Time series of extreme precipitation index for the occurrence of 1-day, 1 in 5-year extreme precipitation, for the Northwest region. The dashed line is a linear fit. Based on daily COOP data from long-term stations in the National Climatic Data Centers Global Historical Climate Network data set. Only stations with less than 10% missing daily precipitation data for the period 1895-2011 are used in this analysis. Events are first identified for each individual station by ranking all daily precipitation values and choosing the top N/5 events, where N is the number of years of data for that particular station. Then, event numbers for each year are averaged for all stations in each 1x1 grid box. Finally, a regional average is determined by averaging the values for the individual grid boxes. This regional average is the extreme precipitation index. The highest number of extreme events occurred in 1996. There is no overall long-term trend.