The Frenchman struck an eight-under-par 63 to take an early two-shot advantage over the field.
Fredrik Andersson Hed and Oliver Fisher shared second place on 65, while India's Anirban Lahiri and Australia's Scott Barr are a shot further away in the event which is co-sanctioned by the European and Asian Tours.
Starting at the 10th hole, Bourdy began with a birdie, turned in 32, and went even better on the final nine holes to dominate in the rain.
“We’ve never had rain like this here before. It was raining even more than it did in Scotland last week,” said Bourdy.
“The forecast for the next couple of days is not great either, but there’s nothing you can do about it. You just have to deal with it, and I managed to do that very well today. I feel much warmer after a 63, that’s for sure,” added the Frenchman.