Some habits don't help when it's cold. Actually, some make it worse.

• Don't leave your beans in the freezer or near a cold window. Cold beans can cause static, clumping, and uneven grind—none of that helps.
• Skip the pre-ground coffee. In winter, pre-ground goes stale faster because of condensation when you open the bag. Grind right before you brew.
• Leave the fancy brew ratios alone for a day. Stop changing three variables at once. If you're also playing with dose, yield, and grind, you'll never figure out what's wrong.
• Don't crank the grinder finer just because it's cold. That's a common mistake. If the shot's bitter, it might be under-extracted, so going finer just makes it worse.