It rains on the average of 4-6 inches in the winter on Kauai. If you are closer to the North Shore or the center of the island, you'll see a lot more.
Kauai Weather That being said, Mount Wai
ʻale
ʻale averages 426 inches of rain a year - and the record year for rain up there was in 1982 - with 683 inches.
Mount Waialeale - Wikipedia, the free encyclopedia The temps run around 72-80 in the winter.
The neat thing about the islands is that it's warm when it rains. If you were born and raised on the mainland, you might associate rain with cold. Not so in Hawaii. Wear clothing that will dry out quickly (in other words, don't pack your good suit - unless it's a bathing suit...) And you'll love it.
Do make a point of going to Waimea Canyon. It's Hawaii's Little Grand Canyon. Breathtaking:
Hawaii's Official Tourism Site -- Waimea Canyon, Kauai