Thunderstorms occur 2 to 5 times a year, chiefly in winter, on the coast of northern California, and 10 to 15 times a year, chiefly in summer, in the Great Basin. In the intervening Central Valley, 5 to 10 thunderstorms occur per year, mostly in late spring; a secondary maximum in early autumn is particularly important in starting forest fires. Correlations between numbers of days with thunderstorms at adjacent Weather Bureau stations are quite low; the highest found is 0.70 for summer thunderstorm days at Sacramento and Blue Canyon, 60 mi apart.
