היא אומרת שאם יהיה מעונן ורק קצת רוח, היא תלבש את המעיל החדש ותצא מהבית בלי מטרייה.

שאם is a combination of two words:

• ש־ = that
• אם = if

So היא אומרת שאם... means she says that if...

This is very common in Hebrew: the conjunction ש־ attaches directly to the next word.

This is a major difference between English and Hebrew.

In Hebrew, when you talk about a real future condition, you normally use the future tense after אם:

• אם יהיה מעונן... היא תלבש...
• literally: if it will be cloudy... she will wear...

That is normal Hebrew. English usually uses present tense in the if clause, but Hebrew usually does not.

Because this is a weather expression.

Hebrew often uses יהיה + adjective for weather:

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

There is no spoken word for English it here. Hebrew just uses an impersonal form.

Also, מעונן is masculine singular because this weather pattern is normally expressed that way. מעוננת would describe a feminine noun, not the general weather.

A fully explicit version could indeed be:

אם יהיה מעונן ותהיה רק קצת רוח...

That would mean if it is cloudy and there is only a little wind...

In your sentence, the verb before רק קצת רוח is omitted because it is understood from context. This kind of shortening is fairly natural, especially in speech or lighter writing.

Also notice that if the verb is stated, it would be תהיה, because רוח is a feminine noun.

It means only a little wind or just a bit of wind.

• רק = only / just
• קצת = a little / a bit
• רוח = wind

So the idea is that the wind will be light, not strong.

Also, קצת does not change for gender or number here.

Both are future tense, third person feminine singular, matching היא.

• תלבש = she will wear / put on
• תצא = she will go out

So Hebrew does not need a pronoun before every verb, because the verb form already shows the person, number, and gender.

The sentence begins with היא אומרת and then has a fairly long if clause. After that, Hebrew often repeats the subject for clarity:

היא אומרת שאם... היא תלבש...

This is very natural. It helps the listener keep track of who will do the action.

You could sometimes omit היא, because תלבש already shows she, but repeating it here sounds clear and normal.

Because את marks a definite direct object.

Here, המעיל החדש means the new coat, which is definite, so Hebrew uses את:

• תלבש את המעיל החדש

Compare:

• תלבש מעיל חדש = she will wear a new coat
• תלבש את המעיל החדש = she will wear the new coat

So את appears when the direct object is definite.

In Hebrew, adjectives usually come after the noun.

So:

• מעיל חדש = a new coat
• המעיל החדש = the new coat

Also, when the noun is definite, the adjective must also be definite. That is why both words have ה־:

• המעיל
• החדש

The adjective also agrees with the noun in gender and number. Here both are masculine singular.

מהבית is made from:

• מ־ = from
• הבית = the house / the home

Together they become מהבית.

So:

• תצא מהבית = she will go out of the house / leave home

This is a very common contraction in Hebrew when מ־ comes before a word with ה־.

Because בלי מטרייה means without an umbrella, in a general or indefinite sense.

• בלי מטרייה = without an umbrella
• בלי המטרייה = without the umbrella

So if no specific umbrella is meant, Hebrew usually leaves out ה־.

Also, after בלי, Hebrew does not use את.

Yes, it is very common.

לצאת means to go out / to leave, and לצאת מהבית often means to leave the house or simply to go out from home.

Depending on context, it can sound very natural in everyday Hebrew, especially when talking about weather, clothes, or whether someone is taking an umbrella.