When temperatures are reasonably warm a drop of rain can be very useful.
Firstly, it saves you having to water your outdoor tomato plants, and secondly, it wets the compost or soil gradually, allowing a more thorough wetting of the root area and helping to prevent blossom end rot.
However, a deluge when there are ripe tomatoes on the plants can cause tomato split. This happens when the soil goes from dry to very wet and the tomato skins cannot expand to the degree of water consumption that the tomatoes absorb.
When this happens I just cut the tomato that's split along the line of the split and nobody knows the difference. I usually cut my cherry toms in half for curries and pasta dishes anyway.
One problem that occurs when it rains heavily is that it brings out the slugs and snails. When the rain stops is a good moment to catch them and throw them into the bushes at the bottom of the garden. At night my wife and I can sometimes hear a crunching sound ... we think it's the hedgehogs having a midnight feast ... every cloud has a silver lining!