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14-1 14. SEVERE WEATHER 14.1 GENERAL BACKGROUND Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. It includes thunderstorms, downbursts, tornadoes, waterspouts, snowstorms, ice storms, and dust storms, among other events. Severe weather is not the same as extreme weather, which refers to unusual weather events at the extremes of the historical distribution for a given area. The most common severe weather events that have historically impacted the planning area are heavy rains (atmospheric rivers), extreme heat, fog, thunderstorms, tornadoes, and windstorms. Public safety power shutoff (PSPS) events represent a newer weather-related phenomenon in California; they are associated with weather conditions suitable for extreme fire risk. For this risk assessment, the term “severe weather” refers to all these event types in aggregate. They are assessed as a single hazard for the following reasons:  Records indicate that each of these weather event types has impacted the planning area to some degree, and all have similar frequencies of occurrence.  None of these weather event types have a clearly defined location, so no quantitative geospatial analysis is available to support exposure or vulnerability analysis; the analyses for this hazard are qualitative. The following sections provide general descriptions of the local weather types of concern, in alphabetical order. 14.1.1 Atmospheric Rivers Atmospheric rivers are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. Those with the largest amounts of water vapor and the strongest winds can create extreme floods if they stall over watersheds vulnerable to flooding. These events can disrupt travel, induce mud slides, and cause catastrophic damage to life and property. However, not all atmospheric rivers cause damage—most are weak, and simply provide beneficial rain or snow that is crucial to water supply. 14.1.2 Extreme Heat Extreme heat affects community members’ safety and increases community costs and energy generation as it continues. Extreme heat events can lead to an increase in heat-related illnesses and deaths, according to the California Department of Public Health’s San Mateo County Profile Climate Change and Health. They can also cause drought, exacerbate wildfires, and impact water supplies. Frequent losses may be associated with the urban heat island effect and overheating of energy, heating, ventilation, and air conditioning systems. Heat can lead to