Hurrican photo by NASA on Unsplash

Hurricane Florence, September 2018

Hurricane Florence is forecast to cause widespread damage due to the storm surge, extreme rainfall and high winds along the US East coast.

We have investigated the possibility of doing a World Weather Attribution (WWA) analysis of the effect of climate change on the rainfall, similar to the one we did for Harvey last year (van Oldenborgh et. al., 2018). This analysis would be more challenging for three reasons:

  • The observational analysis — looking at similar events in the past — will be much harder. There is a gradient in hurricane activity along the coast and there are mountains with strongly varying rainfall amounts. Hurricanes are also rare here so we have to combine different areas along the coast to ascertain whether they fit the distribution of other extreme rain events (like happened in the Gulf) or behave differently than the rest.
  • We have only a single model ready with the required resolution for this region and a solid analysis requires multiple large ensembles of high-resolution models.
  • Lack of manpower in the WWA consortium. We have no stable funding at the moment and cannot do this analysis “on the side” together with the ongoing analysis of the Kerala floods and the normal work. We would strongly welcome partners that can support our work and enable our impactful analyses.

One analysis of the forecast strength of Florence has already been made public by Reed et al. They use a different technique from our regular WWA analyses, which answers a slightly different question. Whereas we compute the change in probability of a class of events, such as all events with extreme rainfall above a certain limit, their study investigates only this specific event (more precisely, one forecast of the event). The results of such a study not only depend on the general effect of climate change on precipitation of hurricanes, but also on the details of this particular hurricane and even the forecast model. More analyses are needed to assess the robustness of this quick analysis, although the basic result that global warming increases the precipitation is a very robust one supported by observations and modelling studies (e.g. Liu et al, 2018).

Note also that in addition to direct impacts of wind and rain, Florence is expected to cause a large storm surge. Such storm surges are of course affected by ongoing sea level rise, but this is not yet part of our attribution assessment.