It looks like the forecast for Saturday/Sunday is a bit 'changeable'. That's OK - yes, ideally you'd like a calm, mild, dry day, but as long as it's not a scorcher or a wash-out, we should be happy enough. The chance of a shower en route underlines the importance of clothing options. So here goes...
In a nutshell - be prepared for hot, cold, dry and wet. I know it's unlikely, but we might start in rain, be basking (basking sounds like something you'd do lying down, so that's perhaps the wrong word) in 20C+ sunshine up the Sloc, battling into a cold headwind at Jurby and finishing in a downpour on the Prom. So, I'd want everything from a vest and shorts to a waterproof coat and even a woolly hat and gloves. Whatever you start the race in, you need to have the confidence that you are equipped for any weather eventuality. Otherwise, at the first soggy realisation that you've got another 60 odd miles to go in the same soaking wet, heavy, t.shirt, the option to stop and head home for a hot bath will become very hard to resist.
So, just get a bag and fill it with anything you might possibly need. If you're unsure - put it in. Far better to not need something you've got, than to need something you haven't got. The Parish is an endurance event and success depends upon energy-efficiency. Getting cold is a sure-fire way to waste energy. The body uses loads of energy trying to stay warm and your performance levels plummet. So, it is essential not to get cold. I know it's late June, but if you get wet - especially if it's after dark and windy too - then get some extra, dry layers on and keep moving. Often, as a walker you don't necessarily notice the onset of a cold evening or the big black cloud on the horizon. This is where your support crew can save the day. Listen to them and do what they tell you (unless they say 'get in the car, we're bored now').
For regularly updated, Parish-specific weather prospects from met-man Adrian Cowin, keep an eye on the manxathletics.com forum.