Why do some storms spin up into a hurricane?

I’ve been playing around with the excellent VAPOR graphics package from the National Center for Atmospheric Research.  Modern meteorological models create massive amounts of data.  Managing it, much less making sense of it, is a challenge.  Here is a view of a simulation of the investigation area off the Southeastern US, AL912014.  This is the forecast for 8pm EDT June 29th, based on data as of 8pm June 28 (in other words, the 48 hour forecast – the label says “00:00 July 1st” because all weather data is in GMT, which is 4 hours ahead of Eastern Daylight Time).

This view shows the path air is taking around the storm in the lowest 500 meters (1500ft) of the atmosphere, color coded to indicate how much water vapor is in the air.  The vertical scale is exaggerated by a factor of 100.  Click to embiggen . . .

You can see air being pulled in at the surface, but the the picture is pretty convoluted. Dry air to the north, and winds criss-crossing the storm are keeping it from spinning up.  By comparison, here is what a mature hurricane looks like (in this case, 1999’s Hurricane Floyd, which passed through the same area).



Notice some key differences – especially the vertical development in the eyewall, and the huge area of organized winds feeding the center with warm moist air.  That is the mechanism by which a hurricane converts the potential energy of warm, moist air in to wind.  By using graphics like this, we can get a good picture of what is going on inside the storm, and hopefully improve our understanding and ability to forecast why some storms become hurricanes and some don’t.


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