A mesoscale model is used to investigate the mesoscale predictability of an extreme precipitation event over central Texas on 29 June 2002 that lasted through 7 July 2002. Both the intrinsic and practical aspects of warm-season predictability, especially quantitative precipitation forecasts up to 36 h, were explored through experiments with various grid resolutions, initial and boundary conditions, physics parameterization schemes, and the addition of small-scale, small-amplitude random initial errors. It is found that the high-resolution convective-resolving simulations (with grid spacing down to 3.3 km) do not produce the best simulation or forecast. It was also found that both the realistic initial condition uncertainty and model errors can result in large forecast errors for this warm-season flooding event. Thus, practically, there is room to gain higher forecast accuracy through improving the initial analysis with better data assimilation techniques or enhanced observations, and through improving the forecast model with better-resolved or -parameterized physical processes. However, even if a perfect forecast model is used, small-scale, small-amplitude initial errors, such as those in the form of undetectable random noise, can grow rapidly and subsequently contaminate the short-term deterministic mesoscale forecast within 36 h. This rapid error growth is caused by moist convection. The limited deterministic predictability of such a heavy precipitation event, both practically and intrinsically, illustrates the need for probabilistic forecasts at the mesoscales.
Extreme rainfall, with storm total precipitation exceeding 500 mm, occurs several times per decade in Texas. According to a compositing analysis, the large-scale weather patterns associated with extreme rainfall events involve a northward deflection of the tropical trade winds into Texas, with deep southerly winds extending into the middle troposphere. One such event, the July 2002 South-Central Texas flood, is examined in detail. This particular event was associated with a stationary upper-level trough over central Texas and northern Mexico that established a steady influx of tropical moisture from the south. While the onset of the event was triggered by destabilization caused by an upper-level vortex moving over the northeast Mexican coast, a succession of upper-level processes allowed the event to become stationary over south-central Texas and produce heavy rain for several days. While the large-scale signatures of such extreme rain events evolve slowly, the many interacting processes at smaller scales make numerical forecasts highly sensitive to details of the simulations. [
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