The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
(s)ten-
DEFINITION:
To thunder. Oldest form *sten2-.1. Zero-grade form *st-.a.thunder; Thursday, from Old English thunor, thunder, Thor; b.blunderbuss, dunderhead, from Middle Dutch doner,donder, thunder; c.Thor, from Old Norse Thrr (older form Thunarr), “thunder,” thunder god. a–c all from Germanic *thunaraz.2. O-grade form *ton-.tornado; astonish, detonate, stun, from Latin tonre, to thunder. (Pokorny 1. (s)ten- 1021.)
-
rr (older form Thunarr), “thunder,” thunder god. a–c all from Germanic *thunaraz. 2. O-grade form *ton
re, to thunder. (Pokorny 1. (s)ten- 1021.) 
