、。
When I woke up, it was snowing.
Literal
Eye [subject-が] wake-up-[and-see-てみる][when-と], snow [subject-が] was-falling.
降っていた ('was falling') is ~ている applied to 降る ('fall'), in past tense. Since 降る is an activity verb, ~ている reads as the ongoing/progressive 'was falling' rather than a resulting state. Past progressive in a subordinate-time context: the snow was already coming down when the waking happened.