A recent study by Carbone et al. 2002 addressed aspects of diurnal and intra-seasonal variability, including the discovery of long-lived precipitation "episodes". The episodes were shown to result from thermal forcing, dynamical self-organization, propagation, and the coherent regeneration of convective systems.

During the past year we have progressed to the point where we can offer new insights at the semi-diurnal and inter-annual periods. At the semi-diurnal frequency, there is a long history of literature attributing a causal relationship between atmospheric tidal or tidal-like forcing to the occurrence of convective precipitation on regional and continental scales. After closely examining a seven-year period of record from the WSR-88D network, we find ourselves in substantive disagreement with this commonly accepted interpretation of warm season precipitation data. We are inclined to declare semi-diurnal forcing at continental-scales a myth for mid-latitude regions.

Semi-diurnal signals are indeed significant in the harmonic analyses of precipitation, however, these signals stem from two sources that are essentially unrelated to tidal influences. The first of these sources is a "delayed-phase" effect from non-local diurnal forcing and the subsequent propagation of rainfall systems for 12 or more hours. The second source is from localized sea/lake/land breezes, principally along the Gulf of Mexico and Atlantic Ocean coasts of the southeastern U.S. Unlike the first example, breezes are a genuine semi-diurnal forcing. Like the first example, breezes are essentially unrelated to tidal scales of motion and are inherently local. We will show diagrams and animations thereof that clearly illustrate both sources of semi-diurnal diurnal signal as derived from the WSR-88D network. The "continental" semi-diurnal signal disappears when localized breeze regions are removed from the domain of computation.

Our period of record spans seven seasons (1996-2002). The statistics from each July exhibit a well-defined "corridor" of precipitation episodes, most prominent across the central U.S. Such "corridors" are vaguely analogous to cool season "storm tracks" and are, at best, only weakly-related to fronts and cyclones. However, marked synoptic scale patterns exist, among these the North American monsoon circulation over the western cordillera. In any given year, a corridor will experience excessive cumulative rainfall while nearby regions are often well below normal. In our presentation, we will show the extent to which major precipitation episodes are focused on these corridors; the year-to-year variability of corridor position; and the relationship of corridor position to the large-scale atmospheric condition.

Reference: Carbone, R. E., J. D. Tuttle, D. Ahijevych, S. B. Trier, 2002: Inferences of Predictability Associated with Warm Season Precipitation Episodes, J. Atmos. Sci., 59, 2033-2056.