Hurricane uncertainties – a personal case study

I am glad to write to you that I had some relaxed holidays during August in the Azores.  For some of you who are unfamiliar with this little place, Azores is mostly known by its natural landscape, moderate climate and relaxed inhabitants. According with a rating done by the Dutch National Geographic, Azores was considered in 2016 the top place to visit (http://www.natgeotraveler.nl/galerij/20-ultieme-bestemmingen-voor-2016/de-azoren).

Anyway, on our way way back there was some news/warning that the Azores would be affected by the Hurricane Gaston. Luckily for me and my family it was going to hit on Saturday, two days after we were going to fly back to Germany (by the way I am originally from the Azores). Regarding natural disasters, Azores is often associated with Earthquakes. Truth is that they have also been hit in the past by few extreme weather events (see for instance Hurricane Gordon, Helene or Alex).

Well given that QUICS focuses on the topic of uncertainty, it does seem appropriate to show you some of the forecasts issued by the US National Hurricane Center for the recent Gaston Hurricane. It shows some of the practical aspects of communicating uncertainty to non-experts. Weather forecasting as we all know is highly uncertain, and hurricane forecasting is not an exception. In particular, I have chosen two examples: the first displays uncertainties of paths, and the second uncertainty of intensity.

jl_fig1
Figure 1: Coastal Watches/Warnings and 5-Day Track Forecast Cone, Hurricane GASTON Advisory #035, 5:00 AM EDT Wed August 31, 2016 (See this NHC page for current map)

Figure 1 shows you the 5 day track forecast issued on the 31st of August superimposed with the one issued on 2nd of September. The black line shows the forecast track on the 31st and the large blue area is the cone of uncertainty in the future track on the 31st. The smaller blue area is the cone of uncertainty on the 2nd. The cone of uncertainty is based on historical data, 60 to 70% of all historical data indicate that the storm center will remain within the cone for the days following the forecast.

jl_fig2
Figure 2: Tropical Storm Force Wind Speed Probabilities, for 5 days from 2nd of September.

The second example deals with wind intensity. Indeed, this is the main issue why hurricane forecasts are issued (see Figure 2 or a current graphic on the NHC site). Winds can reach destructive speeds and are responsible for increasing the wave height considerably (e.g. during hurricane Alex waves up to 14 meters were recorded at high sea). Figure 2 shows the cumulative probability that wind speeds of at least 39 mph will occur during a 120 hour period.  Maps of shorter time periods can be seen on the NHC site (click the different durations or loop in the top right), each graphic provides cumulative probabilities the that wind speeds of at least 39 mph will occur during cumulative time periods at each specific point on the map. Unlike the previous map, here it is possible to compare how intensities are expected to evolve during the five day forecast.

As I mentioned before Azores inhabitants tend to be quite relaxed, and indeed this time it was not too bad. The Hurricane went down to a tropical storm and it did not cause too much damages. A happy ending!

jl_fig3

Since I started with telling you about our holidays in the Azores, it is well suited to finish it with a picture we took. To conclude let me just write that the Azores is also populated with plenty of black and white cows (while some will say this last sentence is useless others may find it useful for understanding the bigger picture).

All the best!

Jorge Leandro (TU Munich and University of Coimbra)

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Hurricane uncertainties – a personal case study

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