I think one of the problems with the success for Storm's solo series is new readers who have come onboard within the last 10 years don't know how awesome she is because she is being written so dismissively in the X-Men team books. I mean, imagine if somebody just started comics last year with Lemire and now Guggenheim and just heard about a Storm solo title coming out. I can see their reaction being, "So what?" Get what I'm saying? They don't know how awesome she is unless they go digging through back issues. Writers have gone out of their way to scale her back so that lesser characters (that includes some of the A-list X-Men, by the way) can shine. People need to be reminded of how great her character is and how powerful she really is.
If she had gotten a solo title in the 80s/90s/2000s, it would have been a huge success. I think her upcoming solo can still be successful, and, if written/marketed well, can grow in success, but I would have had no doubts about this outcome if the book had debuted back then.