Item 26 - Adoption of the City of Santa Ana Emergency Operations Plan

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City of Santa Ana Emergency Operations Plan Part I Basic Plan 2.4.5 Storm/Flood Factors Creating Flood Risk Flooding occurs when weather rainstorms, geology, and hydrology combine to create conditions of water flowing outside its usual course. Seasonal Rainfall Over the last century, the average annual rainfall in Orange County has been just over 13 inches, slightly under 13 inches for the City of Santa Ana. However, the term "average" means very little as the annual rainfall during this period has ranged from 2.19 inches in 2006-07 to 38.2 inches in 1883-84 and 32.14 inches in 1940-41. This makes Orange County a land of extremes in terms of annual precipitation. A regular source for heavy rainfall is from tropical storms, which often coincide with El Nino years. Atmospheric Rivers (See Figure below) Atmospheric rivers are relatively long, narrow regions in the atmosphere — like rivers in the sky — that transport most of the water vapor outside of the tropics. While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water roughly equivalent to the average flow of water at the mouth of the Mississippi River. Exceptionally strong atmospheric rivers can transport up to 15 times that amount. When the atmospheric rivers make landfall, they often release this water in the form of rain or snow. Although atmospheric rivers come in many shapes and sizes, those that contain the largest amounts of water and the strongest winds can create extreme rainfall and floods, often by stalling over watersheds vulnerable to flooding. These events can disrupt travel, induce mudslides and cause catastrophic damage to life and property. A well- known example is the "Pineapple Express," a strong atmospheric river that is capable of bringing moisture from the tropics near Hawaii over to the U.S. West Coast. Not all atmospheric rivers cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply. Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks, particularly in the western United States. A series of atmospheric rivers fueled strong winter storms that battered the U.S. West Coast from western Washington to southern California in December 2010, and again from January through March 2023, producing 11 to 25 inches of rain in certain areas. Tornados Although damaging tornados are very uncommon in Orange County, they are a possible factor associated with severe thunderstorms. Tornados are spawned when there is warm, moist air near the ground, cool air aloft, and winds that speed up and change direction, creating a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornados are capable of tremendous destruction with wind speeds of 250 mph or more and damage paths can be in excess of one mile wide and 50 miles long, but tornados occurring locally are generally weak and short-lived in comparison to larger tornado events occurring in other parts of the country.