I've been trying to find a weather forcasting/charting service
that I can understand. This is the best one I've found: PassageWeather - Sailing Weather - Marine Weather Forecasts for Sailors and Adventurers
It gives you pressure, winds, and wave forecasts for 7-10 days. About as much of a advanced weather warning as you are going to get for the sail to Hawaii.
FWIW, the El Nino years tend to be wet on the Kona Coast. Doesn't seem to be much of an increase in bad weather, just more rainfall on the leeward side of the Big Island. This year has been marked by poorly formed Pacific High which has meant a more weak or non existant tradewind days. No bad weather, not that many Kona winds (southerlies) just not the dependable 15-25mph ENE winds that you can usually count on. Winds have still been mostly from the ENE but lower strength. Looking at the mid ocean weather, winds seem to be consistantly ENE but varying from force 2 to force 4 depending on the day. The high is there but not well organized which is effecting the velocity of the wind
but not the direction.
As far as local winds in the Islands. We've sailed both coasts and to Tahiti
and Back. The strongest winds and nastiest sea conditions we encountered were in the channels between the Islands here. The Alenuihaha has a reputation for being nasty and it's well deserved. It's not that it's survival conditions but gusty 30+mph winds and very steep sided waves are more the rule
than the exception. It doesn't take El Nino to make things interesting, in fact El Nino may not really have all that much effect. Winds in the channel can go from flat calm to putting you on your ear instantly when leaving the lee of an island and gusts in the channel can be fearsome. Reef early and be ready to reef further at the drop of a hat. Coming down the Alenuihaha between Maui and the Big Island is all about the wind and waves from 1,000s of miles of ocean being squeezed through a small area. They are passing between a 10,000'+ mountain on Maui and a 13,000'+ mountain here so don't have any place to go. It's the classic Bernoulli Principle and wind velocity has to increase to get that air through the pass. When I checked the other day, winds in the 'A' channel were 5 mph greater than the open ocean winds. Haven't really studied it but that is probably typical of the wind increase. It's an invigorating sail if you are going west, a slog going west. Made one crossing of the Molokai channel where we played submarine going through, not over, about every 3rd wave. That was my first passage across the channel and thought that was the norm, It's not. It's going to be close hauled in 20+ mph winds and steep 5' or so seas at most other times. Have also crossed in a flat calm, motoring the whole way, but that's very rare. In any case the sun is almost always out which makes it hard to get down on the conditions.
Every winter, we get a couple of storms that pass through from East to West. They are few and far between but could make for some uncomfortable conditions if you are approaching the islands when one of those fronts pass. It's more a case of being unlucky if you run into those conditions, however. These are not the epic storms that make for lurid press. They are probably going to be 30 mph winds with higher gusts that will last for a day or two. These storms have stalled over the Islands extending the bad weather period but that's rare and hasn't lasted much more than 3-4 days when that happens. In the 40 years I've lived on the Kona Coast, we have never had rain/overcast last for more than 3 days in a row and that has happened only 2-3 times.
I wouldn't be all that concerned about making the Passage to Hawaii in the winter. Your biggest challenge will be getting south of the low pressure systems off the Left Coast. If the Pacific High disappears completely because of El Nino, you might want to postpone your passage till it reestablishes itself. Probably won't mean more than a couple days delay. Of course, if you are out there when the High disappears, could make for some interesting conditions but doubt that they would be more than just uncomfortable.