The 30 foot swells and a resulting storm surge in the Bay of Plenty region were expected to result in serious coastal flooding and damage to shore-front structures. But the chief worry from the system, after Debbie’s devastating rains, was more precipitation-related flooding.
(GFS 7 day rainfall forecast for New Zealand shows considerably above average precipitation from Cook. See also GFS rainfall model runs.)
GFS model runs indicated the potential for 4-8 inches of rainfall or more near New Zealand population centers along the path of Cyclone Cook. And for many regions, these totals equal about 1-2 months worth of rainfall at this time of year. Last week, 7.5 inches of rainfall over just two days resulted in a levee breach at Edgecumbe on North Island — flooding the entire town and forcing nearly all the residents to evacuate. And there is some concern that Cook’s follow-on to Debbie will produce similar trouble.
Warming Ocean Waters and High Amplitude Waves in the Jet Stream Feed Storm Pattern
Cook is interacting with a trough to the west of New Zealand in a manner that is broadening the storm — spreading its wind field and rainfall over a larger region than a purely tropical system would typically impact. The trough had dipped down from the Southern Ocean through an extended Jet Stream wave before it became cut-off and linked up with Cook.
(Cook is presently centered between New Zealand’s North and South Islands [roughly under the green circle]. The swirl of clouds and wind to the west of Cook is a second low pressure system that was cut off from a trough sweeping south and west of New Zealand on Tuesday and Wednesday. Cook is interacting with this trough in a manner than is broadening its wind field and enhancing rainfall potentials. Image source: Earth Nullschool.)
The far northward extent of this trough is indicative of higher amplitude Jet Stream waves that have been associated with warming temperatures at the polar regions due to climate change. And the zone south of New Zealand over Antarctica has featured a strong dipole — with well above normal temperatures facing off against a wall of cold air. This dipole has facilitated troughs and facing ridges that extended well into the middle latitudes.
(Cyclone Cook fed on far warmer than normal waters which enabled it maintain intensity as it moved into higher latitudes. Interaction with a trough remnant left over from a high amplitude wave in the Jet Stream also contributed to this extreme weather event. Image source: Earth Nullschool.)
Meanwhile, Cyclone Cook itself fed on 1-2 C warmer than normal sea surface temperatures surrounding New Zealand. These warmer than normal waters allowed Cook to retain strength and to interact with the polar originating trough in a manner that arguably intensified and broadened the scope of this severe weather event.