Bumping and cross referencing from Reddit-
https://www.reddit.com/r/ffxiv/comme...ther_relation/
I do believe the Boss fates are strictly tied to weather and have chances of spawning in those conditions.
I've been doing a lot of work from home so I've been able to do alot of 'AFK Diadem' sessions with my FC. (For those of you not familiar basically it's just entering, running to the The Missing Gem platform and making sure to move your character every 9mins so you don't disconnect 10mins from entrance. I highly do not recommend unless everyone in the party is obviously on board as you will not get any Lore / objectives complete. And probably angry party members if you sit and do nothing while they are trying to farm)
I keep the map open and tab back every so often to check boss fates / move my character and thus far this list has checked out. I've never seen the fates spawn under any other weather condition that isn't mentioned.
Additionally, I've gotten Hyperelectricity as well by simply afking so I'm still not sold on the evaluation being tied to spawning conditions (it very well could be I have just not seen evidence of it when we have 2+ parties from FC 'afking' getting a 0 eval rating. The only absolute I've seen thus far is if I happen to get the same instance back to back I have ALWAYS seen a Emergency Mission spawn. This leads me to believe that 1 Emergency Mission or Hyperelectricity can occur every 80~100-120minutes. So far I need to run this more to be sure of its occurrence, but personally for me I've had this occur on 4 separate instances where I happened to cue into the same instance I had been in for 60mins prior before with a 0 evaluation. I was able to confirm this by other players who were standing AFK at The Missing Gem who had entered after my initial entered time and were still in the same instance when I re-entered and flew back to The Missing Gem.
And while I cannot myself prove this when I enter a Diadem I always shout for "Last known occurance time for EM in this instance?" in cases where I receive shouts back saying 20~40 minutes ago, I have yet to see it pop within my next 60 minutes in the diadem.
I hope this information helps others trying to crack the likelihood of spawn conditions (my above data still does not disprove the evaluation theory but makes me believe it is more time based rather than Evaluation) I hope others can chime in more so we can figure out a better way of pinpointing!
Thanks!