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FutureStarrFlash Flood '
Flash Flood Warnings In Effect As Storm Brings Rain, Thunder, Hail To SouthlandHeavy rain from a late winter storm hit Los Angeles and Ventura counties on Sunday afternoon while the Southland's mountains picked up a significant amount of snow from a late winter storm that promised more precipitation as the night wore on. Suggested Citation:"6 Flash Flood Warning Process in Los Angeles and Ventura Counties." National Research Council. 2005. Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California. Washington, DC: The National Academies Press. doi: 10.17226/11128. Forecasting flash floods in Los Angeles and Ventura Counties is a complex process that begins with an understanding of the geographical makeup of the counties and knowledge of the preferential regions of heavy rainfall (i.e., precipitation climatology) for the area of responsibility. The geography and orography of the region for which the National Weather Service (NWS) Los Angeles-Oxnard (LOX) office is responsible are among the more varied in the United States. The elevation ranges from sea level to 10,500 feet over a mere 50 miles, and the land surface varies from ocean to islands, mountains to valleys, desert to agricultural lands to major metropolitan areas. Mountain chains run not only roughly parallel to the coastline but also nearly east-west, making them almost perpendicular to the moist, southerly flow of approaching winter storms. The forecast process requires a forecaster to be cognizant not only of the antecedent hydrological and current meteorological conditions that may trigger a flash flood, but also of modifications of approaching weather systems as they impinge on the complex, mountainous terrain surrounding the highly populous urban areas in southern California. The impact of urbanization on both weather patterns and water runoff and the potential for recent fire-burned regions to modify watershed response and runoff must also be factored into the flash flood warning process. When it becomes clear that the conditions are ripe for the potential for flash flooding, the NWS Los Angeles-Oxnard Weather Forecast Office (WFO) has the primary responsibility for issuing flash flood warnings to the public. The process for communicating and disseminating warnings to the community is complex, involving not only the NWS, but other government and private agencies as well. The end-to-end process of formulating, issuing, and disseminating flash flood forecasts, watches, and warnings to the Los Angeles and Ventura County communities is documented in the following sections.
During the several days prior to the onset of heavy rainfall in the Los Angeles basin, forecasters at the NWS LOX office monitor weather patterns around the hemisphere to detect weather systems that may affect Southern California. They make heavy use of numerical weather prediction model guidance and their own experience to forecast the timing and amount of upcoming precipitation. During this period, they contact emergency management officials (i.e., California Office of Emergency Services) to alert them to the possibility of an upcoming heavy rainstorm. The LOX office may issue flash flood and flood watches to indicate future risks of severe weather to the public. Watches of this type are disseminated on the National Oceanic . BULLETIN - EAS ACTIVATION REQUESTED Flash Flood Warning National Weather Service Los Angeles/Oxnard CA 1219 AM PDT Fri Sep 10 2021 The National Weather Service in Los Angeles/Oxnard has issued a * Flash Flood Warning for... East Central Los Angeles County in southwestern California... * Until 300 AM PDT. * At 1219 AM PDT, Doppler radar and automated rain gauges indicated line of thunderstorms producing heavy rain across the Bobcat and Ranch 2 burn areas. Flash flooding and mud and debris flow are ongoing or expected to begin shortly. Excessive rainfall over the burn scar will result in mud and debris flows. The debris flows can consist of rock, mud, vegetation and other loose materials. HAZARD...Debris flows are caused by thunderstorms. SOURCE...Radar and automated gauges. IMPACT...Flooding of small creeks and streams, urban areas, highways, streets and underpasses as well as other drainage and low lying areas. * Some locations that will experience flash flooding include... Bobcat and Ranch 2 burn scars, Mount Wilson, Glendora, Monrovia, Angeles Crest Highway between Mount Wilson and Mount Waterman, Sierra Madre, Azusa, and Arcadia. PRECAUTIONARY/PREPAREDNESS ACTIONS... A Flash Flood Warning for a recent burn area means that flooding and debris flows are imminent or occurring. Residents living in or immediately downstream should take immediate precautions to protect life and property. Quickly move away from the burn area only if it is safe to do so, otherwise shelter in place and move to a second story or the highest location in your home to stay out of the path of fast-moving water and debris flows. Turn around, don't drown when encountering flooded roads. Most flood deaths occur in vehicles. Please report observed flooding to local emergency services or law enforcement and request they pass this information to the National Weather Service when you can do so safely. && LAT...LON 3427 11812 3435 11795 3435 11794 3418 11783 3414 11803 FLASH FLOOD...RADAR AND GAUGE INDICATED (Source: www.weatherusa.net)