1611 ET - Natural gas futures post back-to-back gains as summer weather extends well into September, while gains are limited as the season advances and temperatures ease. Still, September heat is above-average and points to higher-than-usual cooling demand through the end of the month, Dennis Kissler of BOK Financial says in a note. And with this week's storage report expected to show another below-average injection, buyers remain active, he says. Nymex natural gas settles up 0.8% at $2.919/mmBtu.