Before leaving, before packing for your trip, do you check the weather at your destination?
You can check it a week or so prior and then again the day before. Will it be exactly as the forecast? Probably not...but at least you'll have some idea of what to pack.
Then, pack accordingly. You can almost always pick up an umbrella from a street vendor in case of unexpected showers. If you plan to dress in layers, you'll probably be ready for most weather changes.