Cyclone COOK compared with the WAHINE storm.

Cyclone COOK compared with GISELLE (The WAHINE storm/'68)
Posted on April 12, 2017

There are a few tropical cyclones who exit from the tropics quicker than they can unravel.
These may be called 'bolters' and Cyclone COOK is one of these.

Usually when a tropical cyclone departs from the warm seas of the tropics, its eye-wall collapses and the systems weakens. Also, the top of the system is often knocked off by a jetstream at around 30degrees south latitude, so usually the system is no longer a tropical system and is downgraded and referred to as ex-tropical cyclone (or extra tropical cyclone, meaning a cyclone out of the tropics).

The feature that once was Tropical Cyclone COOK has now left the tropics so has lost the word "Tropical", but is still spinning with a zone of stronger winds in a ring near its centre, so may still keep its name and be known as Cyclone COOK (for the meantime).

Cyclone COOK is expected to go through a period of deepening tonight (Wednesday night) as it approaches New Zealand. An ensemble of possible tracks and the average of these, may be seen at www.tropicaltidbits.com/storminfo/16P_gefs_latest.png
This "period of deepening" is expected to take place as Cyclone COOK travels south underneath a jetstream and encounters cooler air, the same process as with any mid-latitude depression deepening. This is expected to deepen the central pressure of Cyclone COOK from 994 to 984hPa overnight.

The team at www.tropicaltidbits.com have compared the track of Cyclone COOK with their extensive database of previous cyclones and listed the closest eight analogues. The most recent of these is BECKY in 2007.
It is interesting that the closest on the list is Cyclone GISELLE in April 1968, known as the "WAHINE Storm", which resulted in the sinking of the Inter-Island ferry MV WAHINE. However, Cyclone GISELLE encountered noticeably colder air near Cook Strait and went through a period of rapid deepening there as a mid-latitude system. In comparison, Cyclone COOK is only expected to deepen by 10hPa on Wednesday night and then its central pressure should remain steady on Thursday. Cyclone GISELLE may be the closest analogue in the database to Cyclone COOK, but with an analogue score of only 0.486 this is to be treated as a questionable analogue.

The Track of GISELLE may be seen at australiasevereweather.com/tropical_cyclones/1967_1968/jtwc/tropical_cyclone_giselle.htm

As mentioned in my previous blog, Cyclone COOK has an accompanying tropical depression located near Southern Cooks. See www.met.gov.fj/aifs_prods/0991.jpg. This accomplice is feeding in moist air and Cyclone COOK is turning this into rain. An analysis of the trajectory of the air reaching NZ with Cyclone Cook (from ready.arl.noaa.gov/HYSPLIT_traj.php) shows that it has come from just south of the Sothern Cooks, with a long path across tropically warm seas.

Cyclone BOLA in 1988 had an accomplice like this
- Cyclone BOLA was a 'lingerer', and Cyclone COOK is a 'bolter', so we can expect a different outcome this time; maybe much the same peak intensity of rain, but in different places and only for a day or less, not for three days in one place.

Be aware that the actual path taken by Cyclone COOK may easily depart from the forecast path, and a small change in track leads to large changes in likely conditions.
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