generally leads to smaller track forecast errors in the National Oceanic

and Atmospheric Administration Global Forecasting System (GFS) than

would occur otherwise, not every case is improved. Very large GFS

forecast degradations due to surveillance are investigated. Small

perturbations to model initial conditions may have a large impact

locally or downstream in a short time. In these cases, the

perturbations are due to either erroneous data assimilated into the

models or issues with the complex data assimilation system itself, and

may have caused the forecast degradations. Investigation of forecast

and observing system failures can lead to procedural changes that may

eliminate some causes of future large forecast errors.