Starting the day sitting second, Katsuta leapt into an 11.4s rally lead over Hyundai's Esapekka Lappi after a drama-filled stage four, which resulted in Rovanpera and Ott Tanak retiring from the day's action.
Overnight leader and reigning world champion Rovanpera, making a return to the WRC in a part-time programme for Toyota, was the victim of a snowbank. His rival Tanak suffered a similar fate that damaged the radiator on his Hyundai.
The incidents elevated Toyota's Elfyn Evans to third (+13.3s) ahead of M-Sport Ford's Adrien Fourmaux (+18.7s), with road opener Hyundai's Thierry Neuville in fifth (+40.5s). Oliver Solberg led the WRC2 class in sixth overall (1m09.2s).
Crews faced incredibly difficult conditions for the opening stage of the day as fog and drizzle added to the complexity.
Road position was crucial as conditions improved after each car passed through the #42 Brattby (10.76km), renamed this year in honour of the late Craig Breen, who declared himself the "Mayor of Brattby" after posting a record time last year.
Rally leader Rovanpera made the most of starting seventh on the road to win the stage by 3.2s from Hyundai's Lappi and Tanak. The latter faced the challenge of being third on the road.
Rovanpera revealed at the stage end that he struggled to get a good flow behind the wheel, while Lappi, who followed Rovanpera into the stage, survived a brush with a snowbank that dented his confidence.
"There was one corner where I touched a snowbank quite heavily. After that, I maybe drove a bit slow. Maybe that was not a smart idea. I should have used them instead," said Lappi.
Toyota's Katsuta continued his strong start to the rally after reaching the flying finish, 0.3s shy of the pace. Despite starting second on the road, team-mate Evans lost only 6.1s but was 5.1s faster than road sweeper Neuville.
"Visibility was close to zero with the fog – everything looks the same. I think only the real Mayor of Brattby would have loved that stage, but I didn't," said Neuville.
Fourmaux headed the two M-Sport Fords with the Frenchman 7.9s faster than Rally2 leader Solberg.
Conditions worsened heading into stage three (Norrby, 12.36km) as light rain turned into snow.
Neuville was the first to confront the conditions which the Belgian described as a "nightmare" as he struggled for visibility on his way to the seventh fastest time.
"My wipers are full of s*** so I can't even see at the end of a straight. The grip is zero. I was driving blind most of the time," said Neuville.
Evans managed to eclipse Neuville's effort by 1.7s despite surviving a heart-in-the-mouth, high-speed 360 spin. Luckily, the Toyota driver avoided contact with the snowbanks.
Lappi emerged with the stage win as the Hyundai driver pipped an unhappy Rovanpera by 1.2s. World champion Rovanpera made the rare admission that his driving was "horrible" and "embarrassing."
Katsuta posted the third fastest time, 0.1s quicker than the impressive Tanak, who was the best of the early runners.
But the complexion of the rally was turned on its head in stage four (Floda, 28.25km) which was claimed by Katsuta, who jumped into the rally lead as his rivals faltered.
Third-placed Tanak was the first to hit trouble as he found a snowbank which damaged his i20 N's radiator. He and co-driver Martin Jarveoja inspected the car and continued in EV mode before stopping on the stage.
Unseen by the television cameras, Rovanpera made a rare error which resulted in his Toyota GR Yaris coming to a halt in an access road on the stage.
Adding to the drama, Gregoire Munster suffered a puncture which cost the M-Sport driver four minutes to change.
Lappi also made contact with a snowbank but was able to escape losing 10.5s in the process, the Finn was fourth fastest behind Evans and Fourmaux.
The field will repeat the three stages this afternoon before closing out the day with a second pass through the Umea Sprint stage.
