Hawaii weather is predominantly influenced by the trade winds which in the north Pacific hemisphere brings winds blowing out of the north east onto the Hawaiian island chain, about 85% of the time. This weather pattern caused by the turning of the Earth and the Coriolis Effect is so called because it’s these winds that make easy sailing from the mainland and it’s all down wind sailing for trading ships coming from California to Hawaii. Trade winds regularly blow around 20 knots and very often up to 25 knots. Winds in Hawaii are also significantly affected by the terrain with winds accelerating in the channels and around tall land masses. In the lee of the tallest mountain ridges, such as the Waianae mountain range on O’ahu, the trade winds are often shadowed and create a typical lull area. Other areas in the lee of the islands, such as between Maui and Lana’i the wind can either be very light or very windy depending on the precise strength and direction of the trade winds that day.
To get from Hawaii to California ships have to go north first to get around the mostly stationary north Pacific high pressure area. This high pressure area moves somewhat throughout the year and causes relatively stronger trade winds in the summer months. Hawaii generally has two seasons: summer and winter. While the winter months might bring lighter trade winds, the ocean water temperature is cooler and frontal systems form and extend across the Hawaiian islands which brings a south and west wind weather pattern that people in Hawaii call Kona winds.
When the trade winds are light and Kona winds aren’t present, Hawaiian coastal waters can experience local thermal wind patterns. Especially in the areas that are in the leeward wind shadows of the high mountain ridges, such as Oahu’s Waianae coast, the Big Island’s Kohala coast and in the lee of Maui’s Haleakala, normal trade winds create consistent areas of calm where the only winds come from the difference in changing temperatures between the land and sea throughout the day. So with normal trade winds if you’re planning to sail the west coast of Oahu or the Kona coast of the Big Island you might only have a slight on shore wind in the late afternoon or offshore wind in the early morning.
The seas in Hawaii are made up of wind waves, ocean swells and currents. The wind waves depend on the speed, direction and duration of the wind and can lag the timing of the changing wind conditions. If the winds have been blowing consistently for some time wind waves can build up to serious strength and become a challenge for mariners in Hawaii. Combined with ocean swells that are caused by offshore factors such as far away storms or even tsunamis thousands of miles away, it can create the kind of surf that Hawaii is really known for. Just as the windward sides of the islands get the most wind, the windward sides also can get the biggest seas so generally most of the harbors and best anchorages in Hawaii are in areas that are protected from the windward seas. There are some exceptions though as places like Kaneohe Bay on the windward side of Oahu is also protected by a large reef.
Ocean swells in Hawaii tend to be somewhat seasonal. In the winter months, frequent North Pacific storms create ocean swells that bring Hawaii’s famous north shore surf. In the summer months ocean conditions south of Hawaii often can create bigger south swells, especially when tropical cyclones track south of Hawaii. It’s important to check the swell forecasts, even if if the winds are calm if you are considering anchoring in areas exposed to south swells.
The main ocean current that flows through Hawaii flows with the trade winds at about 1/2 knot and can significantly affect rate of travel through the islands. Currents are more consistent further away from shore. The current tends to accelerate and create reverse or eddy currents in some places near shore and at the points of land, such as Laau pt southwest of Molokai. The area off Molokai that extends southwest into the Kaiwi channel is a sea shelf known as Penguin Bank, where the currents tend to accelerate and seas can get confused in the area where sea depth changes. Hawaii does not experience very drastic tides so tidal current is generally not much of a significant factor.
The official source of weather forecasts in the US is NOAA / National Weather Service and the Hawaiian Coastal Waters Forecast, published four times daily (every six hours) at around 0330, 0930, 1530 and 2130 is the official word. A version of this is read continually on VHF weather channels and is also a good thing to check regularly while you’re out on the water. If you want to dig deeper and think like a meteorologist, the National Weather Service, Forecast Office Honolulu, HI has links to all of the weather analysis products. If you just want a quick and easy visual weather report, everyone has their favorite weather app but it seems the Windy app is most popular in Hawaii, which presents GriB File weather data into an easy to visualize picture of the weather. Be aware though that apps like Windy don’t necessarily take full account of the terrain and funneling of winds in channels and it’s wise to consider wind gusts as what you’ll always experience as consistent wind. A prudent mariner always checks the weather before they go out.