18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Tuesday, 31 July 2001
Experiences from SCRAPE2000—Summer Convective Rainfall in Alabama Prediction Experiment
John T. Bradshaw, NOAA/NWS, Calera, AL; and R. E. Kilduff, R. E. McNeil, K. J. Pence, P. Hart, and W. M. Lapenta
The National Weather Service Strategic Plan emphasizes the improvement of short-term forecast products, particularly those related to convection and quantitative precipitation forecasting (QPF). Today’s forecasters are exploiting an arsenal of observational resources to assess the mesoscale environment, and high resolution model output is providing tantalizing glimpses beyond the 0-3 hour time frame during which convection can be adequately predicted by extrapolative means. Concurrent with these new tools comes increased expectations regarding the accuracy of mesoscale-oriented services. The NWS is challenging itself to provide longer-lead flood warnings, more accurate QPFs for significant rainfall, and better all-around short-term forecasts. But is it realistic to expect improvements in these services, given the observational limitations that currently exist at these temporal and spatial scales?

Prior to planning future NWS products and services, it is important that the current accuracy of short-term forecasts provided by the field be assessed. The following questions also require consideration: Do better tools really yield better mesoscale forecast products? How truly beneficial are mesoscale models to this pursuit? Is it advantageous to produce graphical short-term forecast products, versus the traditional text products that have been a staple of the NWS? To answer these questions, NWS Birmingham has undertaken a multi-year operational exercise known as the Summer Convective Rainfall in Alabama Prediction Experiment, or SCRAPE. The primary goals of SCRAPE are to: a) determine a baseline accuracy for warm season short-term probabilistic QPFs in Alabama; b) quantify the overall value of mesoscale model output to the QPF process; and c) assess the relative utility of observed and model-derived fields in this process.

During Summer 2000, NWS meteorologists produced a total of 51 experimental QPF packages for northern Alabama. Each package consisted of five 6-18 hour graphical probabilistic QPFs. The participants had no operational duties on project days, and were able to devote several uninterrupted hours to the preparation of the forecasts. Following an initial evaluation of observed data sources alone, a preliminary “obs-only” 6-hr QPF was generated. Upon completion of this graphic, a thorough assessment of all available model output was made, followed by the creation of the remaining QPF products. In support of this project, the Global Hydrology and Climate Center (GHCC) in Huntsville, AL established an MM5 mesoscale model domain over the Southeast United States with a horizontal grid spacing of 12 km. The output was made available to WFO Birmingham in real-time, and constituted the primary source of model data for the SCRAPE forecasters. Key fields included three-hour accumulated precipitation, low-level equivalent potential temperature and surface temperature.

Preliminary verification results indicate that: a) the SCRAPE forecasts were superior to their operational counterparts, and b) the introduction of high-resolution MM5 output aided in the short-term QPF process. In particular, a comparison of the “obs-only” QPFs with their model-based counterparts yields interesting insights into the relative importance these data types play in the determination of QPF. These results will be presented in detail at the conference.

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