Status:Ongoing analysis Material from: Linus, David L., Esti,
1. Impact
Hurricane Florence made landfall in North Carolina on 14 September as a category 1 hurricane. The cyclone brought torrential rainfall as it became quasi-stationary for a few days. At least 26 people was killed due to the cyclones.
https://www.bbc.co.uk/news/world-us-canada-45563634
2. Description of the event
3. Predictability
3.1 Data assimilation
The plot below shows the evolution of central pressure in Best Track (dots) and LWDA analysis (solid line). The analysis had difficulties to capture the intense phase on 5 September, but captured the second intensification.
3.2 HRES
3.3 ENS
The plots below show the tracks (ensemble -grey, best track - black), position and intensity on 14 September 00UTC (ensemble - squares, best track - hourglass) in forecasts from 13 September (first plot) to 4 September (last plot). Early forecasts had problems to capture the northward "kink" on 7 September. The forecast from 4 September also had a majority of members turning northward over the Atlantic. This changed on the 5 September, and the sensitivity analysis further down is targeting this change.
The plots below show the 4 most westerly (red) and easterly (green) in the forecast from 4 September 12UTC (first plot), and the normalised difference in z500 between the cluster mean of westerly minus easterly members for 24h, 48h, 72h and 96h. The main sensitivity at 24 and 48 hours are in the easterly extent of the subtropical anticyclone. The anticyclone was weaker in the members going towards U.S.
The plots below shows EFI and SOT for 3-day accumulated precipitation valid 14-16 September. In the earliest forecast presented here (from 8 September), the SOT is already high while EFI is low. This is a sign of a few extreme members in the ensemble. As the forecast lead time decreases, the EFI increases as well.
The plot below shows the evolution of forecasts for 3-day accumulated precipitation (14-16 September) for Wilmington, North Carolina (34.3N,78W). The plot includes ENS (blue box-and-whisker), HRES (red dot) and model climate (red box and whisker). Although a few members highlighted the risk of extreme rainfall before, the majority of the ensemble picked up the signal gradually from 10 September 18UTC and onwards.
3.4 Monthly forecasts
The plots below show weekly strike probability (left) and normalised accumulated cyclone energy from extended-range forecasts, valid for 10-16 September. During the same week as Florence was active, TC Helene was present in the central Atlantic and TC Isaac propagated towards the Caribbean. The earlier part of the hurricane season had been quite, and the extended-range forecast seems to picked up the signal of a more active period 2-3 weeks before.
3.5 Comparison with other centres
3.6 EFAS/GLOFAS
The predicted hydrological impacts of Hurricane Florence and is shown in the GLOFAS forecasts below from 14 September. The rainfall is plotted over the top of the map and the hydrographs highlight the time when the highest river flow is expected.
4. Experience from general performance/other cases