Re: the percentage game has got to go!!!!
Posted by Chris in Tampa on 7/7/2010, 10:18 pm
The NHC percentages are for whether it will become a depression or higher in the next 48 hours. The problem is, what is a depression? What are the rules for upgrading to a depression?

From the NHC glossary...

Tropical Depression:
"A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less."

And now to what is a tropical cyclone...

Tropical Cyclone:
"A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects)."

There is no minimum wind speed for a depression. So it meets that criteria, though most things would. So is it a tropical cyclone? It is tropical. So what is left is if it has a closed surface circulation and deep convection. It does have a circulation that based on the latest non tasked recon is getting better organized at about 12 to 13 thousand feet. And based on dropsondes to the surface, it does have a circulation at the surface. So the only thing left is deep convection.  And that is the issue. At 8:08PM EDT, the NHC released this line in the best track file:

AL, 96, 2010070800,   , BEST,   0, 235N,  938W,  30, 1005, LO,  34, NEQ,    0,    0,    0,    0, 1009,  200,  65,   0,   0,   L,   0,    ,   0,   0,     INVEST, D,

That "D" represents that the NHC thought that the depth of the system was deep. (about decoding best track files) So at the time, it seemed to meet the qualifications for becoming a depression, which is why they said 80%. The problem is, what does the NHC consider deep convection? That gets subjective. Based on infrared I would say no at the moment. Additionally, and I wrongly think the following, the upgrade to this height does not mean something will happen imminently, the percentages actually represent the chances in the next 48 hours which I have to remind myself of. (Although I kept checking the ATCF system for an upgrade because when the chances go up that high sometimes they upgrade right after.) Given the organization based on recon and even on satellite despite the deep colors on infrared, I think they were right on this call despite how it turns out. (But yea, they will look bad if this one does not develop.)

Still wrong the other day when they needed to make the call either way. And the percentages should never be less than 10%, but I do like knowing when the NHC starts thinking something has a better chance. The media just needs to be more responsible with what to do with that information though. That is important to get across to local meteorologists. It's easy to get out by putting it on the front page. The problem, everyone else gets it, and 99.9% of the people seeing it are not meteorologists.

Latest recon image:

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20% of nothing ......gets you 80% of something??? - Doorman, 7/7/2010, 8:49 pm
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