[Fasterthanyou]

Technically the best conditions for max horsepower are the coldest temp, lowest humidity, and highest barometric pressure.
For the SAE J1349 relative horsepower calculations, the standard reference conditions are: Air temp 77 deg F (25 deg C), 29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.
That's how the dyno shops "should" correct for your HP using the above as 100%.
With that being said, right now, even with it being 2 degrees warmer and 60% humidity, I would be making 0.5% more horsepower than at standard reference conditions. The reason is the barometric pressure which right now is 30.02 inHg. Baro has a large effect, temperature is next, humidity last. So whenever the conditions might seem to be good... it might not be depending on the baro. Baro will increase with a cold front so whenever the temperature drops you can almost be sure you've got good pressure, temp, and humidity .
Now with traction, who knows what's best. If the sun isn't out do a longer burn-out 8) to increase your traction on a cool track surface.