Laurefindel
Legend
I presume you mean temperature of minus 40C?Pre-Auril's Snowscapades, Icewind Dale could normally reach temperatures as low as 40℃. We also have to keep in mind that this the same setting in which the Anauroch Desert was created as the side-effect of a magical spat between the Netherese and the Phaerimm and unnaturally persists despite the surrounding environs. Also, the Icewind Dale area is strewn with the ostensibly non-magic substance black ice/chardalyn, the magical fallout of the destruction of an artifact and tends to be suffused with demonic magic (despite not being magical). It's also home to one of the fallen Netherese floating cities (and keep in mind the environmental side-effects of the Netherese magic. Then there was the time that Icewind Dale suffered from a demon-induced environmental disaster. So, with the area's history in mind, perhaps the flora and fauna have had enough exposed to weird magics that they've become unnaturally hardy and adaptive. Perhaps, also, Auril's temper tantrum is being tempered (yes, that was intentional) slightly by a subconscious rational thought about not wanting to destroy her worshipers (and thus the flora and fauna that they depend upon) in her Apokalyptischer Winter des Schicksals (sorry, more Google German).
Sure, this is pure apologia (as I'm trying to work more or less within the framework that we're given). However, I think that, within the context of the rest of the Forgotten Realms, this is one of the lesser eyebrow-raising and questionable occurrences to grace the setting (and that's before considering the Spell Plague and accompanying shenanigans that happened to the Realms during 4e). Of course, one's tolerance of Realmsian BS-ery may vary.
That’s fine, that’d be like northern Russia, Canada, and Scandinavian countries. All of these countries expérience temperature reaching that low, but are far from an average temperature of -45C.
An average below -25C is rare for these countries even in the deepest months of winter. Nevermind an average of -45, which would assume regular spikes of -60C. With the wind factor, that could go down to an equivalent of -80 and worse, which is nearing the temperatures at the equator on Mars.
unless the -45C already include the wind? Anyhow. In itself the temperatures are appropriately apocalyptic; I’d had expected more migration from 24 months of it.
As for the wacky FR-apocalypse of the month; yeah, I’m right there with you...