1. Extratropical Cyclone Track Errors in GFS and its Parallel Guang Ping Lou and Geoff DiMego EMC/NCEP December, 2014

2. Background GFS parallel started on May 15 and it is continuing for four times a day. This stats use data from June 1 through November 30, 2014. Cyclones above 20N/S degree latitude are included in the study. Named tropical storms are not included. The extratropical cyclone tracker is applied over the GFS parallel 1 degree data and low centers are identified in each forecast cycle. Operations GFS tracks were collected in the same time period. Track errors are computed against model self analysis tracks. The verification is homogenous. Therefore, only those tracks that are identified at the same time in both models are used.

3. Parallel GFS has consistent improvements over operations GFS in track errors except for 96hr and 120hr forecasts GFSO (red) – Operations GFS; GFSP (blue) – Parallel GFS On average, more than 4 NM improvements in GFSP over GFSO. Fcst hr01224364860728496108120 Cases692164535738395723911420846507288177110

4. Track errors in the Northern Hemisphere is much smaller than in the Southern Hemisphere. This is due to the error growth rate in the SH is much faster than in the NH. Fcst hr01224364860728496108120 NH Cases3675344130772196137786155135521614498 SH Cases32463012266117611014559295152723312