The severe weather, with winds reaching speeds of over 90 mph (144 km/h) in some areas, created challenges for marine life, driving several creatures far from their homes.
Among them was an exceedingly rare Kemp's Ridley sea turtle that ended up on Talacre beach in North Wales -- around 4,700 miles from its home in the Gulf of Mexico.
The tale of Tally the turtle
According to the National Wildlife Federation, the Kemp's Ridley is the smallest and "most seriously endangered of the sea turtle species," and is listed internationally as being critically endangered.
Matthew Westfield, the coordinator of Marine Environmental Monitoring in the Welsh town of Cardigan, was first alerted about the turtle on November 28, just after Storm Arwen had hit its peak.
"If they get caught in a current, they can be taken into deeper water," Westfield told CNN. "What's probably happened with this one is that it's been floating around for a week or so, and then Storm Arwen hit it and then blew it onto UK waters."
Although there were suspicions the turtle might be dead, he said it was common for the creatures to go into a "cold water shock" mode, resulting in their whole system shutting down.
Westfield said the turtle is suspected to be around two or three years old, as it is the younger ones who tend to wander away from the shore into deeper waters. Its sex is still unknown, as it is difficult to tell at a young age.
The most surprising element of the discovery, however, was its rarity. Westfield said only 72 Kemp's Ridley sea turtles had ever washed up in the UK since 1748, according to records, and only 27 of those had been alive.
The turtle has now been named Tally and is in the care of Anglesey Sea Zoo in Wales, where it is under constant watch and has been placed in an incubator, with the temperature being gradually raised around two degrees each day to offset the cold shock.
"Today and yesterday, with the vet's visit, we're quietly hopeful," said Frankie Hobro, director of the zoo, adding that the cold shock had made Tally "effectively dead" when they found it. "It's going very well at the moment, because it's been very touch and go."
Although it might be the rarest, Tally certainly isn't the only marine creature to have been affected by Storm Arwen -- dozens of seal pups have also washed up on British beaches, with many of them being malnourished.
Em Mayman, the out-of-hours coordinator for the organization British Divers Marine Life Rescue, said Storm Arwen had been "particularly unkind" and had led to increased reports of washed up seal pups.
"These pups are often only a matter of days or weeks old, and have been prematurely separated from their mothers during the critical time in which they normally feed to gain necessary body fat," Mayman told CNN.
"Many of the pups have been malnourished, some under birth weight of around 13 kg (28.6 pounds), indicating that they never had a chance to feed from their mother after birth," she added. "A lucky few have been of a good weight with minimal or no injuries."
Mayman's organization has already received nearly 100 calls from around the country alerting them to rescue cases. She said 2020 broke records, with more than 2,000 calls in total -- and this year looks set to beat it.
The seal pups that are treatable are sent to rehabilitation centers where they are monitored until they gain enough weight to be released. Mayman said it could often be "very emotionally tough" on volunteers to deal with severely injured or unwell pups.
Although Westfield, who took care of Tally, said it was "too soon" to say if the cause of the animals washing up on the shores was due to climate change, Mayman disagrees.
"Extreme weather events like this storm are becoming increasingly frequent due to climate change, with the last one being in 2017 in Wales and southwest England with the loss of up to 75% of pups at some sites," she said.
And zoo director Hobro also thinks Tally won't be the last sea turtle to wash up in the UK, pointing to a similar case of a female sea turtle who was found a few years ago.
"We are having more of these late autumn, early winter storms. We know with global warming we've got the more extreme temperature changes in the oceans," Hobro told CNN. "So without a doubt, that's why we've seen more of these tropical turtle strandings over the last decade or two and I can pretty much guarantee we'll keep seeing them more regularly."