Yep always carry a water temp gauge. Without it, would be like not taking a compass on a wilderness trail.
In drought late summer conditions need to fish at crack of dawn, deep pools, and or where streams enter the river or you know there are underground spring water entering the river (takes experience to find these). If fishing during the day shaded pools and where cooler water enters the river are where the trout will be. Just before dark can be good and cloudy rainy days also.
As for flies in late summer in NY state, I have been gone for twenty years, grew up fishing the catskills rivers. A grass hopper pattern, ant, caddis, jassids were always useful.)
As you are a beginner here is the golden rule for late summer small stream fishing: use very light leaders 1-2 lb tippets and always fish upstream using the above parameters provided.
Tight lines, let us know your conquests as you progress through the flyfishing journey.