Definition of "thunderplump"

The seeds did not germinate in many parts of a row until rains in end of June and thunderplumps in first week of July brought them up later in patches, so that no second sowing was necessary, but singling was done by stages.

1916, Transactions of the Highland and Agricultural Society of Scotland, page 241

There was a real thunderplump. The rain was just coming down in sheets so's the water was dripping over my ears and you couldn't use the Walkman.

2015, Alan Warner, Morvern Callar, page 46