Current predictable?

Stu

Was at the Langford Memorial reef (near the Rankin) at the last full moon and experienced a 0.6 kt. drift. Two weeks later, without being wind blown, it was 3.5 kts. Is this predictable or does the influence of the Stream vary irregularly and erratically? Snapper fishing a small piece of structure in 145' is sure a lot easier when you can spend longer over it!

Thanks.