Warn on the hazard

In this article, Andrew Freedman of Mashable argues that the Saffir-Simpson scale should be changed because it doesn’t measure rainfall, and yet rainfall causes disasters in many hurricanes – including Matthew in North Carolina just now. The Governor of North Carolina has been saying the same thing. The National Weather Service disagrees, and so do I.

In fact the Saffir-Simpson scale used to be based on not just wind, but also pressure and storm surge. It was simplified to just wind a while back because the different variables often wouldn’t match for a single storm – e.g., Sandy would have been a cat 1 due to wind but a cat 3 due to surge. There is no way to make a single number capture all the different hazards without being confusing. You wouldn’t know just from the number what the reason for the number was, and would still have to read the fine print in the forecast to find that out.

In addition, heavy floods can happen from storms that are not tropical cyclones at all. E.g., Louisiana a couple months ago. How would a change to the Saffir-Simpson scale help with those?

The rational approach is to warn on the hazard. That is, de-emphasize the Saffir-Simpson category, and issue warnings about each specific hazard separately. This is in fact the direction the National Weather Service (including the National Hurricane Center) has been moving. For example, they have recently introduced Storm Surge Watches and Warnings.

The NWS system can surely still be improved – apparently, the NWS forecasts of heavy rain didn’t reach everyone in North Carolina, even though they did exist and were quite accurate. So it would be reasonable, for example, for NHC to issue a formal Heavy Rainfall Watch or Warning (or whatever one wants to call them) when it is warranted in association with a tropical cyclone.

But multiple hazards from a tropical cyclone will remain multiple hazards. Combining them into one number wouldn’t eliminate the need for people in harm’s way to understand that. It would just disguise the problem, and disguising problems isn’t usually the best way to solve them.