Can I paint a deck when wet weather is forecasted?
I would paint after the rain and make sure there is a tarp over the deck before it rains.  You need the manufacturer's guidelines but two simple rules to painting outside is not touching a damp surface - and it can take a few days for a deck to dry out and not painting within 48 hours of rain forecast.  At 24 hours after painting you could tarp the deck but really better to have a few clear days.  If rain is constant then you just need to wait.