。
When I woke up, it was snowing.
Literal
Eye [subject-が] wake-up [when-と], snow [subject-が] was-falling.
The conditional/temporal marker と sits between two clauses here ('upon waking, snow was falling'). と and たら both translate as 'when' in this kind of frame, but と gives a more immediate, automatic feel — as soon as the waking happens, the snow is there — while たら is more of a personal recount. 降っていた is the past resultative form of 降る, conveying that the snow was already coming down at the moment of waking.