Considering the fact that Operation Downfall(The Land Invasion) Would have had over 1 million american casualties, 10 million japanese casualties, and god knows what amount of deaths...
Yah it seems pretty justifiable for the first one since they were NOT trying to form a truce. The Battle of Okinawa had at least 319,000 deaths, more than both bombs combined, and the fact that they sent in so many Kamikazies meant that they were not going to surrender. It is also a fact that each civilian would fight to the death againt the "devils" (America, YAY!) for their emperor made it seem completly nessescary for the bomb on Hiroshima to be dropped.
However, the bomb on Nagasaki was not as just. It was 3 days after the first one, and amidst all the chaos the Japanese were unable to prepare for the treaty to be made. Although, on the America's side, three days after the bombing was the only day with good weather for bombing for a long period of time. By the time they had to make a decision, they did not know what the japanese had planned on doing. And the thought of them fighting back was still very possible. It seems like a reasonable choice, judging from the japanese will to fight back.
All in all, the bombings seem justifiable, in order to reduce the expected amount of deaths in total, not just for the United States.