Learn Spanish (Latin America): Como nieva tanto, es hoy cuando esquiamos. - Since it is snowing so much, it is today when we ski.

No, you can't. When como is used to mean "since" or "because" (giving a reason), it must always go at the very beginning of the sentence. If you want to state the action first and the reason second, you need to use a different word like porque.

Could I start the sentence with porque instead of como?

Generally, no. In Spanish, it sounds unnatural to start a sentence with porque to give a reason. Como is the standard word used when you want to put the "because/since" clause at the beginning of your sentence.

Why use the structure es hoy cuando esquiamos instead of just saying hoy esquiamos?

This structure is used for emphasis. Just like saying "it is today that we ski" in English, using es [time] cuando... places special focus on the word hoy. It highlights that today is the specific day you are going skiing, likely because the heavy snow makes it the perfect opportunity.

Why is the simple present nieva translated as the continuous "it is snowing"? Shouldn't it be está nevando?