Hurricane winds: D, S, H and M

The US National Hurricane Center’s forecast maps show wind speeds as D, S, H and M. There is the explanation that D winds are less than 39 miles per hour (63 km/h), S winds are 39-73 mph (63-117 km/h), H winds are 74-110 mph (117-177 km/h) and M winds are more than 117 mph (177 kn/h). I’m sure that people in the forecast path of M winds don’t stop to wonder what these letters mean, but I’m safely on the other side of the planet, so I do. I can’t think of any set of four words beginning with these letters which would describe hurricane-force winds, In other contexts, and by themselves, S might mean strong, H high and M moderate, but that can’t be the definitions here (moderate winds are certainly not the highest category, and what’s the difference between strong winds and high winds?) The US National Weather Center’s website doesn’t have an explanation, and I can’t find anything anywhere else. Any guesses?

I can’t imagine any emergency authority saying ‘Evacuate the area immediately. M winds are forecast for the next 48 hours.’

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