plasticup wrote:I wouldn't be so rough on the NHC. Their 5-day errors shrink every year, but even a few years back they were pretty good. Check out Hurricane Dean, where their 5-day errors were only a few miles from conception to demise. And when the cone is narrow like this, that means they have good confidence. I'd say 90+% chance that some part of Florida gets impact from this.
Just to be clear, the width of the cone is 2/3 of the historical error over the last 5 years at each foreacast point. NHC Director Bill Read talked about what the cone would be for this year at the start of the season in a conference at UM, and I asked NHC staff about the changes for this year to be clear for an article afterward. The NHC does not change the cone for each storm or forecast.
Here is the NHC definition of the cone:
To form the cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96, and 120 h positions, where the size of each circle is set so that it encloses 67% of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles.
Also important to remember that the cone has nothing to do with the size of the storm or the extent of winds. Damaging winds and surge can affect areas outside the cone.