Coming off its 350th episode last week, ABC’s flagship drama Grey’s Anatomy airs its fall finale tonight. It will help set up the January return of spinoff series Station 19 with new showrunner, Grey’s Anatomy‘s Krista Vernoff, who is pulling double duty running both series this season.

The fall finale of Grey’s Anatomy also will mark the medical drama’s last airing at 8 PM for now. ABC just unveiled its winter scheduling plans, which include Grey’s and Station 19 still airing as a 8-10 PM block on Thursday, but switching places, with firefighter drama Station 19 at 8 PM and medical juggernaut Grey’s Anatomy at 9 PM.

It’s been a Grey’s Anatomy staple that a fall or season finale would feature a cataclysmic event and end with a cliffhanger. But instead of tuning in to the next Grey’s premiere to find out what happens next, this time viewers will need to watch the Season 3 premiere of Station 19 on Jan. 23.

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“It felt very organic to launch the premiere of Station 19 off this incident that happens on Grey’s Anatomy,” Vernoff said. “We often do catastrophes or disasters in the season finale of Grey’s Anatomy, and it felt organic to do that and then allow the Station 19 first responders to be on the scene when we come back in January. There is an incident at the end of the Grey’s episode that requires you to come back in January at 8 PM to see how the first responders handle it.”

The storyline will carry through to the Grey’s winter premiere at 9 PM on Jan. 23 to complete the two series’ first crossover this season.

While “this is a big splashy way to premiere the new season of Station 19,” it’s not going to be an every week thing,” Vernoff cautioned about upcoming Station 19-Grey’s crossovers.

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The two shows will interconnect “every few episodes,” she added. “We don’t want to do it every week, we don’t want to create a mold, we don’t want to fall into any kind of pattern, we don’t want people to ever know what to expect. We have found many different, really quite exciting and fun ways to interact between the two shows.”

That would involve more characters from each show appearing on the other. “We have got big casts from both shows and natural ways for them to interact with each other,” Vernoff said. “It’s always really fun when we find ways to bring the Grey’s characters onto Station 19 and vice versa. It feels like our worlds have grown and the possibilities for storytelling have grown.”

That was part of the plan for Vernoff taking over both series.

“The network hired me to run Station 19 so that Station 19 and Grey’s Anatomy could be a more unified world with a more unified timeline and a more unified storytelling,” she said.

Vernoff gave big thumbs-up to the Thursday scheduling change, which will have Station 19 leading to Grey’s Anatomy after airing behind the mothership series for its first two seasons.

“With Station 19 airing at 8 o’clock and Grey’s Anatomy airing at 9 o’clock there are very organic opportunities for Grey’s Anatomy to, for example, inherit patients we see rescued on Station 19,” she said.

This is how Vernoff had envisioned the two series airing from the start.

“Ever since Station 19 premiered, in the writers room we always thought the better progression was from firefighters to the hospital,” she said. “I didn’t think it was going to happen but I was thrilled ABC had made the decision. I literally cheered when they let me know that. It’s really exciting and opens up the storytelling.”

Vernoff also is happy about Grey’s Anatomy’s return to the Thursday 9 PM slot where it had aired for eight seasons before it was moved to the 8 PM time period in fall 2014 to anchor TGIT. There are significant limitations on adult content from 8-9 PM, which is designated as a family viewing hour.

“There are different rules for a 9 PM show than there are for an 8 PM show, and we hope to take advantage of those rules,” Vernoff said. “Grey’s was definitely allowed to be a sexier show when it was on at 9 o’clock. So we are excited by the change back to our original (Thursday) time slot.”

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As for Grey’s successor at 8 PM, Station 19, “I think it’s quite a different show this year,” she said. “It’s messier visually, and it’s messier emotionally than it’s ever been. It was an exciting thing for me to dive into the world of firefighters; it’s very different from the world of surgeons, and to educate myself and talk to these heroes who run towards a fire.”

“I kept coming back to this image of sweat and soot and feeling like that needed to be a thing on this show, that it needed to feel grittier in all ways,” Vernoff continued. “The friendships are really more complicated this season. People are a little less polite this season, a little less kind, a little more competitive and dirtier, literally and metaphorically.”

Vernoff also teased tonight’s Grey’s Season 16 fall finale.

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“It’s Meredith first day back at work after 8 episodes of cleaning services on the side of the road. So she is very excited to be back at work but that is a little bit complicated by the fact that her relationship has hit the skids.”

In a typical Grey’s fashion, “right when you think you know what’s happening and where a thing is landing, you’re wrong, it is full of surprises. It was written by Kiley Donovan, who is doing both Station 19 and Grey’s Anatomy with me, and she found a beautiful way to introduce some of the characters from Station 19 and to intrigue people to want to come visit that world next year.”