Precipitation data were used to identify representative wet, dry and average MJJ periods. Atmospheric conditions during the wet, dry and average years show distinct differences in the synoptic weather patterns. During 1983 (the wet year), the 700hPa level composite MJJ mean is dominated by a trough in the southwestern United States causing southwesterly flow at this level over the Sand Hills. The flow during the dry year (1989) is dominated by a slight ridge to the west of Nebraska, giving rise to a more northwesterly flow over the region. Differences between these two regimes are clearly shown in height anomaly maps with a 10 gpm negative height anomaly during the wet year as compared to a 10 gpm positive height anomaly during the dry year. The pattern for 1986, the average year, is more zonal over the Sand Hills with a weak trough over the western United States. Only small (approximately 5 gpm) positive height anomalies are observed for the Sand Hills region.
For each of the three representative years, three individual precipitation events were selected as case studies. These cases were chosen to represent 1) a synoptic event, 2) a convective event, and 3) a null event for the Sand Hills. The synoptic event is defined as precipitation caused by the passage of a surface front or trough through the region. The convective cases were defined to represent air mass thunderstorms or complexes with upper air ridging in the region. The null category is best described as a situation for which there was extensive precipitation in the region around the Sand Hills, but there was little or no precipitation over most of the Sand Hills. Though beyond the scope of this project, the null cases were chosen to examine the atmospheric controls the Sand Hills exerts on precipitation through modeling sensitivity studies. However, the null cases are still of interest due to synoptic patterns their relationships to the surrounding region. For the most part, the synoptic events occurred in May or June, while the convective and null cases occurred later in the period, June or July. Examination of the atmospheric conditions during different precipitation situations helps us understand the mechanisms responsible for generating the precipitation and gives insight into the variability that exists in the Sand Hills.