Although this information is factual, it is flavored with my own judgments and personal observations. Therefore my comments may not totally match the weather service forecast, or other sources of information you may find.
This document is written with a "Scuba Divers" state of mind - that is, weather and sea states are discussed in terms of diving conditions, rather than land conditions. An example would be the summer trade winds, which tame the hot summer sun and make the humidity bearable on land, but will also create aggressive wind chop and make boat diving less comfortable.
Generally speaking, Hawaii does not have the four typical seasons. I think of the weather on Maui as either Summer or Winter. There is only about a 10 degree (Fahrenheit) difference between the hottest, and the coolest time of year (80 - 90 deg.) The main difference I see between the seasons is that the summer has more wind, and the winter has more rain.
Additionally, I should mention that there are "macro" weather patterns located throughout Maui. The windward side has the most rainfall and cloud cover, while at the same time the leeward side tends to be sunny and hot. As you drive around the island you can experience the difference between sunshine and a tropical downpour in just a few miles.
I am based on Maui Island, and my local knowledge is also Maui based. Although my expressions are true, for the most part, for the other Hawaiian Islands, I can only state that they are accurate (within my perceptions) for Maui.
Summer
The summer "half" of the year begins about May and ends about October. Summer is ushered in by the return of the Trade Winds. Maui is considered to be one of the World's top Windsurfing destinations. I bring this up because it emphasizes the relentless nature of the winds. The trades will blow almost every day during the Summer, dying at night, and then building in intensity during the early part of the next day.
Fortunately, Maui is a high island. Haleakala Crater is over 10,000 feet of wind block! Haleakala, and the West Maui Mountains divert and block the winds, creating a wind shadow along the leeward coastline. Of importance to divers is that this pocket of calm also includes some of the best dive sites!
The mountains are not enough to stop the wind completely, and in the late morning or early afternoon the trades will have built in intensity to the point where they will eventually break through. Lahaina is the only tourist area where the winds can be light throughout the day, but this is not where the best diving is found! The major dive areas are Lanai Island, and the sites adjacent Kihei & Wailea (South Maui), which include Molokini Crater.
The trade winds create a phenomenon that sometimes makes divers on vacation gasp! The departure time for most boats is around 7 A.M.! This is earlier than I have seen in most other dive destinations, but there is an advantage to leaving this early - Mainly, the major part of your diving will take place during the calm, before the winds pick up.
Many of my comments may seem to express a negative feeling - NOT SO! The truth is, I do not get seasick - but I see enough customers hanging over the side to know that I am not a good judge of how sea conditions will affect you. Therefore, I do not want to "gloss over" and play down the sea conditions in Hawaii. I am stating them as I see them. I know that once you get underwater and view our living reef, you will barely remember the state of the surface above.
Summer Statistics
- Water temperature: Climbs from mid 70s and reaches low 80s by August.
- Water visibility: 100'-150' offshore and 50'-100' inshore.
- Storms: Not really a concern, any that do form tend to be short and not very intense.
Winter
I personally prefer the Winter half of the year for diving on Maui. There are definite changes in the wind which make more dive sites available, than are available during the Summer. But the Winter brings in other changes in the weather also.
I consider the winter half of the year as falling between October and May. One of the things I think of in winter is snow, and you might be surprised to find out that Maui does get snow! Of course only at the top of Haleakala Crater (10,025 feet), and only a few weeks out of the year. It is a beautiful sight to view Haleakala with a shinny white snow cap as you lay on the beach baking in the warm Maui sun.
October is considered to be the beginning of the Kona season, which means the winds can shift to a southerly direction. The southerly winds are called Kona winds and strike the normally leeward side of the island, causing high surf and rain. Most years there are only two or three major Kona storms, which may last up to two weeks. During this time, many of the popular dive sites are not available, due to adverse wind and sea conditions. But please read on because it is not as bad as it seems.
I prefer the winter because of the "calm" between the storms. We can sometimes experience total calm for two or three weeks at a time. I am talking glassy, flat water! No wind. No restrictions on where we can take our boats! We have access to the southern end of Maui, and are able to dive remote sites that are inaccessible during the Summer half of the year.
As winter season grows, the water temperature does drop to the low 70s. This is definitely wetsuit temperature for almost every diver. Added to cooler water, is an increase in cloud cover, which makes it harder for divers to warm up between dives. So, if you get cold, plan on bringing your 4,5 or 6mm wetsuit. By the way - there is no thermocline so the surface water is pretty much the same temperature as what you will find at sixty or eighty feet.
Did I mention whales? January - April is Humpback Whale season! Whale shows on the way to the dive site, whale songs during the dives, and the occasional glimpse of a behemoth underwater, if you are lucky. Note - it is illegal to swim with whales, they must come to you!
In recap - I want you to understand that although I have told you we have storms during the winter, I still feel this is a desirable time of year to plan a dive trip. There is some gamble that you may be in Hawaii during a storm, but the chances are highly in favor of having excellent diving conditions. In perspective, we usually only have to cancel 4 or 5 trips each year, due to weather.
Winter Statistics:
- Water Temperature: Mid to low 70s - definite wetsuit temperature for most divers
- Water Visibility: 100' - 150' offshore, 50' - 100' inshore (may be reduced drastically during storms)
- Storms: Usually two or three storms lasting one to two weeks. They can be wet.