Yes

you do need a jacket based on the weather in Eryri National Park (Snowdonia), United Kingdom 1 second ago

Why? Because it's raining in your area and just cool enough for a jacket, but not warm enough to ditch it!
51°F11°C & Rain

Wind is 6 mph9 km/h from the Southwest, gusting to 11 mph18 km/h
September 8
3am52°F11°C6 mph10 km/h SouthwestMostly Cloudy
4am53°F12°C8 mph14 km/h SouthwestMostly Cloudy
5am55°F13°C10 mph15 km/h SouthwestMostly Cloudy
6am56°F13°C10 mph17 km/h SouthwestMostly Cloudy
7am57°F14°C11 mph17 km/h SouthwestMostly Cloudy
8am58°F15°C11 mph18 km/h SouthwestMostly Cloudy
9am58°F14°C11 mph18 km/h SouthwestDrizzle
10am58°F14°C11 mph18 km/h SouthwestDrizzle
11am58°F14°C11 mph18 km/h SouthwestDrizzle
12pm58°F14°C10 mph17 km/h SouthwestPartly Cloudy
1pm56°F14°C9 mph14 km/h SouthwestDrizzle
2pm55°F13°C9 mph14 km/h SouthPartly Cloudy
DateLowHighWindDayNight
September 751°F11°C58°F15°C10 mph16 km/h from SouthwestDrizzleDrizzle
September 850°F10°C62°F17°C10 mph16 km/h from SouthPartly CloudyMostly Cloudy
September 951°F11°C59°F15°C10 mph16 km/h from SouthRainRain
September 1050°F10°C55°F13°C14 mph22 km/h from SouthwestRainRain
September 1148°F9°C55°F13°C13 mph20 km/h from SouthwestDrizzleRain
September 1249°F9°C56°F14°C12 mph19 km/h from SouthwestRainRain
September 1352°F11°C61°F16°C15 mph25 km/h from SouthwestRainRain

Yes

Low of 50°F10°C, High of 62°F17°C
Wind: 10 mph16 km/h
Precipitation: 24% rain

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