The root ᴱ√SISI “singe, etc.” appeared in the Qenya Lexicon of the 1910s with derivatives ᴱQ. sisi- “scorch, singe, fry” and ᴱQ. sisin “parched, scorched” (QL/84). Its derivatives in the contemporaneous Gnomish Lexicon were words like G. thisin “parched, withered” and G. thith “dust” (GL/73), indicating an actual root form ✱ᴱ√ÞISI.
Neo-Eldarin: For purposes of Neo-Eldarin I think it is worth positing a Neo-Root ᴺ√THITH to salvage some of these early words; ᴺ√THITH is better than ✱ᴺ√THIS if we wish retain Early Qenya forms, as otherwise they would become sir- rather than sis-.