Learn Russian: Прогноз был хороший: был снег, но ей было тепло. - The forecast was good: there was snow, but it was warm for her.

прогноз (forecast) and снег (snow) are both masculine nouns, so they use the masculine past tense был. However, the last phrase ей было тепло is an impersonal expression meaning "it was warm to her". Because there is no grammatical subject here, Russian defaults to the neuter past tense было.

Why do we use ей instead of она here?

In English, you say "she was warm" using a regular subject. In Russian, physical states like being warm, cold, or sad are expressed impersonally. You literally say "to her it was warm". ей is the Dative case of она, showing who is receiving or experiencing the feeling.

Could I just say она была тёплая to mean "she was warm"?

No, that would mean something completely different! Saying она была тёплая means her physical body was warm to the touch, like a heated blanket. To describe how a person feels in their environment, you must use the impersonal structure ей было тепло.

In был снег, does был just mean "there was"?

Yes! The verb быть (to be) is often used to show existence in the past. By putting был before the subject снег, the phrase naturally translates to "there was snow".