About 12 hours ago, at 18:30 local time (06:30 GMT) on Thursday, Cyclone Cook roared out of an ocean that has now been considerably warmed by human-forced climate change to made landfall in the Bay of Plenty region of New Zealand. Packing wind gusts of up 90 miles per hour and lashing the region with 1/2 to 1 inch per hour rainfall rates, the storm is the most powerful cyclone to strike New Zealand since 1968.
Most Powerful New Zealand Cyclone in Nearly 50 Years
(New Zealand under a swirl of clouds as Cook makes landfall. Image source: LANCE MODIS.)
The storm raked a region that had already seen saturating, long-lasting, rainfall from the remnants of Cyclone Debbie just one week before. As a result, trees unable to gain purchase in the weakened soils uprooted en-masse. Power was knocked out in Whakatane and in numerous other locations along Cook’s path — cutting off at least 11,000 residences. Locals described gusts like freight trains as hundreds huddled in evacuation shelters. Flights out of Rotorua, Tauranga, Napier and Hamilton in the North Island, and Nelson and Blenheim in the South Island were all canceled.
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.
Hat tip to LeslieGraham (please stay safe!)
Hat tip to wili