Will I be cold?
Will I be cold in Seattle in December?
Verdict
Yes — Yes, cold.
Lows drop to 37°F on average — noticeably cold, especially after sundown.
Day vs. night
Day
46°F
Cool
Sweater weather with the option to add a light shell.
Night
37°F
Cold
Cold enough that a hat and gloves become mandatory.
Why
Seattle in December averages highs of 46°F, lows of 37°F, 17 rainy days.
Plan for real winter kit — an insulated coat, warm hat, and gloves rather than a light jacket.