5 April 2013
Hurricane Forecasts and The Shrinking Cone of Uncertainty
Posted by Dan Satterfield
Two events of note to mention regarding hurricanes and tropical meteorology this week. That familiar cone you see during hurricane season actually has some science to it. The width of the cone is based on the past accuracy of tropical cyclone predictions made by the National Hurricane Center. As the track predictions have improved the cone gets more narrow. Brian McNoldy at the uni. of Miami RSMAS put together a great image showing the difference in the cone from 2008 compared to this coming season.
The cone is made so that the odds are around 67% that the tropical storm or hurricane will be inside that cone during the forecast period. Note that this says nothing about the intensity of the storm, and intensity forecasts remain very problematic. The actual distances are below in this image from NOAA:
Another change worth noting is that after Hurricane Sandy, Hurricane Warnings will be issued for winds over hurricane force even if the storm is turning extratropical. Sandy was technically not a true hurricane at landfall and High Wind Warnings and not Hurricane Warnings were issued farther up the coast. Hurricanes are warm core systems and the pressure drop is caused by the heating of air as water vapor condenses. Extra-tropical lows are cold core systems and use temperature differences in the atmosphere as their fuel.
This is one of those cases where the right thing to do is the wrong one scientifically. Warnings need to be clear to the public, so if hurricane force winds are expected, a hurricane warning will now be issued. I’ve heard little (if any) objections to this in the broadcast meteorology community, or from NOAA forecasters either. We ALL care about clear and accurate forecasts.
This change was actually made very quickly for this type of change, but it was the right thing to do. Does this mean we call an extratropical cold core system with winds over 65 knots a hurricane? No, it’s still an intense low but the public in the path of those winds will receive a hurricane warning.
If you are interested in hurricanes, then I’d be remiss if I did not mention the two best books on the subject ever written. Isaac’s Storm and Divine Wind. Divine Wind is by Dr. Kerry Emanuel, who knows more about the inner workings of these storms than just about anyone on the planet. Both are amazing reads.
Why are the probability circles in the Pacific Basin smaller than the ones in the Atlantic Basin?
Good question. Since the NHC only forecasts for the Western Pacific which is open water I suspect they may not jeep track but I may very well be wrong. Will check.
Why does the NHC only cover hurricanes in the western Pacific when the ones in the Eastern Pacific are the ones that hit the continent?
And if that is so, why are non NHC forcastors better than NHC ones? Could we improve our forecasts by outsourcing them?
Sorry ..meant to say they cover EAST Pacific storms. West Pacific is the Joint Typhoon Warning Center.