Hurricane Katrina has had a devastating impact on the US Gulf Coast, and her effects will be felt for many years. Forecasts of such events, coupled with timely response, can greatly reduce casualties and save billions of dollars. We show how visualizations from storm surge and atmospheric simulations were used to understand the predictions of how strong, where, and when flooding would occur in the hours leading up to Katrina's landfall. Sophisticated surface, flow and volume visualization techniques show these simulation results interleaved with actual observations, including satellite cloud images, GIS aerial maps and LIDAR showing the 3D terrain of New Orleans. The sheer size and complexity of the data in this application also motivated research in efficient data access mechanisms and rendering algorithms. Our goals were to use the resulting animation as a vehicle for raising awareness in the general populace to the true impact of the event, to create a scientifically accurate representation of the storm and its effects, and to develop a workflow to create similar visualizations for future and simulated hurricanes. Screenings of the animation have been well received, both by the general public and by scientists in the field.