The $30,000 was announced on Gamdom's official twitter page like hours before the rain. Was not able to join tho and yes it is diffrent from those where announced weekly from the page. Free rains are not just weekly, every day you get those free rains. And if you want to be part of the bigger rains there are certain amount you need to wagger before you can join. Blizzard rain i think will cost you 20k coins waggered to be eligible to join the rain..
Depends actually, I've seen requirements ranging from 25K up to over 100K, depending on the claim: wager ratio. But what's fixed is 15K for tornado, 25K for Tsunami and Volcanos.