Bob McDavitt's ideas for sailing weather around the South pacific

18 February 2018

Bob Blog 18 Feb 2018



Compiled Sun 18 Feb 2018


Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.


My good wishes to the University students who are crewing on Training Tall Ship SSV ROBERT C SEAMANS.

The vessel visited Auckland last week and sailed to Opua late in the week ( see

Captain, and Professor in Nautical Science, Elliot Rappaport invited me on deck.

I especially like that the students manage a full-time marine lab and also are one of the VOS (Voluntary Observing Ships) that send in regular weather reports using properly calibrated instruments. These observations, around the planet, are part of what helps the global weather models in touch with the real world. See

In a month or so they will be going to Lyttelton then Chathams then French Polynesia, I hope they get to enjoy NZ and then have a Bon Voyage to FP.



We have been having an MJO in the Pacific, a period of extra convection. The passage of this pulse around the world can be tracked with a phase diagram. This diagram shows that the current MJO is fading away. It is expected to continue to fade this week and to revert to the Indian Ocean. A n MJO is often associated with triggering tropical cyclones, and this one helped FEHI and GITA to form. Now that the MJO is moving off we may be able to have a few settled weeks, but there is still the risk of another tropical cyclone forming. The next MJO, around mid to late March, should exacerbate that risk.

The MJO phase diagram may be seen at


GITA brought destructive wind and rain to Tongatapu and surrounding Islands, and got larger and more intense over southern Lau, affecting Ono-i-lau and Vatoa (Cat 4). Since then it has been keeping away from land. It is now weakening as it heads SW across the tropical border and likely to become ex-tropical on Monday, giving wind gusts to around 35 knots to Lord Howe Island.


Then it is expected to curve to S then SE and cross central NZ on Tuesday night and then be south of Chathams by Thursday. May have gusts over 60 knots and swell to over 8 significant metres in its dangerous quadrant as it approaches land, which will then be on its NE side, affecting Taranaki to Kapiti/Nelson. By then the system should be unravelling and its heaviest rain may be in the southern semicircle rather than the NE quadrant, which means Westland/uller /Nelson/Marlborough/ Kaikoura Ranges may well get drenched.

Ensemble runs from the Canadian model, may be seen at


In the past week TC KELVIN (up to CAT 2) formed off NW Australia and made landfall over ‘the’ Kimberley. It is now a rain depression fading as it travels south into the Australian interior.

Satellite imagery of both cyclones may be seen at


There is a small Tropical Low over the COOK Islands vaguely east of Palmerston island and north of Rarotonga. It is expected to travel off to the south over next few days without developing any further, but it does contain squally showers.


Looking at the weekly rain maps from last week and the week before, we can see that the most intense rain around the world is in the South Pacific Convergence zone. It has spread east and is now lapping on western parts of French Polynesia. Further west, it has thinned and shifted north to be over Solomon Islands to Tuvalu/Tokelau.

The track of GITA stands out, but KELVIN’s track is less obvious.  See



SPCZ=South Pacific Convergence zone.

The SPCZ is active across the whole region mainly from Solomons to Tuvalu to Tokelau/Samoa to the Cook Islands and western parts of French Polynesia.


Subtropical ridge (STR)

HIGH to east of NZ at around 160W is quasi-stationary and is expected to travel northeast and end the week near 25S 140W.

The next HIGH is currently in the Australian Bight and is expected to travel across Southern Ocean late this week and across northern NZ next weekend 24/25 Feb with light winds.


Around Tasman Sea

A week to avoid the Tasman Sea, because of ex-GITA.   On Monday  the upper trough moving onto NZ from the southern ocean will encounter this system from the tropics.  The strengthening NW winds aloft will likely capture ex GITA and shear it apart, venting its top half quickly downstream. However, the models are indicating there should still be sufficient rotation  in the surface low to bring wind/swell/rain and storm surge to NZ shores.  Also the injection of cooler air into the system helps it deepen as a mid-latitude low.


Panama to Galapagos

If you are ready now to do this strip, then the next two weeks are looking OK with N to NE winds as far as 5N. After that there are light ESE winds to Galapagos, and a weak convergence zone from 5N to 2N.



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