Nor'west arch

This is the early stage of what we call a nor'west arch. Warm moist air from the Tasman Sea is pushed up by the Southern Alps, causing it to cool rapidly. Much of the moisture is dumped on the West Coast. As the air passes over The Alps, the remaining water vapour becomes visible as a band of cloud. To someone looking from the east, it appears as an arch.

Characteristics of a nor'west arch are low cloud over the mountains, blue sky immediately above, and high cloud bands to the east.

Bingo! That's just what we saw this morning in Christchurch!