Just checked the weather for Kendal, only a 20% chance of rain. Looks ok.

i've taken a gtb apart, i wouldn't trust it as being waterproof, my quantum 2 was "waterproof" potted in resin can't see how you can get more waterproof than that, but it got wet and died.
I think overheating in the wet is fairly unlikely as it's so hard to get the beans down, atleast you lot with GTB's it'll just turn off and make you wait a few seconds

It all comes down to whether you want to take the chance. I would rather trust Novak's water proofing than my own. Especially with the fan making it that much harder. I haven't been able to obtain any balloons either. Will try again in the morning.