According to recent studies, the urban heat island effect and aerosol <br />composition can alter the climate mechanism, which plays an important role in the storm evolution of <br />urbanized regions. Global warming, the other main cause of hydrologic regime change, can induce the <br />acceleration of the water cycle, which can consequently affect the frequency and intensity of future storm <br />events. Research has shown that in the future we may not necessarily see more rainfall, but more rainfall <br />on less days. That is to say that if the monthly average total rainfall is four inches over eight different days, <br />we would now see that four inches come on three or four days. So same amount of rain, just coming more <br />at one time. <br />TOO <br />600 <br />500 <br />co- <br />ur <br />499 <br />is <br />6 <br />w <br />300 <br />3 <br />z <br />200 <br />188 <br />8 <br />--►— Drought Epidemic --e­ Rood Mass Movement Wet i^Mw Storm� <br />198'1.1983 1981.1986 1987-198,9 1997.1992 1993-1995 1996.1998 19199-2001 2002-2004 20105-20072008-2'010 <br />4.2.1.9. Indications and Forecasting <br />Currently, the operational method for forecasting flash floods at the National Weather service is to utilize <br />the Flash Flood Monitoring and Prediction software package to compare rainfall estimates with flood - <br />induced rainfall accumulation thresholds, known as flash flood guidance values. The success of this <br />guidance depends on both accuracy of radar -estimated rainfall rates and the flash -flood guidance values. <br />The National Weather Service Weather Forecast Offices issues all flash -flood advisories, watches, and <br />warnings for their respective county warning areas. The primary indicator used by forecasters to predict <br />onset of flash flooding, is when radar -based rainfall estimates exceed flash flood guidance values over f 1, <br />3, or 6 hours. Flash -flood guidance is defined as the threshold rainfall required to initiate flooding on small <br />streams that respond to rainfall within a few hours. <br />35 <br />