According to the Collin County Master Gardeners you need to get them in the ground by August 15th.

Let me guess what you're be doing this weekend!

Here it is Aug the 8th and the temp is 104. Is it still possible to plant fall potatoes in this kind of heat. I'm located in Canton,Tx.

I am planting fall potatoes here in SoCal over the next few weeks. Our season is probably a bit longer than yours.

But probably the worst case scenario for you is that they haven't sized up by the time cold weather rolls in and kills the tops. So, you dig up a bunch of little spuds, maybe mostly seed potatoes, maybe a few big enough to eat. You probably end up with more seed potatoes than you started with and you wait til spring and have another go. Or just leave the little ones in the ground, they will likely come back on their own come springtime.

Generally, potatoes need about 3 months( plus /minus 2 weeks) .
So I would plant the fall crop about 90 to110 days before my first frost date. Also, consider planting early varieties so that you won't be caught by surprise if you get a real early frost.

P.S>
I WOULD USE THE BEST OF ANY THING FOR PLANTING, JUST LIKE BREEDING IN ANIMALS WORLD.

Yukon Gold is an early variety of potato, which means you should have plenty of time to get a crop in.

Also keep in mind that the relevant factor is soil temp, not air temp. Plant them deep and use cold water, also a mulch to keep the soil cool and damp.