Without reading all the previous posts, life is too short, I think that you will do no better than use the NWS (NOAA) GRIB files - the GFS. Since the enhancement last year the GFS now gives good guidance up to 4 or 5 days ahead. Along with the UKMO, this is the best operational numerical weather prediction model. The ECMWF will do a little better for days3 or 4 onwards but comes available several hours later than the GFs and is not, as fas as I know, available in GRIB form to most sailors. Anyone giving advice from the shore is going to be depending on the GFS or similar models.
How you get the GFS is, of course, another matter. With internet
access, zyGrib is the best service
that I know with the quickest downloads, good topography and many data options. CAPE is particularly useful in indicating areas of strong convection, therefore thunderstorm/lightning risk.
At sea, the email
from Saildocs with its repeat request facility is probably the most sufficient. Otherwise MailASail. You can still use the zyGrib viewer. Obviously, HF radio
is the cheapest option but with careful choice of such as gri spacing of the data, area, time intervals, costs over Iridium
should not be too great.
Obviously (?) I should have added, whatever GMDSS service
you can receive via VHF, MF.HF and, INMARSAT.