Coming from the aviation site: What is the westerly index?
The westerly index is:
... the Trenberth Z1 index (Trenberth 1976), which is the departure of the monthly mean pressure difference between Auckland and Christchurch (near 44°S) from its mean over the 1961–90 period.
Those two places are ~750 km apart (not a small distance), and judging by the METAR reports, the QNH difference in the past 72 hours varies from 4 to 7 millibars or so. Is that unusual? What's the significance of that index?