Yes

you do need a jacket based on the weather in Nancy Downs, Watford WD19 4NF, UK 1 second ago

Why? Because it's not very warm out and it's on the windy side!
55°F13°C & Cloudy

Wind is 11 mph17 km/h from the South, gusting to 21 mph33 km/h
November 9
4pm55°F13°C10 mph17 km/h SouthCloudy
5pm54°F12°C10 mph16 km/h SouthCloudy
6pm54°F12°C10 mph16 km/h SouthCloudy
7pm53°F12°C10 mph17 km/h SouthCloudy
8pm53°F12°C11 mph18 km/h SouthCloudy
9pm54°F12°C12 mph19 km/h SouthDrizzle
10pm54°F12°C12 mph20 km/h SouthDrizzle
11pm54°F12°C13 mph21 km/h SouthDrizzle
November 10
12am54°F12°C13 mph21 km/h SouthDrizzle
1am54°F12°C13 mph21 km/h SouthDrizzle
2am54°F12°C13 mph21 km/h SouthDrizzle
3am54°F12°C13 mph21 km/h SouthDrizzle
DateLowHighWindDayNight
November 941°F5°C57°F14°C7 mph11 km/h from SouthMostly CloudyDrizzle
November 1048°F9°C57°F14°C10 mph17 km/h from SouthRainMostly Cloudy
November 1146°F8°C59°F15°C14 mph23 km/h from SouthDrizzleDrizzle
November 1254°F12°C59°F15°C11 mph17 km/h from SouthDrizzleCloudy
November 1354°F12°C61°F16°C6 mph9 km/h from SouthDrizzleDrizzle
November 1452°F11°C59°F15°C10 mph15 km/h from EastRainDrizzle
November 1546°F8°C53°F12°C9 mph14 km/h from EastDrizzleMostly Cloudy

Yes

Low of 47°F9°C, High of 56°F14°C
Wind: 10 mph17 km/h
Precipitation: 78% rain

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