Trends in Extreme Daily Surface Temperatures in California, 1950–2005

Daily observations of maximum and minimum temperatures from NWS COOP stations are used to examine trends in the magnitude and frequency of extreme daily maximum (TMAX) and minimum (TMIN) surface temperatures in California for the period 1950–2005. Significant trends are found in both TMAX and TMIN but are more prevalent in minimum temperature. Across the state, the coldest days of the year are becoming warmer, and extremely cold wintertime events are becoming slightly less frequent. The hottest days of the year are warming at relatively few stations, and frequencies of summertime extreme heat events show only limited positive trends. Annual frequencies of extreme cold events are decreasing faster (approximately 1 fewer day per year) than hot events frequencies are increasing (approximately 0.5 more days per year). Urban sites in particular are experiencing significant decreases in the annual frequency of extreme cold events, mainly in winter and spring, and are also seeing increases in the frequencies of unusually warm spring and summer TMINs. Significant trends in temperature extremes are well distributed across the state, with little clustering. Urban sites have larger numbers of significant trends, but trend magnitudes are generally similar at urban and rural sites.