While Florence was crossing the Atlantic, there was a period from 1200 UTC 02 September through 1200 UTC 04 September when the NHC forecast track errors became much larger than the five-year averages. Operational numerical model forecasts for the track of Florence experienced much larger errors in this same period. Figure 1 shows the 72 hour forecast track errors (nm) for the official NHC forecast (OFCL), the NCEP Global Forecast System ensemble mean (AEMN), the Hurricane WRF Model (HWRF), the Canadian global model (CMC), the UK Meteorological Office global model (UKM) and the European Center for Medium-range Weather Forecasts (ECME) models for this period. The bold red line shows the five-year average NHC track error for 72-hour forecasts (102 nautical miles).
Figure 2 shows the forecast tracks themselves plotted with the NHC Best Track for the 5-day forecasts initialized at 0000 UTC 04 September for the same models shown in Fig. 1. The model forecast tracks are all similar to one another, but all of them forecast Florence to curve to the northwest, while the actual storm track turns towards the west. This presentation will document the meteorological patterns that caused all the forecast models to miss the turn to the west.
Stewart, S. R, and R. Berg, 2019: Tropical cyclone report: Hurricane Florence (AL062018) 31 August – 17 September 2018. National Hurricane Center, 98 pp., https://www.nhc.noaa.gov/data/tcr/AL062018_Florence.pdf