Analysis of diurnal patterns in winter precipitation across the conterminous United States

Hourly winter (November-March) precipitation data were assembled for nearly 5000 stations in the conterminous United States over the period 1948-98. Despite a potential observation bias in the 24th hour, a general tendency for winter precipitation events was found to occur more frequently near sunrise than for any other time of the day. Based on the standardized amplitude of the first harmonic wave, the pattern is most pronounced in Texas and in an area surrounding Colorado and Wyoming. The pattern also appears significant in the southeastern United States and in northern California based on the variance explained by the first harmonic fit. It is suggested that the diurnal patterns seen in the conterminous United States are related to increased relative humidity values occurring near sunrise and increased wind speeds in the warm sector of cyclonic storms.