|snow n||(ice crystal precipitation)||ثلج|
|We had 10 cm of snow yesterday.|
|تساقط 10 سنتم من الثلوج أمس.|
|snow⇒ vi||(snow: fall)||يثلج|
|Is it snowing outside yet?|
|هل ما زالت تثلج بالخارج؟|
1.) Technically "we had 10 cm of snow yesterday" doesn't necessarily mean that it all fell yesterday. It means that there was 10 cm of snow on the ground, regardless of when it snowed.
2.) I don't see a need to say الثلوج in the plural.
I suggest: كان هناك ١٠ سنتم من الثلج أمس. You could perhaps add something like في منطقتنا to convey the "we" part.
3.) The second translation is completely wrong.
Maybe هل بدأت تثلج خارجًا أم ليس بعد؟ (there may be something better)