Summer surface flow characteristics over northeast Colorado

Surface wind data from the Program for Regional Observing and Forecasting Services (PROFS) have been analyzed to investigate the diurnal wind flow pattern over the broad drainage area of the South Platte River in northeastern Colorado. A consistent diurnal pattern appears in monthly averages as well as on most undisturbed individual days, and is similar to the classic descriptions of mountain-valley wind flows. It is observed that, rather than occurring simultaneously at all elevations, downslope-to-upslope and upslope-to-downslope surface flow transitions along the Front Range of northeastern Colorado begin near the foothills of the Rocky Mts. and propagate eastward across the plains. During the summer months, local confluence is found at midday along major E-W ridges in the region (e.g., Cheyenne Ridge and Palmer Lake Divide). Consequently, in addition to the N-S Continental Divide, these E-W ridges are preferred regions for initial afternoon thunderstorm development. The late afternoon transition to downslope flow often appears to be associated with the propagation of thunderstorms from the mountains and ridges eastward to the plains.