Status:Ongoing analysis Material from: Linus, Esti
1. Impact
Helene was the eighth named storm of the 2024 Atlantic hurricane season, intensifying into a Category 4 hurricane with winds of 140 mph (225 km/h) before making landfall near Perry, Florida, on September 26. After moving inland, Helene weakened, transitioning into a post-tropical cyclone over Tennessee before dissipating on September 29 (yesterday). Prior to landfall, Florida and Georgia declared states of emergency, anticipating severe storm surges, hurricane-force winds, and inland flooding. The storm caused widespread devastation, particularly in North Carolina and Tennessee, where heavy rainfall triggered catastrophic flooding. As of September 29, Helene was responsible for 118 deaths, with an additional 1,073 people reported missing.
The major rainfall in central-eastern U.S was due to the interaction between the cyclone and a mid-latitude trough.
https://en.wikipedia.org/wiki/Hurricane_Helene
2. Description of the event
The evaluation of the rainfall will focus on 72-hour rainfall on 26 September 00UTC - 29 September 00UTC in a 0.5x0.5 degree box centred on Asheville (35.5N, 82.2W).
The plots below show analyses of MSLP and 6 hour rainfall from 24 September 00UTC to 29 September 00UTC, every 12th hour. One can note that the major rainfall in Georgia and North Carolina started well before the arrival of the (ex)-TC Helene to the region.
The plots below show analyses of z500 and T850 from 23 September to 29 September, every 24th hour.
3. Predictability
3.1 Data assimilation
3.2 HRES
The plots below show 72-hour precipitation in observations (first plot) and concatenated short forecasts (second plot) and NEXRAD (3rd plot).
The plots below show 72-hour precipitation in ENS control forecasts with different lead times.
The plots below show 72-hour precipitation in DestinE 4.4km with different lead times.
The plots below show 72-hour precipitation in AIFS with different lead times.
The plots below show 72-hour precipitation in AIFS0-ENS member 1 with different lead times.
The plots below show 72-hour precipitation in 49r1 ENS member 1 with different lead times.
3.3 ENS
The plots below show EFI for 3-day precipitation (26 September -28 September) from different initial dates.
The plot below shows the forecast evolution plot for 72-hour precipitation around Asheville. The value for NEXRAD is 356 mm/72h.
Analysis - green dot
ENS control–red
ENS - blue box-and-whisker
DestinE4.4 - evergreen dot
AIFS - cyan dot
AIFS-ENS - grey box-and-whisker
Model climate – cyan box-and-whisker
Ensemble mean as black diamonds. Triangle marks the maximum in the model climate based on 1200 forecasts.
The plots below show the tropical cyclone track for TC Helene for the operational ECMWF forecasts from 26 September 00UTC (first plot) to 21 September 00UTC (last plot). The symbols shows the position on 27 September 00UTC (hourglass for BestTrack). HRES (red), ENS CF (blue), ENS PF (grey) and BestTrack (black). AIFS is included in green.
The plots below show the same as above but with DestinE in purple.
The plots below show the same as above but for 49r1 e-suite.
The plots below show the tropical cyclone intensity (central pressure - top, maximum wind -bottom) for TC Helene for the operational ECMWF forecasts from 26 September 00UTC (first plot) to 21 September 00UTC (last plot). HRES (red), ENS CF (blue), ENS PF (grey) and BestTrack (black). AIFS is included in green.
The plots below show the same as above but with DestinE in purple.
The plots below show the same as above but for 49r1 e-suite.
3.4 Monthly forecasts
3.5 Comparison with other centres
4. Experience from general performance/other cases
5. Good and bad aspects of the forecasts for the event
- The path of the cyclone and the risk for extreme rainfall in N. Carolina was captured from the 22 September - 4-6 days ahead of the rainfall.
- The AIFS ensemble underestimated the event, while the deterministic AIFS performed similar to the IFS control forecast.