The thing is, when soil is very dry, it at first repels water, then only starts to readily absorb it once it has become a bit moist. So when a heavy downpour falls on dry soil, much of the water is lost to runoff rather than soaking into the earth and irrigating your plants. And often you really needed that water!
Therefore if your soil is very dry and weather reports announce that rain is on the way, the best thing to do is to get out there and water a few hours earlier. This watering doesn’t have to be abundant: you just want to deposit a bit of moisture on the soil surface, enough so that the soil becomes water absorbant rather than repellent. Then when it really does start to rain, you’ll discover that almost all the water will descend into the soil… where your plants can take full advantage of it!