« on: April 22, 2015, 10:56:41 am »
There is what I would consider an ideal timeframe to leave Yorktown on the TransAm (which is the first half of May), but you can really leave anytime, understanding that the farther from the ideal timeframe you leave the greater the chance of weather problems. It isn't like there is some drop-dead date, like leaving on June 8 would be fantastic but leaving on June 9 would be a disaster. Furthermore, it depends on whether you plan to take 4 weeks for the trip or 20 weeks for the trip (how long do you plan to take anyway?).
Personally, I left in early May from Yorktown and did not find the heat to be a problem. If you want to avoid heat, I would suggest you leave earlier than that (April) rather than later. Even though you tolerate cold well, however, you might not tolerate riding through a foot of snow very well. Furthermore, even if you tolerate cold, do you tolerate cold and wet together?
Also, there is heat and there is heat. The temperatures in June might be higher than the temperatures in August in many places, but the humidity in June might be less than the humidity in August, making June more pleasant than August.
Since you ask your question about "latest", I assume that you cannot leave early. If you're not too slow, and you have enough time to wait out a snowstorm or two, I think you could wait as late as August to leave. You may have to skip McKenzie Pass (regrettable), but there is an alternative. Also, if you're planning to camp (are you?), you may find some campgrounds closed after the beginning of September.