When most people think of flash flooding, they think of a convective event where too much rain falls too quickly. However, in Central Iowa that is not half the story. Here, ice jams along rivers are also common causes of flash flooding and are not just the concern of hydrologists. Meteorologists - especially broadcasters who are required to play several roles - must recognize the atmospheric setups that lead to ice jamming. In some cases precipitation may not be involved at all. While convective storms with heavy rain are more common in warmer times of the year, iced rivers and frozen ground obviously threaten during cooler months. Spring and summer convective storms are the usual suspect when it comes to locally heavy rain. In this case, a mid-latitude cyclone may provide the energy. Transitional periods between cooler and warmer seasons present particularly strong temperature gradients, powering even more lift and stronger storms with heavier rain. Heavy rain results from large, slow moving storms or train-echo storms where a series of storms move repeatedly across the same area. If storms under these circumstances come early enough in the year to bring heavy rain onto frozen ground and rivers, flash flooding could occur on an even higher magnitude. Heavy rain on solid frozen water and ice is possible, but rarely occurs as far south as Iowa. Ice jamming on river and water basins commonly occurs in early spring. They can occur with or without flooding. A river's ice jamming history provides a near-certain prediction it will happen again. Certain rivers and water basins are known to jam and flood every year, often enough to lightheartedly earn the designation "ice magnets" from those that observe them. These are, of course, in areas where flash flooding is a nuisance rather than a threat. Officials tend to advise flash flooding, rather than issue warnings for these places where such floods are common and do not threaten lives or property. But if ice breaks and blocks a major waterway, it can do so unexpectedly and disastrously. This creates two flash flood dangers: one upstream of the bridge where water and ice collect and can not pass, the other downstream of the bridge where flood waters rush quickly once the jam breaks. In these situations, areas upstream are first warned of flash flooding, while downstream areas are issued a watch. A sudden release of built-up water can cause a large crest wiping out most everything in its path including boulders, cars, trees, and homes. Surprisingly, flash flood deaths exceed other storm-related fatalities. An average of 77 deaths occur each year due to the direct result of flash flooding and flood events in the United States. This may be because the public does not take the threat of flooding seriously enough. They underestimate the force and power of water, sometimes choosing to drive into deadly floods. Additional flash flooding events regularly occur due to increasing urbanization. Water can't soak into paved ground. In Central Iowa, more than an inch of rain in a short time will flood some roads in urban areas. Meanwhile, agricultural land with mature corn (Central Iowa's traditionally most common crop) can easily absorb up to eight inches of rain without flooding. Accurate temperature forecasting is required simply because ice melts and breaks as temperatures warm. The quicker ice melts, the more likely jams will occur. Meteorologists must pay special attention as temperatures transition from below to above freezing. It's important to recognize that in Central Iowa, a few days of highs near 60 degrees with lows not significantly below freezing will break ice too fast, likely causing flash flooding from ice jams. Flash floods from ice jams are common during transitional weather and rarely come as a complete surprise. Hydrologists monitor river level observations and can tell if an ice jam is beginning when water readings rise upstream, and fall downstream. At this point they collect eyewitness reports on the condition of the ice and severity of the jam to determine how long it may last and how high water may rise. Action is taken to keep lives and property safe, not forgetting those directly downstream. Heavy equipment may be used to break the jam. In extreme cases, explosives are used, sometimes destroying the bridge if the threat is great enough. Fewer options exist in terms of heavy rain. Potential for flash flooding is assessed when forecasting precipitation. This may be several days in advance when a potent storm system is expected to deliver thunderstorms or long periods of rain. Sometimes a flash flood threat is not realized until smaller storms are observed bringing concentrated rain to local areas. This is one of the most difficult situations to predict precisely and accurately because of the spatial limitations of models and the time it takes to calculate and prepare a forecast. Another difficulty is the lack of historical data on flash floods. They often they occur in small, ungauged areas. The local National Weather Service office has computer software that creates a three dimensional model of water on the ground. Rainfall for all areas is determined by calibrating radar estimates with physical rain gauges. River and