This forum has many posts that say “It rains more on the North Shore than in Poipu in the winter.”
There’s no doubt that’s a true statement. There are lots of statistics to prove it - total rainfall by month, and daily tabulations of rain-gauges.
I know that most rain in the Hawaiian Islands comes in the form of “Trade Wind Showers” which lead to the ubiquitous forecast of “Night and Morning Showers” on the ten o’clock news, but don’t often ruin your day. I also know that much of the rain in the winter comes in the form of winter storms that often make for heavy rains all over all the islands during their passage. On the days when those storms are over Kauai or Oahu it usually rains all over the island.
I’m an Engineer by training and experience, so that makes me want to gather data, and I did so this last month. There is a web site (http://tinyurl.com/2gy2hbu) with four real-time webcams – in Hanalei, Kapaa, Lihue, and Poipu – that clearly show (after a little practice) whether it is raining, cloudy, or sunny in each location. I recorded those webcams daily in December 2010. Then I tabulated the results.
The resulting video is up on YouTube, so you can see for yourself, but the bottom line is that for the 25 days that I recorded (I missed a few days over the holidays) there were only two more instances of rain in Hanalei than in Poipu (6 vs 4). Here’s the link (click the icon at the lower right corner to watch in full screen). The tally is shown at the end, so at any time you can stop the player, slide the Progress Bar to the right, and resume, to see the tally.