A cold storm is expected to bring rain to Diamond Bar and Walnut on Wednesday and snow in the mountains north of the San Gabriel Valley.
The rain chance is 30 percent Wednesday and 90 percent Wednesday night, according to the National Weather Service.
A cold low pressure system from the Gulf of Alaska is expected to produce wintry weather in the mountains Wednesday evening through most of Thursday, the Weather Service advised in an update Tuesday afternoon.
"The system will bring widespread rain . . . mountain snow . . . and gusty onshore winds in the mountains and deserts," the Weather Service advised. "The brunt of precipitation is expected Wednesday night into Thursday morning. Very cold air aloft may cause isolated thunderstorms to form during the day Thursday."
Daytime highs Wednesday for Diamond Bar and Walnut are forecast around 60, with Wednesday night lows around 50. Daytime Thursday highs are expected in the mid 50s. Thursday night is expected colder, with an overnight low around 41. Daytime highs Friday could peak in the mid 50s.
In Mount Baldy Village, the daytime high Wednesday is forecast around 50, with Wednesday night lows of 34. Daytime highs Thursday are forecast around 42 and Thursday night lows could dip to 30, according to the Weather Service. Friday is expected a bit clearer and the daytime high in Mount Baldy Village is forecast around 44.