An Analysis of NMC's Nested Grid Model Forecasts of Alberta Clippers

Abstract

Forecasts of Alberta Clipper?type cyclones defined as cyclones that move southeastward from regions of western Canada into south-central Canada or the north-central United States before moving eastward to the coast of North America, were studied using the National Meteorological Center's Nested Grid Model (NGM). All available NGM forecasts between 1 January 1988 and 31 December 1992, excluding the months of June, July, and August, were analyzed. The 5-yr sample included 59 Alberta Clippers. For each cast, the cyclone center position and central pressure were recorded for the NGM's initialization (0 h) and for 12-, 24-, 36-, and 48-h forecasts for each available run. Average cyclone position errors ranged from 140 km, with a standard deviation of 183 km at 12 h, to 339 km, with a standard deviation of 407 km at 48 h. Average central pressure errors ranged from 0.0 hPa, with a standard deviation of 2.5 hPa at 12 h, to 1.0 hPa, with a standard deviation of 5.7 hPa at 48 h. East of 84.0°W, cyclones verified farther east than forecast. West of 84.0°W, the NGM tended to forecast the cyclones farther north and slightly farther west than they were seen to verify. Positional and pressure forecast errors over and near the Great Lakes were found to be smaller than in other areas.