Request for Comments - Proposed Modification to SSHWS
Proposed Modification to the Saffir-Simpson Hurricane Wind Scale
The National Hurricane Center is considering a minor change to the Saffir-Simpson Hurricane Wind Scale in order to resolve awkwardness associated with conversions among the various units used for wind speed. Currently, Category 4 is defined to be 131-155 mph (114-135 kt, 210-249 kph). The proposal broadens the Category 4 wind speed range by one mph at each end of the range, giving a new range of 130-156 mph (113-136 kt, 209-251 kph). Because NHC assigns intensity using 5-kt increments, neither storms in the historical record nor any future storms would have their SSHWS category changed as a result of this adjustment.
NHC's assigned intensities in 5-kt increments are converted to other units in advisory products, and the converted intensities are also rounded to the nearest 5 mph or 5 kph. However, the current Category 4 range is problematic when a hurricane's intensity is 115 kt (132.3 mph). Although 115 kt falls within the Category 4 range, the equivalent wind speed in miles per hour rounds to 130 mph, which falls in the Category 3 range. In order to show the hurricane as Category 4 in both sets of units, NHC must incorrectly convert 115 kt to 135 mph in its advisory products. A similar problem occurs at the current category 4/5 boundary when 135 kt is converted to kph.
Changing the Category 4 range to 130-156 mph, 113-136 kt, and 209-251 kph would allow all unit conversions from knots to be done correctly and keep storms in the correct category regardless of the units used.
Summary of the proposed change:
Category Existing range Proposed range ------------------------------------------------------------ 1 74-95 mph 74-95 mph 64-82 kt 64-82 kt 119-153 kph 119-153 kph 2 96-110 mph 96-110 mph 83-95 kt 83-95 kt 154-177 kph 154-177 kt 3 111-130 mph 111-129 mph 96-113 kt 96-112 kt 178-209 kph 178-208 kph 4 131-155 mph 130-156 mph 114-135 kt 113-136 kt 210-249 kph 209-251 kph 5 156 mph or higher 157 mph or higher 136 kt or higher 137 kt or higher 250 kph or higher 252 kph or higher