Yes

you do need a jacket based on the weather in Seattle, WA, USA 1 second ago

Why? Because the cold's there, but it's just a reminder, not an invasion!
38°F3°C & Cloudy

Wind is 3 mph5 km/h from the South, gusting to 4 mph7 km/h
December 13
10am38°F3°C3 mph5 km/h SouthCloudy
11am38°F3°C3 mph4 km/h SoutheastCloudy
12pm39°F4°C2 mph4 km/h SoutheastCloudy
1pm41°F5°C3 mph5 km/h SoutheastCloudy
2pm43°F6°C4 mph7 km/h SoutheastCloudy
3pm44°F7°C4 mph7 km/h SoutheastDrizzle
4pm46°F8°C4 mph7 km/h SoutheastDrizzle
5pm46°F8°C5 mph9 km/h SoutheastDrizzle
6pm46°F8°C6 mph10 km/h SoutheastDrizzle
7pm45°F7°C7 mph11 km/h SoutheastDrizzle
8pm45°F7°C7 mph11 km/h SoutheastDrizzle
9pm45°F7°C8 mph12 km/h SoutheastRain
DateLowHighWindDayNight
December 1338°F3°C46°F8°C5 mph8 km/h from SoutheastRainRain
December 1440°F4°C48°F9°C16 mph26 km/h from SouthWindyMostly Cloudy
December 1538°F3°C46°F8°C6 mph9 km/h from SouthMostly CloudyRain
December 1639°F4°C44°F7°C4 mph6 km/h from SoutheastRainDrizzle
December 1741°F5°C49°F10°C6 mph9 km/h from SouthDrizzleDrizzle
December 1843°F6°C51°F11°C9 mph15 km/h from SouthMostly CloudyMostly Cloudy
December 1941°F5°C49°F9°C2 mph4 km/h from NortheastMostly CloudyMostly Cloudy

Yes

Low of 40°F5°C, High of 48°F9°C
Wind: 16 mph26 km/h
Precipitation: None

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