Barring means “except for” or “unless something happens.” When a weather forecaster says “barring any storms, it should be a beautiful weekend,” she means the weekend will be nice unless a storm arrives. If a coach tells the team “barring any injuries, we'll start the same lineup,” he means the lineup will stay the same unless someone gets hurt.

The word signals a possible exception to what you're saying. It acknowledges that circumstances could change your plans or predictions. A teacher might say “barring a snow day, we'll have the test on Friday,” meaning the test happens Friday unless school gets canceled.

When you say “barring bad weather,” you're essentially saying “except for the possibility of bad weather” or “unless bad weather happens.”

You'll often see this word in forecasts, plans, and predictions where people want to be clear about what could go wrong. “Barring any complications, the project will finish on time” means everything should work out, but the speaker is being honest that unexpected problems could change things.