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I don't think I've ever noticed M before.

H and S are Hurricane and Tropical Storm.

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The Legend helps... S39-73mph H74-110mph & M >110mph - M for Major is likely though NASA launched a weather satellite called NOAA-M, later renamed (NOAA)-17(M). A clue maybe? earthobservatory.nasa.gov/Newsroom/view.php?old=200206249448 –  Mapperz Aug 26 '10 at 14:42
    
So it was right there in front of me! –  Jay Cummins Aug 26 '10 at 15:13
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up vote 5 down vote accepted

M must be new this year - I hadn't noticed it before either. From the Forecast Positions legend on the map, it looks like M denotes winds predicted to exceed 110mph at a forecast position. Since 111mph is the lower bound of Category 3 on the Saffir-Simpson Hurricane Scale, my guess is that M stands for "Major Hurricane".

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Major. Makes sense, although I was hoping for "Monster" or "Mega". I re-googled and found an alternative map using Google: nhc.noaa.gov/refresh/graphics_at1+shtml/143812.shtml?gm_track And it has a legend! nhc.noaa.gov/gifs/forecast_legend_v6.png –  Jay Cummins Aug 26 '10 at 13:46
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I believe that it is a 'Major Hurricane'. Major hurricanes are defined as being of Category 3 or greater. The wind speeds in the definition of a Category 3 hurricane on this page http://www.nhc.noaa.gov/aboutgloss.shtml matches the wind speed classes in the legend on the map.

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