water basin data is also assessed to map a predicted path of water runoff. This current and common method is improved through more precise prediction of local heavy rain events and more accurate radar rainfall estimates. New research attempts to predict this rainfall estimate map before it falls. Some new models use satellite imagery to get a handle on rainfall estimates for the near future. The satellite-based rainfall data provides a spatially-even gridded dataset remotely. Rainfall may be determined by infrared data assuming colder cloud temperatures indicate those producing more rainfall. Microwave sensors can estimate rainfall from liquid droplets or suspended ice particles. That data can be correlated to what may soon reach the ground. The idea is to quickly model this data to "first guess" potential flash flooding. Although these attempts may yield only minutes more lead-time, its life-saving value improves considerably. Better flash flood forecasting requires awareness on the part of meteorologists on how temperature impacts frozen waterways. Recently, observation and distribution of real-time data has become cheaper allowing greater coverage. But faster precipitation forecasts with higher spatial resolution is still a huge step in predicting flash floods faster. Software models are not yet available to comprehensively model water runoff in urban areas. Experts must rely on loose historical data to guess what roads or buildings may flood in heavy rain. All citizens need to be aware of the serious danger of flash floods, especially those who live in areas like Central Iowa where the threat comes not only from precipitation but temperature change. Meteorologists who share their forecasts with the public should use that opportunity to stress the importance of flash flood preparedness. Flash floods are difficult to forecast because they're highly local and occur quickly. Scientists are developing new techniques to alert of flash flood dangers with greater lead-time. However, early warning is no help if ignored. References 1. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 2. Flash Floods and Hail: property and personal devastation, WW2010. Retrieved April 10, 2008 from http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/dngr/flood.rxml 3. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 4. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 5. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 6. Severe Weather Awareness - Flash Floods. Retrieved April 10, 2008, from http://www.crh.noaa.gov/mkx/?n=taw-part11-flash_floods 7. Don't Drown Turn Around, NWS Southern Region. Retrieved April 10, 2008, from http://www.srh.noaa.gov/tadd/ 8. Severe Weather Awareness - Flash Floods. Retrieved April 10, 2008, from http://www.crh.noaa.gov/mkx/?n=taw-part11-flash_floods 9. Flash Floods and Hail: property and personal devastation, WW2010. Retrieved April 10, 2008 from http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/dngr/flood.rxml 10. Don't Drown Turn Around, NWS Southern Region. Retrieved April 10, 2008, from http://www.srh.noaa.gov/tadd/ 11. Flash Floods and Hail: property and personal devastation, WW2010. Retrieved April 10, 2008 from http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/dngr/flood.rxml 12. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 13. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 14. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 15. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. 16. Meon, Gunter. (2006) Past and present challenges in flash flood forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%201/Ab_08_Meon.pdf 17. Meon, Gunter. (2006) Past and present challenges in flash flood forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%201/Ab_08_Meon.pdf 18. Mecikalski, John R. (2006) Satellite-based short-term thunderstorm forecasting toward flash flood predictions: Recent developments for Mesoamerica in the context of SERVIR. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_30_Mecikalski.pdf 19. Love, Timothy B.. (2006) An overview of Satellite-Based Precipitation Estimation With Respect to Flash Flood Modeling and Forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_17_Love.pdf 20. Love, Timothy B.. (2006) An overview of Satellite-Based Precipitation Estimation With Respect to Flash Flood Modeling and Forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_17_Love.pdf 21. Love, Timothy B.. (2006) An overview of Satellite-Based Precipitation Estimation With Respect to Flash Flood Modeling and Forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_17_Love.pdf 22. Love, Timothy B.. (2006) An overview of Satellite-Based Precipitation Estimation With Respect to Flash Flood Modeling and Forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_17_Love.pdf 23. Valverde, Angel Lios Aldana. (2006) Advances in predicting flash floods. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%202/Ab_14_Aldana%20(eng).pdf 24. Meon, Gunter. (2006) Past and present challenges in flash flood forecasting. Retrieved April 7, 2008 from http://www.nws.noaa.gov/iao/FFW/2006/Presentations/Session%201/Ab_08_Meon.pdf 25. Marian Baker, NOAA Hydrologist for Central Iowa, personal communication, April 9, 2008. |