In short
See also: why forecasts can be wrong.
When they nail it, enjoy it.
For the flip side (and a straighter explanation), see Why are the forecasts so wrong?.
Why are the forecasts so accurate?
More Weather Help
What is the Beaufort Scale?
What's the difference between Swell, Waves & Seas?
How to read the Wind graphs
What is the difference between waves and swell?
How to read wind forecasts
How does the weather work?
How do "My Graphs" / My Weather layouts work?
Slow, Jittery or Lagging drawing of the weather graphs