65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Diagnosis of Operational Model Track Forecast Error for Hurricane Ike (2008)
Michael J. Brennan 1 and Sharanya J. Majumdar 2
(Michael.brennan@noaa.gov)
1 NOAA/NWS/NCEP/NHC; 2 RSMAS Division of Meteorology and Physical Oceanography,
University of Miami
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 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.
Examination of the 500-hPa height analyses from the members of the NCEP Global
Ensemble Forecast System (GEFS) at 0000 UTC 9 September 2008 show significant spread in
the strength of the mid-level subtropical ridge over the southern United States north of Ike.
Ensemble members that showed a stronger (weaker) ridge north of Ike during the model
integration moved Ike on a track farther to the left (right) of the track of the GEFS ensemble
mean, which was close to the actual track of Ike. Given these observations, an experiment was
performed where the initial conditions in the GFS run from 0000 UTC 9 September were altered
to slightly weaken the ridge north of Ike. Results from this experiment showed Ike moving
significantly to the right of the operational GFS forecast from that time and closer to the
observed track of the hurricane. Additional experiments perturbing other synoptic scale features
possibly impacting the track of Ike had limited impact on the hurricane’s track. These results
suggest that the evolution of the subtropical ridge was critical to determining Ike’s ultimate
track. This case also represents an instance where a single-model ensemble system provided
value over the typically superior multi-model consensus aids for TC track forecasting.
Poster Session – Page 20