A note from the Kumamoto International Scary Things Desk (which, in Japan, is jishin, kaminari, kaji, oyaji 『地震雷火事おやじ』): The thunder you're currently hearing is likely cloud-to-cloud, which explains the absence of visible lightning. The cool, dry jet stream.from the west is scooting above the warm, wet air from the south with a high speed differential, creating electrically charged regions between the two cloud layers which must be equalized; hence, my dog spending the morning under the table (for every action, there is a reaction). Since it's another rainy Sunday, one may as well ponder this. Below is a chart to help you start. - William