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

17 July 2016

Bob Blog 17 July 2016b

Issued 17 July 2016
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.

I'm in mid-winter holiday mode this week in Bay of Islands. Have visited my
nephew's family in Kerikeri today (he's now got a lovely daughter). Will be
visiting Northland radio on Monday, and the remainder of
week is my own (apart from 18 yachts still on my joblist)

This week I shall comment on the latest readings of the tropical parameters for
El Nino/La Nina as seen at

This shows that the Southern Oscillation index is now slightly positive-we are
in neutral territory between an El Nino and a La Nina. The climate forecasts
are for a La Nina to kick in during the next few months. If so that should
encourage a northeasterly low between Tonga and New Zealand. Well as you saw in
last week's blog, we have been having that for months now anyway.. So maybe the
seasonal forecast is for more of the same.

Looking at the heat-store in the ocean, a slightly different story may be told.
This graph is extended backwards so we can compare the recent large El Nino with
the one in 1998. We can see that the 1998 El Nino as far as the ocean heat-store
was concerned, dove quickly into a La Nina. We can also see that our recent El
Nino was stronger and longer, and is not cooling off as fast. Maybe things
will be slow to change. I suppose that's another way of saying expect more of
the same.

The Tropics
CELIA is set to visit northern parts of Hawaii over next few days, but just as a
low , not a cyclone.
Lining up in the eastern winds for Hawaii are Cyclone DARBY and maybe ESTILLE

We can see from
that the eastern north pacific is the breeding ground this month looking at the
weekly rain maps. These also show interesting developments in the South Indian
Ocean and over eastern Australia (brief trough).

SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain draped from Coral Sea to north of Vanuatu to Tokelau.
A smaller convergence zone is likely to bother the area between Marquesas and
Tuamotu islands this week, and a passing trough should affect Southern Cooks
from local mid-week and move onto Tahiti next weekend.
The trough over Cairns to Mackay tonight is expected to fade over next two days.

This week's SPCZ may be seen on, select rain accumulation 10 days.

STR= Sub-tropical Ridge
The weak HIGH over the New South Wales area tonight is expected to stall on
Monday and Tuesday awaiting reinforcements and then travel east across Tasman
Sea on Wednesday and over northern NZ late Thursday early Friday. The next weak
ridge should cross northern NZ on Late Monday/early Tuesday 25/26 July.

Voyage Outlooks:
Tahiti to the West
It should be a reasonably quiet and dry weak in Tahiti this week - but may be
more active next week with a passing trough. As for travelling west, well such
a voyage is likely to encounter that passing trough as some stage, but this is
expected to be minor. From Sat 23 July, as that HIGH travels east of New
Zealand, there may be a squash zone of enhanced trade winds between 20S and

Between NZ and the tropics
Disturbed SW winds over northern NZ this week, there are breaks in this flow on
Monday, Thursday, and maybe on Sunday. Increasing NW winds on Friday and
Saturday ahead of this week's most active front on Saturday night.

Snow is forecast on State Highway across central North Island tonight for first
time this winter. There is a parameter that measures how far north the SW winds
from the Southern Ocean can get over NZ. It is very blunt the southern Annular
Mode or SAM, and the only measure I can find for it is the AAO (AntArctic
Oscialltion) at
, which covers the entire Southern Hemisphere (not just NZ). This tells
an interesting story: that its been positive last few months, has dived briefly
negative mid-July, and is expected to go positive again after today. Not good
for skiing.

See my yotpak at for terms used.
See my website for information on tailor-made voyage forecasts
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