Forecasting Hurricane Impacts/Damage: why it’s tricky

Forecasting hurricanes can be difficult.  You have to get both where the storm is going (the track) and how strong it will be when it gets there (the intensity).  Forecasting the damage a storm will cause is even harder.  The reason is that damage is proportional to the cube of the wind speed (if the wind speed is v, damage is related to v^3, or v*v*v).  So slight differences in the track or intensity can make a huge difference in damage.  At 70 mph sustained winds, we would expect a large number of average (not hurricane resistant) wood frame houses with shingle roofs to have damage costing  3.7% of total value (mostly roof damage).  At 75 mph, that goes up to 5.8%.  So small changes in winds make a big difference in damage.

Compare these simulations of Tropical Storm Arthur as of this morning.  Here is the official NHC forecast track and wind swath from my TARU model:
al01_ofcl_2jul_am
On this track and intensity, my ISTANU model says the impact should be about $30 Million dollars.  Now compare that to the forecast using the HWRF objective track/intensity model:

al01_hwrf_2jul_am
 On this track and intensity, impact top out at almost $400 Million dollars.  By contrast, the GFDL model is only $25 Million,mostly due to disruption, not physical damage:

al01_gfdl_2jul_am

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 . . .
al91_48hrfcst_29june

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).

floyd_vapor2

 

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.