When tomatoes (and all ripening fruits) have a sudden fluctuation in their water levels, they are bound to react. After a somewhat dry summer (and with a consistent watering schedule), a sudden downpour allows plants to drink up way more water than usual. As they take up water, the fruits expand, causing the skins of tomatoes to crack and split. This is particularly offensive in the case of almost-ripe tomatoes; They are already super moist and so have a greater tendency to split right before harvest – a total bummer.
The good news is, if you get out in the garden and harvest almost-ripe tomatoes BEFORE forecasted rain, you can save them. Just keep them on a windowsill or countertop to continue ripening. (Any time you can ripen fruit in the sun, hence a windowsill, it will produce better-tasting fruit.) If you miss the window and rain comes, harvest cracked and split tomatoes asap and ripen them in the same fashion. While ugly, the cracks don’t do a lot to alter the flavor, and for any purist, you can cut around the tough skin and compost that portion.