Could it snow in Sigil...?

Well, anything the Lady wills ... I can't imagine it would be pretty snow, however; heavy flakes of smog stained sludge, staining clothes and making footing treacherous - the type of snow that counts falling on the unsuspecting among its greatest pleasures and freezing bums to death just another part of its job. (no great fanfare berk, just another bubber what got caught out late picklin' in the frost)

I would say - sure - any weather is possible given the right combination of belief, mischief, magic, xaos, and the Lady's good (or not so good) grace. The important thing would be to make it not only bizarre, but flavored some way for Sigil itself ...

I don't like to think of the Lady of Pain controlling all the weather. She believes in chaos as well as law (despite her listed alignment in the Planar Handbook) and I imagine she leaves some things to chance.

Snow would depend on what conjunctions of portals are active in Sigil and in Sigil's sky. A door to the Paraelemental Plane of Ice in the clouds would obviously create a good chance of snow, as would portals to Stygia, Cania, or Pelion (and Ysgard, the Iron Wastes in the Abyss, Mungoth in Gehenna, etc. etc.).

Even if the Lady sets in motion all of the timetables for the portals (as opposed to relying on natural planar conjunctions to determine some of this), weather is a chaotic enough system that whether or not any snow actually reached the ground would be somewhat a matter of chance, I think. The Lady might influence the weather accidentally as well, the local conditions depending on her moods and dreams.

I don't like the idea of her using weather as an agency of her wrath because her wrath already has agencies (mazing and flaying), and I think giving Her Serenity too many superpowers risks overusing her. Of course, some Cagers are likely to attribute everything to the Lady of Pain, while others will speak of her only in hushed whispers or not at all, preferring more prosaic explanations.

Really exotic weather (Pandemonic winds, Avernian fireballs, storms of paraelemental ash, ghost winds, shadowstorms, Astral thought-gales) probably make it through occasionally as well. Not too often, though; Sigil's supposed to feel gloomy and polluted most of the time, not meteorologically insane. But I think being a planar nexus should mean that Sigil occasionally gets planar weather.

I kind of like the idea that semi-regularly (say, every five to ten years or so, or perhaps every century so that they're not too prepared when it happens), Sigil gets a monsoon.

I echo this, the general weather of Sigil, smog, acid tainted rain, and so forth, is supposed to echo the chief irony of the place: that the most important nexus in the entire multiverse is actually largely a trashy dump that no one would live in by choice. This fits with the ultimate purpose of the Lady of Pain, to spread Pain and suffering throughout the multiverse.

Snow would in many ways be too harsh. The structure of Sigil means that importing large quantities of fuel for heating would be extremely difficult and so a significant and prolonged drop in air temperature below freezing would inflict considerable suffering and quite possibly death on some of the members of Sigil's poorer, cold-sensitive residents. The reverse, a prolonged heat wave, would do likewise.

Um, no, it's straight out of Pages of Pain, which, as much as perhaps some people would rather not, is a canon source.

Well, generally anything introduced in a canon source is held to stand until it is overturned by a later canon source. Thus, certain portions of the 2e planescape canon were overturned by the various 3e supplements dealing with the planes and ultimately, by this site's own 3E/3.5 PSCS (which is considered canon because planewalker was given official permission to produce it).

However, I've yet to see anything that overturns the significant components of Pages of Pain which ultimately only consist of a relative small number of things regarding the Lady (she's very much still a mystery at the end of the novel and the novel's central thesis is deliberately inconclusive). That the Lady functions to spread the pains of the multiverse is the only truly major assertion and is mechanically meaningless anyway. Troy Denning has certainly been given clearance to determine the fate of campaign worlds before: Forgotten Realms twice, Dark Sun once, and the Star Wars expanded universe repeatedly, so I would imagine considerable consultation went into the various revelations within Pages of Pain.

As far as weather the novel does support that the Lady's moods and emotions influence Sigil's weather, and I quote: "my dread wells, and claps of thunder roll through the Clerk's Ward. The strand rounds me again, and the Market Ward shudders with my trepidation."

Reasoning from such a basis one would imagine Sigil's general weather of overcast and drab vs. crummy drizzle to reflect the Lady's prevailing moods, which I would imagine as stoic and dour. More exciting weather would reflect that she had seen or experienced something that piqued her limited emotional palate.

If Pages of Pain is considered canonical, would the Blood Wars Trilogy be considered canonical as well?

I would hope not...

why do people hate the trilogy so much? i thought it was very imaginative and one of the better fantasy trilogies.

My feeling is something like this:

It rarely snows in Sigil.

My fair lady could stop it or cause it either consciously or even unconsciously but I don't think that she does the former except, e.g., in cases where she closes the whole damn shop, and that indirectly stops the flurries or whatever else.

At all events, we might as well say then that the weather in Sigil is a matter of the probability of certain events overlapping with each other.

With the disclaimer that I haven't read the series, I simply think that this seems a fairly reductive role to place such a central and enigmatic figure in Planescape. I've always imagined her as occupying the role of a metaphysical litmus test in the setting, so I've been of the opinion that her role should either be left undefined, or defined in a truly noteworthy fashion. The Lady is both seperate and central, so it makes sense that any explanation for her should be beyond the typical mythologizing of Gods. Pain is an interesting philosophical problem, but is only really of major significance philosophically from the perspective of monotheistic religions that envision a good or benevolent god. In a setting like Planescape, explaining the role of pain in the multiverse seems more suitable a role for a major diety since the problem in a pantheistic world is more one of causal consideration than it is metaphysical.

The best non-canon idea I've heard trying to summarize the role of the Lady was to sort of make her embody aspects of Nietzsche's works, specifically Thus Spoke Zarathustra (you could also throw in some Goethe and some Wagner while you are at it, to really play of the Ring angle). I think that is appropriate simply because Nietzsche's philosophy was both transcendent, in that it went beyond both deism and nihilism, and it was a bit paradoxical in its search for an immortal creative freedom in a world bounded by the prison of mortality. Is the Lady the multiverse's only Ubermensch, or a prisoner of her own nature? Is there a distinction between a thirst for creative freedom and a thirst for power? Is the soul truly immortal, or is time the ultimate tyrant? I guess I like an answer that makes you ask even bigger questions, or perhaps that gives some insight into something greater, and I feel like the Lady is a perfect vehicle for phrasing those kinds of questions.

The problem of suffering is an oldie and a goodie, I just hope that wasn't intended to be the extent of the explanation for the Lady. I haven't read the book, so I guess I can't really judge, but that was why I voiced my discontent.