The internet has many good sources of free information for the weather. If you really want to get the most reliable weather window for a passage, you need to compare several weather models (GFS, ECMWF,NAM,..) and look for agreement among them. Fortunately, many weather services make this easy to do. My favorite weather sources are:
Below is an example from Windy of a unique situation where the weather models strongly agree on the wind strength and direction for several days.
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