The 2019 Atlantic hurricane season produced two Category 5 hurricanes on the Saffir–Simpson scale: Dorian and Lorenzo. Dorian formed south‐ east of Barbados on 24 August. It became a Category 5 storm before hitting the Bahamas, where it stalled and caused extensive damage. The cyclone then swept north along the US East Coast. Lorenzo formed on 23 September south of Cape Verde. It reached its maximum intensity on 29 September as the easternmost Atlantic Category 5 storm on record. The storm later weakened as it moved northward but caused problems on the Azores on 2 October, when the harbour on the Flores Island was severely damaged by high waves. Finally, the cyclone hit Ireland on 3 October as an extratropical system.
ECMWF’s medium‐range forecasts predicted Lorenzo’s genesis much better than Dorian’s, and early track forecasts gave a good indication of the cyclone’s northward turn. However, the forecasts underestimated Lorenzo’s propagation speed and there was great uncertainty in track forecasts as the storm approached Europe. Wave forecasts provided early indications of high waves around the Azores.
Genesis and track
As shown in the first figure, the genesis of Lorenzo was predicted by ECMWF ensemble forecasts exceptionally early. While the genesis of Lorenzo was predicted with high confidence more than a week in advance, ECMWF forecasts captured the genesis of Dorian only two days in advance. Big differences in predictability were also seen for major hurricanes in 2017. For example, the genesis of Irma was much more predictable than that of Harvey or Maria.
Ensemble forecasts for Lorenzo from before 22 September favoured a more westward track than the outcome. However, as illustrated in the second figure, the cyclone track forecast for Lorenzo from 24 September 00 UTC, issued just after the cyclone was classified as a tropical storm, already predicted a northward turn over the central Atlantic and provided an early indication that the storm might hit the Azores. However, comparing the position in the forecasts valid on 2 October with the observed position, we find that all members placed Lorenzo too far south, indicating a too slow propagation in all ensemble members.
In the ensemble forecast from 28 September, there was good agreement between all ensemble members on a path towards the western Azores. However, here too we find that in most ensemble members the propagation speed was too slow and the predicted position of Lorenzo on 2 October was too far south compared to the corresponding observation. The forecast from 28 September also missed Lorenzo’s rapid intensification the following day (not shown).
After passing over the Azores, medium‐range forecasts indicated large uncertainties in the path. In the forecast from 28 September 00 UTC, we find one group of members going towards the Bay of Biscay, another group towards Ireland and some members even further to the west over the central‐northern Atlantic. This injected a lot of uncertainty into the forecast for western Europe at this point. The cyclone eventually hit Ireland as an extratropical cyclone on 3 October with strong winds.
During Lorenzo’s passage over the Azores, waves were a major issue. The risk for extreme significant wave height south of the Azores between 1 and 3 October was picked up early in the forecast. As the bottom figure shows, as early as 25 October, i.e. a week in advance, the ensemble median was well outside the 99th percentile of the model climate distribution. Forecasts closer to the time predicted a significant wave height of around 14 metres (significant wave height roughly corresponds to the average height of the highest one third of waves). As shown in the figure, this corresponds well to altimeter satellite data available for about 04 UTC on 2 October close to the centre of the storm.
The forecasts for Lorenzo exemplify several challenges. The excellent prediction of the genesis for Lorenzo contrasts with the much poorer forecast for Dorian. This raises the question of how the predictability of tropical cyclone genesis depends on the wider meteorological situation. Lorenzo’s track was well predicted overall, but its predicted propagation speed was too slow compared to the observed speed, and forecasts missed a period of rapid intensification. These are issues which we have seen in several previous cases. Finally, Lorenzo injected a lot of uncertainty into forecasts for Europe during its extratropical transition. There are plans to study all these aspects of hurricane forecasting in greater detail in the coming years to enable us to further improve our medium‐range predictions of tropical cyclones and the weather in Europe.