Learn Hardcore Japanese: sugu ame ga hutte kuru kamo siremasen.

This can sound like you’ve already been talking about rain, or contrasting it with something else.
In a neutral prediction about the weather, 雨が is the default and most natural.

What nuance does すぐ add here? Does it mean “right now” or just “soon”?

すぐ can mean both “immediately / right away” and “soon / in a short time,” depending on context.

In this sentence, すぐ雨が降ってくるかもしれません, it’s best understood as:

“It might start to rain soon,” or
“It might start to rain any minute.”

Compare with:

すぐ行きます。 – “I’ll go right away / I’ll be there soon.”
もうすぐ雨が降ります。 – “It’s going to rain very soon.”
- もうすぐ emphasizes “almost now, very soon.”
- すぐ is also “soon,” and in this context they’re very similar, but もうすぐ can sound a bit more like the change is very close at hand.

Here, すぐ is natural and gives a sense of “in the near future, not long from now.”

Why is it 降ってくる instead of just 降る? What does ～てくる do?

降る by itself simply means “to fall” (for rain, snow, etc.).

降ってくる is 降る in the て-form plus くる (“to come”).
This ～てくる pattern often expresses:

A change or action that starts and moves toward the speaker (in space or time)
- 友だちが走ってきた。
  → “My friend came running (toward me).”
- 涙が出てきた。
  → “Tears started coming out (I started to cry).”
More abstractly, something beginning or arriving from now toward us.

With weather, 降ってくる suggests:

“The rain (will) start falling, coming here (to where we are).”

So:

雨が降るかもしれません。
→ “It might rain.” (neutral)
雨が降ってくるかもしれません。
→ “It might start raining (on us / here).”

Both are correct, but 降ってくる adds a bit of feeling that the rain is going to come in on us, start up, and arrive at our location.

What exactly does かも mean here, and how is it different from かもしれません or かもしれない?

The core expression is かもしれない, which means “might / may / possibly” and expresses uncertainty.

かもしれない = plain form (informal)
かもしれません = polite form
かも is a shortened, very casual spoken form of かもしれない.

Relationships:

雨が降るかもしれない。
→ “It might rain.” (casual)
雨が降るかもしれません。
→ “It might rain.” (polite)
雨が降るかも。
→ “Might rain.” (very casual, often with friends)

In your sentence:

すぐ雨が降ってくるかも is casual speech.
すぐ雨が降ってくるかもしれません is polite speech.

So here, かも alone has the same “maybe / might” meaning, but is more colloquial and less formal than かもしれません.

Why is it しれません and not 知りません? Aren’t both from “to know”?

They look similar, but in this expression they are not interchangeable.

The verb 知る (“to know”) does exist.
Its potential form is 知れる (“can be known”), and the 連用形 (conjunctive form) is 知れ.

Historically, かもしれない comes from:

かも
- 知れない → “(even) the possibility cannot be known for sure”
  But in modern Japanese, かもしれない / かもしれません is a set phrase that just means “might / may”.

You never say:

✕ かも知りません

It must be:

かもしれません (polite)
かもしれない (plain)

So しれません here is just the fixed form from 知れる, and you should treat かもしれません as one chunk meaning “might / may.”

Could I also say すぐ雨が降るかもしれません? What’s the difference from 降ってくるかもしれません?

Yes, you can absolutely say:

すぐ雨が降るかもしれません。
→ also “It might rain soon.”

Differences in nuance:

降る (plain):
- Neutral statement: it might rain.
- No special sense of movement towards you.
降ってくる:
- Suggests the start of the rain and its coming (arriving) here.
- A bit more vivid: “It might start raining on us / over here soon.”

In everyday conversation, both versions are common and natural; the difference is quite subtle. If you focus on the idea of “it might start any minute around here,” 降ってくる fits that feeling a bit more strongly.

How would this sentence look in very casual speech between friends?

You would typically:

Drop ます / です for plain form.
Possibly shorten かもしれない to かも.
Maybe drop the particle が as well.

Common casual versions:

すぐ雨が降ってくるかもしれない。
すぐ雨が降ってくるかも。
すぐ雨降ってくるかもね。
- Adding ね makes it sound like “Looks like it might start raining soon, huh.”

All of these are natural among friends. The original すぐ雨が降ってくるかもしれません。 is the polite version you’d use in more formal situations or with people you’re not close to.

Can すぐ go somewhere else in the sentence, like 雨がすぐ降ってくるかもしれません? Does that change the meaning?

Yes, すぐ is somewhat flexible in position, and the basic meaning stays similar, though nuance can shift slightly.

Possible orders:

すぐ雨が降ってくるかもしれません。
- Very natural.
- “Soon, it might start to rain.”
- Slight emphasis on “soon” as the setting for the whole sentence.
雨がすぐ降ってくるかもしれません。
- Also grammatical.
- Can feel a bit more like “The rain might start falling soon” with emphasis closer to “the rain will (be the thing that) soon starts falling.”
- In practice, the difference is tiny; many speakers wouldn’t feel a strong distinction.
In casual speech: すぐ雨降ってくるかも。
- Very natural.

The most textbook-like and common for this kind of statement is exactly what you have: すぐ雨が降ってくるかもしれません。

In English we say “it might rain soon.” Why is there no “it” in the Japanese sentence?

Japanese doesn’t need (and basically doesn’t have) a dummy subject like English “it” in weather sentences.

English: It rains. (dummy “it”)
Japanese: 雨が降る – literally “rain falls.”

So in すぐ雨が降ってくるかもしれません:

雨 is the actual subject (the thing that falls).
There is no separate “it” because Japanese doesn’t require a placeholder subject.

This is why you’ll often see weather sentences like:

雨が降っています。 – “It is raining.”
雪が降るかもしれません。 – “It might snow.”

Always remember: Japanese simply treats 雨 / 雪 / 風 themselves as the subjects of the verb, without any dummy “it.”

What’s the level of certainty with かもしれません? Is it like “maybe” or “probably”?

かもしれません expresses possibility with a fairly low to medium certainty. It’s closest to English “might / may / maybe”, not “probably.”

On a rough feel scale:

たぶん雨が降ります。
→ “It will probably rain.” (speaker thinks it’s quite likely)
雨が降るかもしれません。
→ “It might rain.” (speaker is unsure; it’s one real possibility)

You can even combine them (especially in speech) for nuance:

たぶん雨が降るかもしれません。
→ Sounds like “It might well rain / there’s a pretty good chance it’ll rain,”
though grammatically it’s a bit redundant.

In your sentence, かもしれません tells us the speaker is not sure; rain is just a real possibility: “It might (possibly) start raining soon.”