| Summary: | We developed a procedure using the chi-square statistic to determine the effect of El Nino- Southern Oscillation on the frequency of extreme daily weather occurrence. Its application is demonstrated for a site in southwestern Montana, located east of the Continental Divide 970 km from the Pacific Ocean. The study used daily weather data focused on a 29-wk period from 3 Dec to 23 Jun in a 100-yr weather record at Montana State University (Bozeman) and compared this weather in relation to November-March sea-surface temperature anomalies in an area of the eastern tropical Pacific Ocean. Daily weather extremes were compared between 25 El Nino years and 50 ‘normal’ years. During El Nino years, December-June weather at Bozeman was characterized by more days of extreme high maximum temperatures, fewer days of extreme low minimum temperatures, fewer days of high precipitation amounts, and fewer days with small diurnal temperature ranges. For the 29-wk period, we determined the difference between El Nino years and normal years to be about 20 percent for each of these four extreme daily weather conditions. An increase or decrease of extreme daily weather occurrences can impact natural resources and a wide range of human activities including agriculture, forestry, recreation, construction, and other businesses.
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