Where and when would a 737-700 be taking off at an altitude of 19 kft?
As Ron points out in the comments the question is not so much where as it is when. Aircrafts see the density altitude of the air around it not the true altitude they are at. On a very hot day at a very high altitude airport the density altitude may very well be higher than the true altitude by a significant amount which affects takeoff performance. The FAA provides a nice overview of density altitude here. SO even though there are no airports at 19,000 MSL a lower altitude airport that routinely sees very high temperatures can yield the same situation.