City of Irvine
2020 Local Hazard Mitigation Plan
this happens, air will flow dramatically, creating high- speed winds. The most common wind events in southern California are the “Santa Ana” wind conditions that typically occur in the fall and winter. When winds are fast enough, they can cause property damage to homes, public facilities, utilities, and other infrastructure. They can also uproot or topple mature trees or pick up debris and send it careening through the air. This debris can injure or even kill bystanders who may find themselves stranded outside. High- speed winds can also deposit this debris in the middle of rights-of-way, such as roads, freeways, and railways, blocking exit routes for would-be evacuees or impeding access to first responders trying to reach wounded people.
Santa Ana Wind Events
Location and Extent
Extreme Heat Extreme heat events are not limited to any part of the City. They occur with the same intensity and duration at the same time across all locations in Irvine. The minimum threshold for an extreme heat day in Irvine is 93.2°F. The minimum threshold for a warm night in Irvine is 66.4°F. 42 F94 F 97 These thresholds are based on a 2% probability event. Heavy Rain The location and size of a rain event vary depending on regional geography as well as regional and global weather events. For example, small precipitation events may occur in only one section of Irvine. In contrast, a large rain event could inundate a majority of Orange County as well as other parts of southern California. California’s precipitation varies from year to year, depending on how much moisture the state receives from atmospheric rivers. Atmospheric rivers are corridors along which wet air travels from the tropics to continents. When the moisture arrives in California, it may precipitate as rain or snow. One of the most known atmospheric rivers in California is the “Pineapple Express,” which brings moist air from the ocean surrounding Hawai’i to California. During certain years, an immense amount of moisture may be transported along the atmospheric rivers that cross over California, leading to severe rains. 98 Another weather phenomenon influencing rainfall in southern California is “El Niño,” officially referred to as the “Southern Oscillation” or “El Niño -Southern Oscillation (ENSO) .” ENSO can cause increased rainfall, particularly during the winter months, which is caused by warming of the surface of the eastern tropical Pacific Ocean, leading to evaporation of warm, moist air into the atmosphere. Winds bring this moisture to the eastern Pacific and the American continents, where it falls as rain. ENSO does not always lead to increased rainfall by default, but in general, it can increase the chances for a winter with higher-than- usual precipitation. 99,100 Rain events are usually measured by the amount of precipitation that falls. Table 3-16 categorizes rain events by the amount of precipitation per hour.
77
Made with FlippingBook HTML5