Are you looking for local forecasts or passage
forecasts? If local, windguru does good ones based on the GFS model and the app is fairly tolerant of low bandwidth connections. Most apps will give forecasts only based on computer models. For something that's been analysed by a meteorologist I don't know any apps, but you can can get nadi text forecasts as well as request grib files through saildocs. Take a look at their website for details. Its a little tricky to use but really really versatile once you get the hang of it.
In the Caribbean
I would usually just use windguru for quick forecasts and predictwind for hopping between the islands.