The diurnal variability of precipitation across the central Rockies and adjacent Great Plains

The diurnal variation of precipitation across Wyoming, Colorado, South Dakota, Nebraska, and Kansas has been studied by a harmonic analysis of 35 yr of hourly precipitation data for 334 stations and a regional probability of precipitation analysis for grouped stations. For all measurable (>0.25-mm) precipitation events, the phase of the diurnal cycle tends to peak between 0200 and 0600 hr LST in winter; 0000 and 0600 hr LST in spring; 1600 and 0400 hr LST in summer; and 2100 and 0600 hr LST in autumn. The earlier times occur in the mountain regions, and the later times near the eastern edge of the district. The well-known afternoon mountain and nocturnal Great Plains convective signal is seen for hourly precipitation intensities of 2.5 and 10.0 mm. New findings beyond previous work include 1) a pervasive 0300-hr LST maximum for the precipitation category >0.25 mm, which is most prominent during the cooler half of the year and partially masked in summer; 2) the transition from winter to spring (March/April) is accompanied by an increase in measurable precipitation frequency, but a decrease in precipitation frequency for rainfall amounts >2.5 mm; and 3) the summer rainfall regime is made up of distinct local and mountain-generated signals. The summer heavier precipitation events tend to occur 1-4 hr earlier than all measurable rainfall events, particularly on the plains east of 101 degrees W. The implication of these results is that 1) the winter regime is affected by large-scale circulation features as the 0300-hr LST maximum is found elsewhere, e.g., the northeastern U.S.; 2) dynamically significant precipitation systems, although infrequent, affect the five-state region in winter; and 3) heavy summer nocturnal precipitation systems over the eastern plains cannot be explained solely by the eastward propagation of mountain-generated systems from the previous afternoon.