。
When I woke up, it was snowing.
Literal
Eye [subject-が] awakened-[if/when-たら] snow [subject-が] was-falling.
The たら conditional here ('when I woke up') gives a more specific/personal narrative feel than と would — a recount of what happened on this one occasion rather than a habitual pattern. Paired with the past progressive 降っていた, the scene is set: you open your eyes, and snow is already coming down.