Why did Volcanologist David A. Johnston stay so close to Mt. Saint Helens on and before May 18, 1980?

I recently visited Johnston Ridge (formerly known as Coldwater Ridge) less than 4 miles from Mt. Saint Helens. In the days leading up to the eruption on May 18th 1980, it was though by many volcanologists that the mountain would erupt characteristically vertically in the atmosphere. However, Johnston was one of the few, who was on the site monitoring the situation, and observed the buldge growing on the north slope. He felt that the eruption would occure as it eventually did, that is latterally, and to the north. He knew the that the mountain was due to blow and he even knew the potential power. Yet he was located directly in line with the north-face buldge and in a dangerously close proximity. I've tried to look up more about his personal thoughts on the situation but to no avail. As a geologist myself, I can't understand why he would so blatantly place himself in that situation if he knew the risks. Does anyone know the particulars behind him being where at was, at the time he was?