It's often 100° outside for 3-4 months of the year here in Texas. 110° isn't too rare either. I've had an iPhone battery expand out (basically pushes the screen out of the shell) just from being with me on a walk through the neighborhood. If I owned an Apple Watch, I don't know if I'd even chance it on a summer jog.
Well, you could put a sprinkle of water on it once and awhile and that would be enough to keep it OK ;-). No joke, it truly would work.
The internal components of the watch (and possibly the battery) can work at more than 35C (external) temp, the problem is if the temp outside is 35C, the CPU, or battery inside will likely be significantly higher (local spots possibly at 45-60C) if the watch is used heavily for awhile, because heat transfer will be completely inefficient and there is no fans in this.
A Lithium battery stored 3 months at 60 Celcius and 100% charge, will lose 40% of its capacity, even if does nothing at all. At 50 Celcius internally, you'd need 6-9 months to get the same results (that's probably constant active operation at 35C+ external temp). The Apple watch is not likely to be put in such a situation (unless deliberately). So, even for significant quick heat damage you need to work at it. In this case, the watch would even need to be plugged in outside (in the sun?) to keep its 100% charge!
The fact that Apple talks of possible battery degradation in high heat, makes me think that the internal temp if could reach 55-60C if the device is used intensely in such temps. This would cause relatively rapid battery degradation at 100% charge (but not at 50% charge). Apple, not knowing the charge level or usage level, is being being cautious (it also doesn't want to invite abuse). The watch has the same operating range as the Iphone, which considering how small that thing is, is pretty spectacular.
(some battery info here)
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
If you have to sometimes use lithium batteries in very high heat (35C+), use them at half charge.
The average max temp in Dallas is over 35C=94F (nighttime 24 C=74F) from mid June to early September; off course, there are days were it is higher and lower than this. The temp exceeds 100F 4% in late July, early august (1h average per day, probably 3-4h in late afternoon on the days were the temp is above 100F).
If the watch wasn't wasn't doing any heavy lifting before you go out, and your just using the data collection (not playing music), internal heat production should be small, so it is mostly the external heat that will slowly seep in. You would be OK to jog before probably 1 pm (or after 8pm) even on scorcher days. But, who on earth would even want to jog in 100F heat anyway

.