Diagnosis of operational model track forecast error for Hurricane Ike (2008)

The National Hurricane Center (NHC) issues track forecasts every six hours for all active tropical cyclones in its area of responsibility. These forecasts are made at projection times of 12, 24, 36, 48, 72, 96, and 120 hours. On 9-10 September 2008 track forecast errors from numerical model track guidance and the NHC's official forecast for Hurricane Ike increased substantially compared to errors from forecasts during the previous couple of days. This period of elevated track error occurred three to four days prior to Ike's Texas landfall early on 13 September. During this time, much of the track model guidance and the official NHC forecasts demonstrated a southward bias, taking the center of Ike to a landfall location along the central or southern Texas coast, instead of the upper Texas coast near Houston/Galveston where landfall ultimately occurred.

The average error of the NHC 72-h track forecasts issued on 9 September was 92.8 nm, a 38% increase in the average error compared to the NHC 96-h track forecasts issued on the previous day (66.8 nm). Errors from many of the track models were even larger; the average 72-h forecast error for the interpolated GFS (GFSI) model on 9 September was 131.5 nm. This increase in track forecast error occurred during a critical time when federal, state, and local government agencies were making mitigation and disaster response decisions. Additionally, at the time when the track forecast error increased, Ike was located in a relatively data-rich environment, with data from synoptic surveillance missions being conducted by the NOAA Gulfstream-IV aircraft and special 06 UTC and 18 UTC radiosondes in the southern United States being assimilated into the numerical models.

The noted increase in track forecast error appears to be related to subtle differences in the evolution of the mid-level subtropical ridge over the southern United States north of Ike. The features critical to the eventual erosion of the ridge appear to be a pair of mid- to upper-tropospheric shortwaves that originated well upstream from Ike over the north-central Pacific Ocean on 8-9 September. These shortwaves moved over a longwave ridge in the eastern North Pacific and into western North America by 12-13 September, and their interaction and subsequent impact on the mid -to upper-level flow over North America appears to be important to the evolution of the mid-level ridge north of Ike. The origin of these shortwaves will be examined and various deterministic and ensemble model solutions will be compared, with the goal of understanding why such a large shift in the model guidance occurred at this critical time.