First off, I can't see every post on here all the time, so I don't always know if something has already been addressed. Having said that, there is absolutely no statistical discrepancy between the two products.

The HHC product is one based off of simple thermodynamical principles relating temperature to energy content. If you take the temperature over an oceanic depth (100m) and convert it to an energy using those equations, you can obtain the HHC product. Substantial values generally arise for SSTs >27C; non-zero values arise for SSTs >= 26C. I have the two plots up on my browser right now -- the only area where you may see a discrepancy is near the 26C isotherm on the edges, and that is solely because of the color scale used that has all HHC values from about 0-2 kcal/cm^2 as the same color. Those waters drop off in temperature fast below the surface and you'd expect the contributions to the HHC to be near-zero.

I'm not sure what you mean by there being no systems in the western or SW Atlantic this season to churn up the waters...Ophelia did a number on the Gulf Stream waters not all that long ago and the area near and just south/southwest of Bermuda all the way to Florida has seen many, many storms this season -- Tammy, Rita, Katrina, Nate, the aforementioned Ophelia, Irene, Franklin, and Harvey. The near-shore waters -- between the shore and the Gulf stream -- are also going to be cooler than you might expect due to Ekman-related divergence of the waters away from the SE US coast.

Finally, that product is limited in that it doesn't really account for cases like Vince -- the extratropical-subtropical-tropical conversion cases over typically sub-standard SSTs/atmospheric conditions. The only one that really does is MPI, and that doesn't account for shear at all and is generally an overestimator of a storm's actual intensity by 20mb or greater. There is no single product and all of them should be used in concert with each other to balance out their weaknesses. However, for a case in the NW Caribbean such as future-Wilma, it is a pretty useful product to show the energy available to the system. Unlike in the Atlantic, these waters have not been tapped since July with Dennis and Emily.

Simply put, there is no perfect utility to use -- and as I and others have always said, they are just that, utilities. For a storm in the tropics, they work reasonably well. In the subtropics, not as well.

And trust me, I know the differences between all of the types of storms and what they can/cannot do...and I also know that model representation of tropical cyclones AND extratropical transition often borders on pathetic. Intensity forecasting is not an exact science.

As for the climate indices...color me unconvinced. They are more reflections of what is actually occurring as opposed to indicators of what is to come (other than those such as ENSO). The NAO is derived from projecting surface pressures onto rotated EOFs across the northern hemisphere, with different modes from the EOFs proving to be more important than others during particular times of the year. The leading mode only accounts for about 20% of the variance in the entire data set. It's good to know, but once again -- just a utility, not a robust forecast product. And, ultimately, for extratropical transition, we're finding that patterns more consistant with the Arctic oscillation may be more important than the NAO...again though, it's still in the formative stages and only just a utility.