Boeing published a book some years ago that shows the winds on various city pair airway/jet routes based on annual, seasonal and monthly conditions. The wind levels are expressed as a function of probability.
An 80% probability headwind would mean you could expect to encounter a wind of that level or lower 80% of the time. The wind would be higher 20% of the time.
An 80% probability tailwind would mean you could expect to encounter a wind of that level or higher 80% of the time. The wind would be lower 20% of the time.
The book has been converted to a computer program that has been updated over the years for new routes and observed weather conditions.