Riis volleyed home a cross from Brad Potts with five minutes of stoppage-time played at Deepdale to make it 2-2.
The Robins had taken a 12th minute lead through Chris Martin, with Riis pulling PNE level early in the second half.
Antonie Semenyo restored City's lead in the 81st minute and it looked like it would give them victory until Riis produced his late volley.
Lowe said: "I didn't expect him to score from there, I'm not going to lie because that is a Premier League goal - I don't think Mo Salah would have scored from there!
"At one point I was wondering whether the ball was going to stay in because it was a great pass and Brad Potts was opening his legs up to get to it.
"As a striker I would probably have been running across the six-yard box to meet it not pulling away towards the edge of the box.
"What a fantastic strike and a fantastic run from Pottsy."
Lowe accepted that North End's performance in the first half had been way below what he wanted and expected from his side.
The visitors were deserving of their half-time lead and could have been further ahead.
During the interval Lowe made two substitutions, bringing on Ali McCann and Josh Earl in place of Ryan Ledson and Greg Cunningham.
Lowe said: "It was a good second-half response, a good second-half performance.
"The first half we weren't quite happy with but they are allowed one of those when they give it their all in the second half.
"I was disappointed with large parts of the first half, we didn't play enough football, the turnovers were too quick which allowed Bristol City to be on the front foot.
"I think I can erase the first half out of my memory but I won't accept the first-half performance, the lads know that.
"We freshened things up for the second half, made a couple of changes and I was really pleased with the reaction to come back and get a point.
"We just changed shape a little bit. Ryan had played in a two with Ben Whiteman at West Brom, with Alan Browne as a 10.
"We just wanted a bit more freshness in there so we went back to a three in midfield, brought Ali on to play that right side and put Browney on the left.
"Greg was more as a back five so we wanted someone with a bit more energy higher up the pitch.
"We got higher up the pitch with Josh, and Ali gave us that energy.
"The shape changed a bit, I was more happy with Ali being there as an eight rather than Ryan.
"They were the reasons for the changes, sometimes I have to make those kind of changes to get us higher up the pitch and on the front foot."