Verification of Extratropical Cyclones within the NCEP Operational Models. Part I: Analysis Errors and Short-Term NAM and GFS Forecasts

Abstract

Abstract This paper verifies extratropical cyclones around North America and the adjacent oceans within the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) and North American Mesoscale (NAM) models during the 2002–07 cool seasons (October–March). The analyzed cyclones in the Global Forecast System (GFS) model, North American Mesoscale (NAM) model, and the North American Regional Reanalysis (NARR) were also compared against sea level pressure (SLP) observations around extratropical cyclones. The GFS analysis of SLP was clearly superior to the NAM and NARR analyses. The analyzed cyclone pressures in the NAM improved in 2006–07 when its data assimilation was switched to the Gridpoint Statistical Interpolation (GSI). The NCEP GFS has more skillful cyclone intensity and position forecasts than the NAM over the continental United States and adjacent oceans, especially over the eastern Pacific, where the NAM has a large positive (underdeepening) bias in cyclone central pressure. For the short-term (0–60 h) forecasts, the GFS and NAM cyclone errors over the eastern Pacific are larger than the other regions to the east. There are relatively large biases in cyclone position for both models, which vary spatially around North America. The eastern Pacific has four to eight cyclone events per year on average, with errors >10 mb at hour 48 in the GFS; this number has not decreased in recent years. There has been little improvement in the 0–2-day cyclone forecasts during the past 5 yr over the eastern United States, while there has been a relatively large improvement in the cyclone pressure predictions over the eastern Pacific in the NAM.