"It rains cats and dogs"
"it never rains but it pours"
"to be under the weather"
"Every cloud has it's silver lining"
"Don't rain on my parade"
Less frequently used:
“Red sky at night, sailor’s delight. Red sky in the morning, sailor, take warning.”
“A year of snow, a year of plenty.”
“Halo around the sun or moon, rain or snow soon.”
“A wind from the south has rain in its mouth.”
“Seagull, seagull, sit on the sand. It’s never good weather when you’re on the land.”
"In the morning mountains, in the afternoon fountains"
"Cold is the night when the stars shine bright"