Why don't we see many thunderstorms during the winter months?
Thunderstorms develop either when there's strong 'shear' (a strong increase in winds with altitutde or a sizable change in wind direction) or when there's a sharp decrease in temperature with altitude. The first condition certainly can still occur during the winter (strong shear), but the second demands fairly warm temperatures near the surface and much colder temperatures aloft. The problem is that we typically don't see surface temperatures warm enough to 'trigger' the rising air that produces thunderstorms. Because we're also 'downwind' from the Pacific ocean, which moderates air temperatures, the rate of cooling with increasing altitude that's needed to cause the instability rarely develops.That's why thunderstorms are less common during the winter months here in the Pacific Northwest.