From the HOME screen:
Press [CATEGORY SEARCH]
Select "Init"
Select "Init Normal (AWM2)"
Press [ENTER]

Touch the "Type/Name" box in the screen at Part 1
The pop-in will appear, touch "Edit"
Touch the "Element 1" to view its parameters
Highlight the Waveform box, it currently reads "CFX Stretch..." (the default wave at 0001)

Touch that Waveform box to see the Waveform Search screen
Under the Magnifying Glass type "thunder"
Set the Category to "All" or "Sound FX"

The thunder Waveform should appear... select it.
Add copious amounts of Reverb to taste, multiple notes at different regions to vary tone...

Thank you! I see a lot of other wave forms I can use. Appreciate the help and detailed instructions.

Hi Bob i did created a complete sound set with all the NATURE sounds and all single nature sound presets are playable. on each key is a Nature sound to can play & record in the montage performance recorder without need a Daw for your music music project !! . Check the sounds & recordings compilation demo >>

https://soundcloud.com/motifmoxsoundheaven/yamaha-motif-montage-thesounds-of-nature

The fun in working with a synthesizer is the creating them yourself! Nothing beats it!

Totally agree on that !!. Because Many people who own a modern synthesizer seem to have forgotten that a [ SYNTHESIZER ] is fully designed
just special to design / produce / change / edit / create sounds and that the standard factory sound presets are just a basic
starting point to go to work on. you have to delve into some more with the almost endless options, that nowadays a modern synthesizer like the Yamaha Motif
and Yamaha Montage series offers, with the powerfull unlimited sound/effects processor engines . !!!