Hostile mobs spawning in regular rain weather

I was running around a plains biome and it was a regular rainy weather with no thunder or lightning then a ton of creepers and zombies started spawning. Let me know if I'm wrong of this. But it did become a thunderstorm a couple minutes after I experienced the hostile mobs spawning; do mobs spawn in rainy weather moments before the thunderstorm even starts? Thank for reading and hopefully answering.