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2021 Multijurisdictional Local Hazard Mitigation Plan Microclimates are significant in the case of fog events because some locations can experience fog while clear skies predominate only a few miles away. Western breezes may bring fog from the ocean, but it will be blocked from passing certain points by mountainous ridges. Even the type of fog in microclimates may vary; some regions are more prone to experience radiation fog, while others only receive a canopy of high fog. This is usually based on the proximity of the location to mountains, ridges, fault lines, and water sources, among other factors. Heavy Rain The frequency of heavy rain events remained fairly consistent between 1910 and the 1980s; however, it has risen substantially since then. Certain locations have noted more significant increases in heavy rain events than others. Most notably, the Northeast and Midwest have experienced the greatest changes, although the Southeast, Great Plains, Northwest, Alaska, and Southwest have also noted increases. Although San Mateo County experiences heavy rain events, it is at a reduced level compared to other parts of the country. Thunderstorms Thunderstorms affect relatively small localized areas, rather than large regions like winter storms and extreme temperature events. Thunderstorms can strike in all regions of the United States; however, they are most common in the central and southern states. The atmospheric conditions in these regions of the country are ideal for generating these powerful storms. It is estimated that there are as many as 40,000 thunderstorms each day worldwide. The most thunderstorms are seen in the southeast United States, with Florida having the highest incidences (80 to over 100 thunderstorm days each year). San Mateo County can experience an average of 10 thunderstorm days each year. The entire extent of San Mateo County is exposed to some degree of lightning hazard, though exposed points of high elevation have significantly higher frequency of occurrence. As noted earlier, lightning instances in the County have only been associated with other storm events and not as a standalone hazard. 14.2.3 Frequency All Events The planning area can expect to experience adverse impacts from some type of severe weather event at least annually. Using the historical data presented in Appendix F, recurrence probabilities for the primary local types of severe weather events are as shown in Table 14-4. Table 14-4. Recurrence Probabilities for Severe Weather Events Severe Weather Event Time Frame (Years) # of Events over Timeframe Recurrence Interval % Annual Chance Heavy Rain (Atmospheric River) 70 18 233 years 0.43 Extreme Heat 14 5 32 years 3 Fog 70 8 601 years 0.17 Public Safety Power Shutoffs *3 *7 1 years 100 Thunderstorms 70 23 175 years 0.57 Tornados 70 4 1,132 years 0.09 Windstorms 70 118 1 year 100 Based on PG&E Statistics for the Bay Area 14-12