Yes

you do need a jacket based on the weather in San Dimas, CA, USA 1 second ago

Why? Because it's cool. Pleasantly cool, but cool.
52°F11°C & Mostly Cloudy

Wind is 2 mph4 km/h from the W, gusting to 6 mph10 km/h
April 26
11am52°F11°C3 mph5 km/h SWCloudy
12pm53°F12°C5 mph7 km/h SWMostly Cloudy
1pm55°F13°C5 mph8 km/h SWMostly Cloudy
2pm57°F14°C7 mph11 km/h SWMostly Cloudy
3pm60°F15°C9 mph15 km/h SWPartly Cloudy
4pm63°F17°C12 mph19 km/h SWPartly Cloudy
5pm65°F18°C13 mph21 km/h SWPartly Cloudy
6pm65°F19°C13 mph22 km/h SWPartly Cloudy
7pm66°F19°C12 mph20 km/h SWMostly Clear
8pm67°F19°C11 mph18 km/h SWMostly Clear
9pm66°F19°C9 mph15 km/h SWMostly Clear
10pm65°F18°C7 mph12 km/h SWMostly Clear
DateLowHighWindDayNight
April 2651°F10°C67°F19°C9 mph14 km/h from SWPartly CloudyMostly Clear
April 2747°F9°C69°F21°C5 mph9 km/h from SWMostly ClearClear
April 2850°F10°C74°F24°C5 mph8 km/h from SWClearClear
April 2953°F12°C78°F26°C4 mph7 km/h from SWClearClear
April 3054°F12°C78°F25°C5 mph8 km/h from SWClearPartly Cloudy
May 154°F12°C73°F23°C5 mph7 km/h from SWMostly ClearMostly Clear
May 253°F12°C73°F23°C5 mph8 km/h from SWMostly ClearPartly Cloudy

Yes

Low of 47°F9°C, High of 70°F21°C
Wind: 5 mph9 km/h
Precipitation: None

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