It is not unheard of.

One year I had Big Boy setting fruit at insanely high levels in the first and second week of August when our high temps were approximately 108-112 and our low temps were 80-84. During that time we were in Exceptional drought (we only had a smidgen over 18" of rain for the entire year) and our afternoon humidities were in the teens and even the single digits. I believe that when it is exceptionally hot some tomato plants will set fruit well when the relative humidity is low, and not as well when the relative humidity is high. I do have some varieties still setting fruit here, and our afternoons have seen high temps from 108-111 and relative humidity bottoms out at about 16-18%. Even in the mornings, whatever humidity we have does not last long and we are usually in the 20s around lunch time.

Some Garden Web members who garden in hot areas that tend to be dry with low RH, like Sacramento, CA, report great fruit set from some varieties even in hot weather as well. I was relieved when I read some posts from one of them, because when my plants were setting fruit in insane heat, I thought I was losing my mind.

My plants look so terrible, I don't think I can keep them alive long enough to harvest them. But I guess I'll just keep watering and see what happens.

All plants look pretty terrible by late July and early August. Even in a normal year they have had to endure a lot of weather and pest stress by this point in the growing season, and this year is so much worse than an average year.

If you can water them enough to just keep them hanging on and clinging to life, they'll produce pretty well in the fall, most of the time. I don't know if that will be as true this year because of the extreme temps we're having.

I've been trying to keep a few alive, but am about to give up. This was our fifth consecutive day at or over 110 (was 112 today) and the plants and I both are just sick of this heat, and it shows.