This. There may be weather conditions present or seasonal changes in effect, totally different than what would be natural, within the domains. I think there is one domain where it is constantly winter, with soem werewolf. But it is one of the older modules, form 2e or 1e.
Here's how "The Beast" (DDAL04-02) describes the first big storm of winter in Barovia:
A massive storm front has been building for the last several days, and as the characters enter the cave the sky breaks loose—in addition to the heavy winds, a thick freezing rain begins to fall in torrential bursts, interwoven with inch-wide hailstones. Staying outside will almost certainly bring death to whomever is foolish enough to do so. (p. 21)
The storm causes the temperature to drop very quickly. Characters outside the wolf den take 1d4 points of bludgeoning and cold damage at the start of each of their turns due to the hail and freezing rain; characters wearing cold weather gear take half of this damage. (p. 21)
Once the storm hits, at the beginning of each encounter each character must succeed on a DC 9 Constitution saving throw or gain a level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold environments. (p. 11)
Also (as mentioned above), a major plot point of the entire AL season is about how the village of Orașnou may not have enough food to get it through the winter. A later adventure, "The Broken One" (DDAL04-08), mentions that a field of turnips was caught in an early freeze and cannot be harvested.
I'd have to re-read it but I think the Black Box and Domains of Dread Entries describes the spring and the summer, and mentions long winters (though I suppose it suggests more about the winter by saying the snow melts in the warmer seasons and it starts to get green in the spring). Also I think they mention that the snow caps remain year round. I might be misremembering. I got the impression winters would be harsh and there was a lot of alpine climate going on
EDIT: also if you look at the old boxed set map (the realm of terror boxed set) the map shows the elevation levels so that also might give you an indication of things like weather.