High Temp at Startup after Upgrading to High Sierra
In iStat Menus, check the figure for 'relative to ProcHot' - so long as this is a positive value, all is OK in theory, though I would have expected the fans to ramp before that temperature was reached.

Maybe try resetting the NVRAM &amp; SMC - Really confused about how to reset SMC and PRAM and NVRAM on my Macs and iPad and iPhone? has a comprehensive answer on how to do this for all Mac models.

Picture of where is shows on my machine, but as it's a dual processor Mac Pro no doubt it will look very different. [bottom half menu cropped off, to save space]