tiānqìyùbào shuō huì xiàyǔ.

It breaks down like this:

• 天气预报 = the weather forecast
• 说 = says
• 会下雨 = will rain / is going to rain

So the structure is:

[noun] + 说 + [clause]

Very literally, it is The weather forecast says [it will rain].

Chinese often uses 说 to introduce reported information, much like English say.

In Chinese, 说 is often used more broadly than just for a person speaking out loud. It can also be used for things like:

• reports
• signs
• notices
• forecasts
• messages

So 天气预报说…… is perfectly natural and works like English the weather forecast says...

A slightly more formal alternative would be 据天气预报 = according to the weather forecast.

Here, 会 is a modal verb showing a predicted future event. In this sentence, it means something like:

• will
• is expected to
• is likely to

So 会下雨 means it will rain.

This is not the 会 that means can / know how to in sentences like 我会说中文.

Chinese does not use a dummy subject like English it in weather expressions.

English says:

• It is raining
• It will rain

Chinese just says:

• 下雨
• 会下雨

So there is no need for a word corresponding to English it.

This is very common in Mandarin with weather verbs, for example:

• 下雪 = to snow
• 刮风 = to be windy / the wind blows

下雨 is best learned as a fixed weather expression meaning to rain.

Historically and literally, 下 has the idea of falling downward, so 下雨 is roughly rain falls. But for learners, the easiest approach is to treat 下雨 as a single verb phrase meaning to rain.

The same pattern appears in:

• 下雪 = to snow
• 下冰雹 = to hail

So in this sentence, 会下雨 is simply will rain.

You may hear shortened expressions like that in very casual or compressed speech, but 天气预报说会下雨 is more standard and clearer.

Why? Because 会 marks the prediction clearly. Without it, the sentence can sound a bit clipped.

So for a learner, 天气预报说会下雨 is the safer and more natural choice.

Because then 会 would attach to 天气预报 as the subject, making the sentence sound like the weather forecast will rain, which does not make sense.

The correct structure is:

• 天气预报 = subject
• 说 = main verb
• 会下雨 = what it says

So the sentence must keep 说 to introduce the content clause.

It is a compound noun made from:

• 天气 = weather
• 预报 = forecast

Together, 天气预报 means weather forecast.

So yes, it is built from two parts, but it functions as one noun phrase in the sentence.

This is a very common question.

• 会下雨 = it will rain / it is expected to rain
  This is a neutral prediction.

• 要下雨了 = it is about to rain
  This usually suggests the signs are already visible and the rain seems imminent.

So in a forecast sentence, 会下雨 is the normal choice. 要下雨了 sounds more like someone looking at the sky and saying rain is coming soon.

The pronunciation is:

tiānqì yùbào shuō huì xiàyǔ

Tones:

• 天 tiān — 1st tone
• 气 qì — 4th tone
• 预 yù — 4th tone
• 报 bào — 4th tone
• 说 shuō — 1st tone
• 会 huì — 4th tone
• 下 xià — 4th tone
• 雨 yǔ — 3rd tone

A natural rhythm is:

天气预报 / 说 / 会下雨。

You can make a small pause after 天气预报, because that whole part is the subject.