Analyses of warm season rainfall over the United States have revealed long-lived precipitation episodes that exhibit strong spatial-temporal coherence and autocovariance properties. Sequences of summertime precipitation maintain their identity for long periods as they march eastward across the North American continent. This occurs primarily in the mid- to late summer when synoptic disturbances are less prominent and predictive skill is at a minimum. The existence of these episodes speaks to a heretofore unseen predictability inherent in warm season rainfall in the the 6-24 h range. Therefore, a better documentation and understanding of these coherent features is warranted.

The 2002 warm season provided a venue for studying the influence of topography on precipitation episode initiation and rainfall coherence. For the six-season climatology prior to 2002, the diurnal cycle features a distinct afternoon maximum in rainfall starting at 105 W that shifts eastward into the Plains during the late evening. Given the same domain of interest (30-48 N), the time-longitude behavior of the 2002 season appears markedly different. No coherent phase shift is readily apparent. However, a simple spatial transformation of the rainfall data restores the general pattern. North of 42N, where the Plains extend further west, the coordinate system may be skewed to align the spine of the Rocky Mountains approximately north-south. The eastward tracking diurnal rainfall peak resurfaces when the distance dimension is transformed in this manner.

This supports the theory that elevated terrain governs downstream precipitation initiation. The westward jog of the mountains and northward bias of the storm track in 2002 allowed this hypothesis to be indirectly tested.