I won't get to run this adventure for some time (not even halfway through one campaign and only recently started another). That said, I too find the timeline implausible (and told WotC as much in my playtest feedback).
I know the
Sword Coast Adventurer's Guide isn't that popular, but it's one of my favorite 5e books, and it provides a handy summary of the events of the Second Sundering (the event that took the Realms from 4e to 5e). The book includes this relatively minor detail that I nevertheless have incorporated into every FR game I've run:
Throughout much of Faerûn, the winter of 1487 and 1488 lasted longer than any on record. The solstices and equinoxes had somehow drifted. Later seasons followed suit, with each starting and ending later than expected.
Rime is meant to take place in 1489 DR or later. I think, by then, people would have gotten a little more used to the delayed seasons.
So my plan is to start the adventure on what used to be midsummer. There was no spring, but people weren't too bothered by this because of the way things have been over the past few years:
"Spring hasn't sprung yet, but no matter. It was late last year, and the year before that."
However, by midsummer, there's
still been no sign of the sun, food is starting to running out and people are getting desperate. The Sundering and the Era of Upheaval are meant to have ended by now, so
something is definitely not right. It's not just that summer is delayed ... it doesn't seem to be coming at all!
Furthermore, the Spy secret mentions the Harper spy, Beldora, who poses as a homeless woman on the streets of Bryn Shander. She is given more detail in
Storm King's Thunder, as she is one of the playable NPCs for the Bryn Shander opening segment. In SKT, she possesses a
sending stone that she uses to keep in touch with a fellow Harper based in Hundelstone, at the south end of the pass into Icewind Dale. Although Rime theoretically takes place before SKT, I'd say it would make sense that Beldora would nevertheless have brought the
sending stone with her when she first went to Icewind Dale. That would mean there's at least
one person in Icewind Dale who would know that summer has arrived elsewhere (and who could pass on the message to the rest of the world that things aren't right in the dale). But she wouldn't necessarily want to blow her cover by revealing that she knows this to just anyone. She'd look for allies who could spread the news for her.
As an aside, I also intend to just have cold resistance / cold weather gear grant advantage on saves. I did that when I ran
Legacy of the Crystal Shard, and I did the reverse in the jungles of Chult with my TOA campaign. (I sure would not want to wear a suit of boiled leather in the jungle any more than I'd want to wear a suit of full plate.)