Fig. 2. Plots of the final points on the trajectory of a shallow-water, barotropic, hurricane-like vortex moving for 240 h on a beta plane in anticyclonic shear and cyclonic shear of magnitude 0.1 f in a circular domain 4000 km in radius. The in the former instance, the motion converges to within a few kilometers for truncation at wavenumber 4. The system exhibits similar behavior for uniform environmental flows and a quiescent environment on a beta plane. In a cyclonically sheared environmental flow, convergence is more gradual, attaining convergence to within a few tens of kilometers only for truncations > 15. Still, at wavenumber 4 truncation, the distance from the center of the spiral is 135 km. If the error grows linearly with time, the expected error due to truncation at n=4 for a 72-84 h forecast would be less than 10% of a typical operational forecast of the same duration.