First step is to check the weather again on Friday - the weather bureau stand at least some chance of knowing what is going on by then (this is in no way a guaranteed outcome - even if they do look out the window to check, which they usually don't). Your boat is well and truly big enough to handle the conditions - as to the occupants - everyone's threshold is different. 15 to 20 Kph will probably be ok. If it's 15 to 20 knots, investigate "plan B" as it will be uncomfortable. Watch your wind direction - many a young player has headed across thinking "this ain't so bad" only to discover they were running with the sea and that once they turn to run into it to come home, it's not so much fun. Also be aware that wind against tide can make the seas stand up.