Tropical Storm Debby formed in the south-central Gulf of Mexico in June, 2012 (Kimberlain, 2013). Formation was slow, but after the merger of a weak surface low pressure area, and a tropical wave, the resulting low pressure trough strengthened into a tropical storm by 12 UTC 23 June 2012. Debby drifted slowly northward for the next 24 hours and then slowly northeastward for the following 24 hours, before finally moving off to the east after 12 UTC 25 June.
Model forecasts for the movement of Debby from 12 UTC 23 June through 00 UTC 25 June, were not in agreement about whether Debby would turn west and head for the Texas Gulf Coast, or turn east and head for northern Florida. A sample of model forecasts from 00 UTC 24 June 2012 appears in the figure below and the disparity is very clearly shown. The Official National Hurricane Center (NHC) forecast at this time was similar to the yellow track (labeled HWRF) in the figure. During this period, Debby was located in a col between two ridges to the north; the storm motion depended in part on the exact location and strength of these ridges.
The NHC forecasters examined the tracks from the European Center for Medium-Range Forecasting (ECMWF) 50 member ensemble (Molteni et al., 1996, Leutbecher and Palmer, 2008), as well as the 20 member global ensemble forecast system (GEFS) from the National Centers for Environmental Protection (Toth et al., 2012). The ECMWF deterministic model and ensembles mostly took the storm west, while most of the 20 GEFS members took the storm to the east.
We have split the GEFS member forecasts into three groups: westward moving, northward moving and eastward moving storms. The 500 hPa patterns associated with each group show subtle but clear differences in the locations and strengths of the ridges to the north. Further examination of the GEFS member forecasts reveals a strong correlation between the forecast longitude and the storm strength (maximum wind speed or minimum sea level pressure). Stronger storms have a clear tendency to turn eastward, while weaker storms turn westward. The correlation coefficients between forecast west longitude and maximum wind for forecasts made on 00 UTC 24 June, for 12-60 hour lead times, range between -.77 to -.89. Similar correlation coefficients are present in forecasts made through 00 UTC 25 June, and to a slightly lesser extent for forecasts through 12 UTC 26 June. Debby moved onshore near 21 UTC 26 June 2012 (Kimberlain, 2013).
Cross sections through each GEFS member storm show that the westward moving storms had shallower circulations. Steering flows were calculated from the data, using the average wind at 3o of latitude distance from the storm centers, averaged over shallow (1000 500 hPa), medium (1000 300 hPa) and deep (1000 250 hPa) layers. For the GEFS member storms that moved west, the deep layer steering flows were still towards the east. This result is consistent with weaker storms moving west, since weaker storms will be shallower, and thus influenced less by the upper-level westerly flow.
Work is ongoing to determine how often ensemble member storm strength is strongly related to the storm direction. Furthermore, the ECMWF ensemble members are being examined to look for similar behavior. Preliminary results suggest that the ECMWF ensemble members do not show the same correlation between storm strength and longitude. Early looks at the steering currents and cross sections suggest that the simulated storms in the ECMWF ensemble members do not show much variability in the height of the circulations, and that the storms that moved west, did so because the steering currents were westward.
Acknowledgements:
Data for ensembles from THORPEX Interactive Grand Global Ensemble project. http://apps.ecmwf.int/datasets/data/tigge/.
Forecast data for GEFS members provided by James Franklin, NHC, Miami, FL.
Figure courtesy of Clark Evans, University of Wisconsin Milwaukee, from his website: http://derecho.math.uwm.edu/models/.
Figure shows forecast tracks for Tropical Storm Debby from model runs initialized at 00 UTC 24 June 2012. Models shown are Canadian Global Model (CWC), North American Model (NAM), U.S. Navy NOGAPS Model (NGX), Global Forecast System Model (AVNO), Geophysical Fluid Dynamics Laboratory Model (GFDT), Hurricane Weather Research and Forecasting Model (HWRF), and Medium Range Forecast (old Global Forecast System MRFO).
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner