An investigation into changes in the spatial and temporal precipitation patterns over the past 50 years was completed to better understand how precipitation patterns might have varied with recent climate change. This study will also demonstrate how precipitation intensity has changed over time. Twenty-four Cooperative Observer Program (COOP) Stations within the Great Plains region, roughly northern Texas to the U.S.-Canadian border between 103°W and 95°W, were chosen based on spatial distribution and completion, all stations must have less than 2% missing data for the 50 year period. The study region experiences both large scale synoptic storms with steady precipitation over long periods of time and smaller mesoscale convective storms with high intensity precipitation over shorter periods of time. Daily precipitation data were obtained from the U.S. Historical Climatology Network (USHCN). Daily totals were placed into six different amount categories: category 1: 0.0 mm-0.25 mm, category 2: 0.25-5.00, category 3: 5.00-15.00, category 4: 15.00-25.00, category 5: 25.00-50.00 and category 6: greater than 50.00 mm. Statistical analyses were preformed for monthly, seasonal and annual periods for each of the six categories.

Stations in the southern plains, Oklahoma and Texas, have two monthly precipitation average maximums, May and October. The stations north of Oklahoma have only one monthly maximum during late spring or early summer depending on the latitude of the city. Willow City, ND (48°N), the furthest most north station, has the latest monthly precipitation maximum, June. The northern stations also experience more days within precipitation categories 2 and 3 compared to the southern stations. The southern stations have the greatest number of category 5 and 6 precipitation days. Overall, the season with the highest precipitation total is summer except for the most southern stations, when it is fall. The annual precipitation totals have not changed between the beginning and end of the study period for the entire region. However, the distribution of events in each precipitation category has changed. For example, there is an increase in the number of days within precipitation categories 5 and 6 and an increase in days with no precipitation for Texas and southern Oklahoma, resulting in greater variability in the intensity of events. The annual totals have not changed due to the increase in heavy precipitation days balancing out the increase in non precipitation days. The northern stations experience an increase in precipitation days in amount categories 2 and 3 while days with amount categories 5 and 6 and non precipitation days have decreased slightly, still maintaining the annual average. Overall, daily precipitation at the southern stations has become more extreme, meaning more days with intense precipitation or no precipitation and fewer days with light to moderate precipitation amounts. However, at the northern stations the light to moderate precipitation amounts have increased meaning precipitation amounts are becoming more constant with less extreme events.