Friday, August 29, 2008

A little backstory...

I have been watching the progression of [now] Hurricane Gustav with the same mild detachment as I watched Katrina and Rita. This is certainly not to my credit.  

But I began to wonder about the maps of projected paths I pored over.  What was the basis for these predictions?  Should I fall for them again?

This graphic shows an approximate representation of coastal areas under a hurricane warning (red), hurricane watch (pink), tropical storm warning (blue) and tropical storm watch (yellow). The orange circle indicates the current position of the center of the tropical cyclone. The black line and dots show the National Hurricane Center (NHC) forecast track of the center at the times indicated. The dot indicating the forecast center location will be black if the cyclone is forecast to be tropical and will be white with a black outline if the cyclone is forecast to be extratropical. If only an L is displayed, then the system is forecast to be a remnant low. The letter inside the dot indicates the NHC's forecast intensity for that time.

NHC forecast tracks of the center can be in error; track forecast errors in recent years were used to construct the areas of uncertainty for the first 3 days (solid white area) and for days 4 and 5 (white stippled area). These areas of uncertainty are formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The historical data indicate the entire 5-day path of the center of the tropical cyclone will remain within the outer uncertainty area about 60-70% of the time. There is also uncertainty in the NHC intensity forecasts. The Maximum 1-minute Wind Speed Probability Table provides intensity forecast and uncertainty information.

It is also important to realize that a tropical cyclone is not a point. Their effects can span many hundreds of miles from the center. The area experiencing hurricane force (one-minute average wind speeds of at least 74 mph) and tropical storm force (one-minute average wind speeds of 39-73 mph) winds can extend well beyond the white areas shown enclosing the most likely track area of the center. The distribution of hurricane and tropical storm force winds in this tropical cyclone can be seen in the Wind History graphic linked above.   [More]
It is not the responsibility of NOAA to educate me on the graphical information they provide, but I appreciate the ability to learn more Indeed, if more of us just asked where numbers were coming from, I think we'd all be able to make better choices.

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