tenki wo yosousita.

を marks the direct object of the verb. Here, 天気 is the thing being predicted, so 天気を予想した means something like predicted the weather.

In very simple terms:

• 天気 = weather
• を = marks what the action is done to
• 予想した = predicted

Because 天気 is the object of 予想した, not the topic of the sentence.

• 天気を予想した = predicted the weather
• 天気は... would make weather the topic, and the sentence would usually continue in a different way

For example:

• 天気はどうなるかな。 = As for the weather, I wonder what will happen.

In your sentence, the speaker is talking about the action of predicting, so を is the natural choice.

予想した is the plain past form of 予想する.

Breakdown:

• 予想 = prediction / expectation
• する = to do
• 予想する = to predict
• 予想した = predicted

So this sentence is in the past tense.

Grammatically, it is a suru-verb:

• 予想 is a noun meaning prediction
• add する and it becomes the verb to predict

This is very common in Japanese. Other examples:

• 勉強する = to study
• 運動する = to exercise
• 予想する = to predict

So 予想した is the past form of that suru-verb.

Japanese often leaves out the subject when it is understood from context.

So 天気を予想した。 could mean:

• I predicted the weather.
• He predicted the weather.
• She predicted the weather.
• They predicted the weather.

The exact subject depends on the situation. This is very normal in Japanese.

Because Japanese usually follows subject-object-verb order, and the main verb typically comes at the end of the sentence.

So:

• 天気 = weather
• を = object marker
• 予想した = predicted

Literally, it is closer to weather predicted than to natural English word order.

It is read:

てんきを よそうした。

More specifically:

• 天気 = てんき
• 予想した = よそうした

So the full sentence is:

てんきを よそうした。

Yes. 予想した is the plain past form, so it sounds casual or neutral.

A more polite version would be:

天気を予想しました。

That would be appropriate in more formal situations.

Not exactly.

予想する means to predict / to expect / to guess.
予報する means to forecast, especially in a more official weather-forecast sense.

So:

• 天気を予想した = predicted the weather
• 天気を予報した = forecast the weather

Your sentence is grammatically fine, but depending on context, 予報した might sound more natural if you mean a formal weather forecast.

In this sentence, 天気 most naturally means the weather.

It can refer generally to weather conditions, such as whether it will be sunny, rainy, and so on. So 天気を予想した suggests predicting what the weather would be like.