Hurricane Wind Forecasting (1) - What and How?

Steve's picture

Hurricanes are one of the most damaging natural hazards that exist. While we can’t stop hurricanes from happening, we can take steps to mitigate their consequences. To formulate a balanced mitigation program however, we need a hazard forecast.

Let’s talk for a moment about hurricane wind hazard ---  just WHAT do we want to forecast?  The most useful forecast address the questions - “How strong of wind do I expect in time interval X and at what probability?”

Two types of forecasts speak to this:  a Long Term Wind Exceedence Probability and a Short Term Wind Exceedence Probability. The former is generic (time independent), typically a multi-decade average. The latter is time-specific covering a few days in advance of a particular storm.

So HOW do scientists make Hurricane Wind Forecasts?  The most complex ones are strictly physics-based. These employ the full “weather forecasting” equations that link evolve temperature, pressure, and wind starting with their current conditions.  Needless to say, these forecasts are Super Computer Scale.

Simpler, but equally valid, forecasts are statistics-based. These employ historical hurricane behaviors plus a few empirical relations. These forecasts interest me because they are Laptop Computer Scale.

Luckily for us, starting from 1851 the Weather Service has compiled track (location and time) and strength (central pressure) information for ~1400 hurricanes and tropical storms in the North Atlantic.  This data, together with a Storm Wind Model that takes central pressure to a wind velocity at distance D from the central eye, provides all that we need to follow this Long Term Forecast recipe:

(1) Run all existing storms 1851-Present.

(2) Keep count of the number of times that the model wind velocity any given location exceeds a threshold.

(3) Divide the catalog period by that number to estimate mean recurrence interval.

(4) Assume Poissonian statistics to calculate probability of exceedence for any desired time interval.

 

In a future blog,  I’ll bring in the additional ingredients needed for Short Term Forecasts.  Meanwhile, to get a jump on the subject and to appreciate the “quirkiness” of hurricane behaviors, you might watch this movie  “Storms of the 70’s”     http://www.youtube.com/watch?v=daBmxkncaXE

 

Steven N. Ward,   Santa Cruz

 

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