The Particle Separator will work in rain conditions. Rain will not be an issue, with water being heavier than air, it will separate the rain like it does with dust. There's also a drain before the stock intake to let any water drain out that is not separated before hitting the air box. You should not however completely cover the intake opening, for example if a wave of water splashing over the roof completely covering the front of the separator.
The Scavenge Fan is IP55 rated and was designed to endure extremely harsh conditions which includes exposure to water and dust for over 50,000 hours of operation. S&B warranties the fan for 3 years from the date of purchase.