I think you guys are missing the point of using hot water. The roof of the Metrodome is just that: a dome. We get rain in Minnesota too; but that rain drains away just like the snow does when it melts. Using hot water to melt snow starting at the edges of the dome and working up to the top presents less stress than a heavy thunderstorm would during the summer months. Melting the snow creates only water to flow off the dome.
As long as there is differential pressure to keep the dome inflated, there is no problem. Look at the video and notice how forcefully some of the snow is blown out of the tear in the roof. There were some unusual stresses on that roof with 35 MPH winds and snowfall rates at 2" per hour. (It was around 20 degs. at the time and the snow was very dry.)
Think of an above ground swimming pool. The pool holds all the water in just fine - but sometimes someone, or something, bumps the side, and it splits open causing the water to cascade out to the open yard. Same thing here: something caused a roof panel to split; snow, water, wind, whatever. When it did, the roof starts to collapse with no way to stop it.
757 Guy (Still digging out)