מחר לא יהיה גשם, אבל בבוקר יהיה מעונן.

יהיה is the 3rd person masculine singular future form of להיות (to be).

In this sentence it is used in two slightly different but very common weather patterns:

• יהיה גשם = there will be rain / it will rain
• יהיה מעונן = it will be cloudy

So it appears twice because both parts of the sentence are making predictions about the weather.

Hebrew usually does not use a separate word for it in weather sentences.

English says:

• It will rain
• It will be cloudy

Hebrew often says literally:

• Will be rain
• Will be cloudy

The verb form itself is enough, and weather expressions are often treated as impersonal.

לא is the normal Hebrew word for not, and it usually comes before the verb or verbal phrase it negates.

So:

• יהיה = will be
• לא יהיה = will not be

That is why מחר לא יהיה גשם means tomorrow there will not be rain.

Hebrew can do both.

Two common ways to say it will rain are:

• יהיה גשם = literally there will be rain
• ירד גשם = literally rain will fall

Both are natural. In this sentence, יהיה is especially convenient because the second half also uses יהיה:

• יהיה גשם
• יהיה מעונן

That creates a nice parallel structure.

מעונן means cloudy or overcast.

It is masculine singular because weather expressions in Hebrew often use the masculine singular as a kind of default form:

• יהיה חם = it will be hot
• יהיה קר = it will be cold
• יהיה מעונן = it will be cloudy

If you used an explicit noun, the adjective would agree with that noun. For example:

• השמיים יהיו מעוננים = the skies will be cloudy

But in impersonal weather sentences, masculine singular is very common.

בבוקר means in the morning.

It is made from:

• ב־ = in
• הבוקר = the morning

When ב־ attaches to a word with ה־ (the), Hebrew combines them into one form:

• ב + הבוקר = בבוקר

So בבוקר is the normal way to say in the morning.

In this sentence, it is understood as tomorrow morning because the sentence already begins with מחר (tomorrow).

So the time frame is already set:

• מחר = tomorrow
• בבוקר = in the morning

Together, the meaning is naturally understood as tomorrow morning, even though מחר is not repeated.

Hebrew word order is flexible, and putting מחר first is very natural when giving a forecast or setting the time frame.

Starting with מחר tells the listener right away when the whole statement is about:

• מחר לא יהיה גשם... = Tomorrow, there will be no rain...

You could also say something like לא יהיה גשם מחר, but beginning with מחר sounds very natural in this kind of sentence.

גשם here means rain in a general sense, not the rain as some specific known rain.

So:

• גשם = rain
• הגשם = the rain

In weather forecasts, Hebrew usually uses גשם without the definite article when speaking generally:

• יהיה גשם = there will be rain

More or less, yes:

• מחר = tomorrow
• לא = not
• יהיה = will be
• גשם = rain
• אבל = but
• בבוקר = in the morning
• יהיה = will be
• מעונן = cloudy

A very literal translation would be:

Tomorrow not will-be rain, but in-the-morning will-be cloudy.

That is not natural English, but it helps show how Hebrew builds the sentence.

Yes. אבל is the standard everyday word for but.

So in this sentence it simply connects two contrasting weather statements:

• לא יהיה גשם = there will be no rain
• בבוקר יהיה מעונן = in the morning it will be cloudy

The contrast is: no rain, but still cloudy in the morning.