Friday's first practice session effectively only lasted 39 minutes as track workers needed to drain the circuit of standing water following an intense hailstorm.
After some intermediate running a dry line formed just in time for a late batch of soft tyre laps, while FP2 started on slicks on a damp surface before conditions worsened again.
With the weather forecast predicting more mixed conditions on both Saturday and Sunday, Friday's pacesetter Alonso said the weekend would revolve on which teams made the best bets on tyre choices and set-ups.
"It's going to be a gamble always on which tyre to put on at which moment," Alonso said after topping FP2 ahead of Mercedes man George Russell and Aston Martin team-mate Lance Stroll. “Let's see if we get it right.
"It was a tricky Friday for everyone. Not many laps in FP1, not many laps in FP2, no proper laps in dry conditions and no proper laps in wet conditions. We were in the middle of nowhere.
"But it could be like this in qualifying and the race, so it's still very useful information. We need to analyse a little bit the data and be very sharp.
"I think the right decision can gain you five seconds, The wrong decision, you are out of the race."
An additional element is the Circuit Gilles Villeneuve's brand-new asphalt, but Alonso gave the new surface the thumbs of so far.
"It feels okay," the Spaniard said. "Obviously, it's not very clean.
“Also after the storm it's quite dusty, but at the same time, it feels quite good grip, so I think the new Tarmac is good.
"We just need to test it in proper wet conditions, if there is any aquaplaning or something like that, but apart of that it's going to be interesting weekend."
Alpine's Pierre Gasly, who scored his first points of the campaign two weeks ago in Monaco, is embracing the changing conditions as another opportunity to sneak into the top 10.
"It's going to make the rest of the weekend more exciting," the Frenchman said. "Obviously, a lot of question marks with this new tarmac and how the degradation will be like on Sunday, and we'll have to go a bit into the unknown.
"But from the position we are in, I see it as opportunistic weekend. We know where we are. We know Monaco was a one-off and a very particular track.
"Hopefully the conditions can spice things up and open more opportunities for us."
