Will I be cold?
Will I be cold in Washington DC in December?
Verdict
Yes — Yes, cold.
Lows drop to 32°F on average — noticeably cold, especially after sundown.
Day vs. night
Day
47°F
Cool
Sweater weather with the option to add a light shell.
Night
32°F
Cold
Cold enough that a hat and gloves become mandatory.
Why
Washington DC in December averages highs of 47°F, lows of 32°F, 9 rainy days.
Plan for real winter kit — an insulated coat, warm hat, and gloves rather than a light jacket.