The morning session was cancelled when snowfall prevented the medical helicopter from flying, forcing officials to show the red flag until noon.
Once conditions cleared, officials confirmed the remainder of the days proceedings would run uninterrupted until 1800 in an effort to recoup some of the lost time.
Track conditions remained far from ideal, with heavy rain continuing to fall.
Those who ventured out completed only installation laps before returning to pit lane, Alonso the only one to set a timed lap.
His effort of 2:18.545s, the best part of a minute off the pace set on Tuesday, stood as the day's fastest.
“It's good to be the car with the most laps on the board today,” said Alonso, who recorded a total of 11 laps.
Daniel Ricciardo (Red Bull) and Brendon Hartley (Toro Rosso) managed just two installation laps during the day, with Marcus Ericsson (Sauber) and Robert Kubica (Williams) completing a solitary lap each.
“The poor weather conditions made things difficult all day long – we even had snow – so it was definitely not a normal day of testing and many people decided not to run,” Alonso continued.
“We still had some checks to do on the car as it's very early days for us with this new package, and we are learning things with every lap.
“We opted to do some background checks this morning in the garage – maybe not crucial to the car's performance, but still good learning for us.”
Rain is set to remain as teams enter the final day of the opening test tomorrow, though is expected to ease.