Learn Hardcore Japanese: kyou ha kanari samui ga, yuki ni naru wake de ha nai. - It’s quite cold today, but it’s not like it will snow.

It is the conjunctive が meaning "but/although," linking a contrast between the two clauses: 今日はかなり寒い (it’s quite cold) vs. 雪になるわけではない (that doesn’t mean it will turn to snow). It is not the subject-marking が.

Why is は after 今日, and why is it pronounced "wa"?

は marks the topic (今日は = as for today). In this use, the particle は is pronounced "wa" even though it is written with the kana for "ha." This is just a convention of the topic particle.

What exactly does わけではない mean?

It means "it’s not the case that..." or "not necessarily." It denies an inference or an overgeneralization rather than stating an absolute impossibility. Here: even though it’s quite cold, that doesn’t automatically mean it will become snow.

What are the parts inside わけではない? Why では?

わけ = "reason; conclusion; the upshot," used to nominalize the preceding idea.
ではない is the negative of the copula だ: だ → ではない (with は adding a contrastive feel). Variants:
わけでない (possible; a bit lighter, less contrastive)
わけじゃない (casual)
わけではありません (polite)

Why is に after 雪 in 雪になる?

With なる, the result state is marked by に: "become X" = X になる. So 雪になる = "become snow / turn to snow."

What’s the nuance of 雪になる vs. 雪が降る?

雪が降る = "it snows / snow falls" (describes the act of snowfall).
雪になる = "it will be (or turn into) snow" (often used when talking about the type of precipitation or a change, e.g., rain turning to snow). Weather forecasts commonly use 〜になる for precipitation type.

Would 雪は降らない mean the same thing?

Not exactly.

雪は降らない = "It won’t snow" (plain denial of snowfall).
雪になるわけではない = "It’s not the case that it will (necessarily) become snow" (softer; denies a conclusion drawn from the cold).
雪にはならない (see next Q) emphasizes "won’t become snow (as opposed to some other form)."

How does 雪にはならない differ from 雪になるわけではない?

雪にはならない uses contrastive は after に to stress "as for becoming snow, that won’t happen" (stronger, more categorical).
雪になるわけではない is hedging: it rejects the inference but doesn’t claim absolute impossibility.

Why isn’t なる in the negative? Shouldn’t it be ならない?

The negation is handled by わけではない, which negates the entire proposition 雪になる. If you said 雪にならないわけではない, that would be a double negative: "It’s not that it won’t become snow" (i.e., it might).

How strong is わけがない compared to わけではない?

わけではない = "not necessarily; not the case that..." (soft denial).
わけがない = "there’s no way; impossible" (strong categorical denial). So 雪になるわけがない = "There’s absolutely no way it will turn to snow."

Can I replace が with けど / けれど(も) / でも / しかし?

Yes, with register differences:

けど / けれど(も): more casual/neutral (けれども is a bit more formal).
が: neutral to slightly formal/written.
でも: starts a new sentence; feels conversational.
しかし: formal/written. Example: 今日はかなり寒いけど、雪になるわけではない。 / 今日はかなり寒い。でも、雪になるわけではない。

How does かなり compare to とても / けっこう / すごく?

かなり: considerably, quite (somewhat objective, steady emphasis).
とても: very (strong, neutral).
けっこう: fairly/quite, often "more than expected."
すごく: very (colloquial, emotive). All can fit, but かなり sounds matter-of-fact and fits forecasts or objective statements well.

Who or what is the subject of 寒い here?

Japanese doesn’t need an explicit subject for weather. 今日は sets "today" as the topic; 寒い is simply "is cold (today/the weather)." There’s no dummy "it" like in English.

Are spaces normally written between words like that?

No. The normal writing is: 今日はかなり寒いが、雪になるわけではない。 The spaced version is for learners to see word boundaries.

How can I make this more polite or forecast-like?

Polite: 今日はかなり寒いですが、雪になるわけではありません。
Forecast/guess: 今日はかなり寒いですが、雪にはならないでしょう。 Casual: 今日はかなり寒いけど、雪になるわけじゃない。